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In polymer chemistry, radical polymerization (RP) is a method of polymerization by which a polymer forms by the successive addition of a radical to building blocks (repeat units). Radicals can be formed by a number of different mechanisms, usually involving separate initiator molecules. Following its generation, the initiating radical adds (nonradical) monomer units, thereby growing the polymer chain.
Radical polymerization is a key synthesis route for obtaining a wide variety of different polymers and materials composites. The relatively non-specific nature of radical chemical interactions makes this one of the most versatile forms of polymerization available and allows facile reactions of polymeric radical chain ends and other chemicals or substrates. In 2001, 40 billion of the 110 billion pounds of polymers produced in the United States were produced by radical polymerization.[1]
Initiation is the first step of the polymerization process. During initiation, an active center is created from which a polymer chain is generated. Not all monomers are susceptible to all types of initiators. Radical initiation works best on the carbon–carbon double bond of vinyl monomers and the carbon–oxygen double bond in aldehydes and ketones.[1] Initiation has two steps. In the first step, one or two radicals are created from the initiating molecules. In the second step, radicals are transferred from the initiator molecules to the monomer units present. Several choices are available for these initiators.
The initiator is heated until a bond is homolytically cleaved, producing two radicals (Figure 1). This method is used most often with organic peroxides or azo compounds.[2]Figure 1: Thermal decomposition of dicumyl peroxide
Radiation cleaves a bond homolytically, producing two radicals (Figure 2). This method is used most often with metal iodides, metal alkyls, and azo compounds.[2]Figure 2: Photolysis of azoisobutylnitrile (AIBN) Photoinitiation can also occur by bi-molecular H abstraction when the radical is in its lowest triplet excited state.[3] An acceptable photoinitiator system should fulfill the following requirements:[3]
Reduction of hydrogen peroxide or an alkyl hydrogen peroxide by iron (Figure 3).[2] Other reductants such as Cr2+, V2+, Ti3+, Co2+, and Cu+ can be employed in place of ferrous ion in many instances.[1]
Figure 3: Redox reaction of hydrogen peroxide and iron.
The dissociation of a persulfate in the aqueous phase (Figure 4). This method is useful in emulsion polymerizations, in which the radical propagates initially in the water phase before entry into polymer particles (or self-collapsing to nucleate a particle).[2]Figure 4: Thermal degradation of a persulfate
α-, β-, γ-, or x-rays cause ejection of an electron from the initiating species, followed by dissociation and electron capture to produce a radical (Figure 5).[2]Figure 5: The three steps involved in ionizing radiation: ejection, dissociation, and electron-capture
Electrolysis of a solution containing both monomer and electrolyte. A monomer molecule will receive an electron at the cathode to become a radical anion, and a monomer molecule will give up an electron at the anode to form a radical cation (Figure 6). The radical ions then initiate free radical (and/or ionic) polymerization. This type of initiation is especially useful for coating metal surfaces with polymer films.[4]Figure 6: (Top) Formation of radical anion at the cathode; (bottom) formation of radical cation at the anode
A gaseous monomer is placed in an electric discharge at low pressures under conditions where a plasma (ionized gaseous molecules) is created. In some cases, the system is heated and/or placed in a radiofrequency field to assist in creating the plasma.[1]
High-intensity ultrasound at frequencies beyond the range of human hearing (16 kHz) can be applied to a monomer. Initiation results from the effects of cavitation (the formation and collapse of cavities in the liquid). The collapse of the cavities generates very high local temperatures and pressures. This results in the formation of excited electronic states, which in turn lead to bond breakage and radical formation.[1]
Ternary initiators
A ternary initiator is the combination of several types of initiators into one initiating system. The types of initiators are chosen based on the properties they are known to induce in the polymers they produce. For example, poly(methyl methacrylate) has been synthesized by the ternary system benzoyl peroxide and 3,6-bis(o-carboxybenzoyl)-N-isopropylcarbazole and di-η5-indenylzirconium dichloride (Figure 7).[5][6]Figure 7: benzoyl peroxide + 3,6-bis(o-carboxybenzoyl)-N-isopropylcarbazole + di-η5-indenylzicronium dichlorideThis type of initiating system contains a metallocene, an initiator, and a heteroaromaticdiketocarboxylic acid. Metallocenes in combination with initiators accelerate polymerization of poly(methyl methacrylate) and produce a polymer with a narrower molecular weight distribution. The example shown here consists of indenylzirconium (a metallocene) and benzoyl peroxide (an initiator). Also, initiating systems containing heteroaromatic diketo carboxylic acids, such as 3,6-bis(o-carboxybenzoyl)-N-isopropylcarbazole in this example, are known to catalyze the decomposition of benzoyl peroxide. Initiating systems with this particular heteroaromatic diket carboxylic acid are also known to have effects on the microstructure of the polymer. The combination of all of these components—a metallocene, an initiator, and a heteroaromatic diketo carboxylic acid—yields a ternary initiating system that was shown to accelerate the polymerization and produce polymers with enhanced heat resistance and regular microstructure.[5][6]
Due to side reactions, not all radicals formed by the dissociation of initiator molecules actually add monomers to form polymer chains. The efficiency factor f is defined as the fraction of the original initiator which contributes to the polymerization reaction. The maximal value of f is 1, but typical values range from 0.3 to 0.8.[7]
The following types of reactions can decrease the efficiency of the initiator.
Primary recombination
Two radicals recombine before initiating a chain (Figure 8). This occurs within the solvent cage, meaning that no solvent has yet come between the new radicals.[2]
Figure 8: Primary recombination of BPO; brackets indicate that the reaction is happening within the solvent cage
Other recombination pathways
Two radical initiators recombine before initiating a chain, but not in the solvent cage (Figure 9).[2]
Figure 9: Recombination of phenyl radicals from the initiation of BPO outside the solvent cage
Side reactions
One radical is produced instead of the three radicals that could be produced (Figure 10).[2]
Figure 10: Reaction of polymer chain R with other species in reaction
During polymerization, a polymer spends most of its time in increasing its chain length, or propagating. After the radical initiator is formed, it attacks a monomer (Figure 11).[8] In an ethene monomer, one electron pair is held securely between the two carbons in a sigma bond. The other is more loosely held in a pi bond. The free radical uses one electron from the pi bond to form a more stable bond with the carbon atom. The other electron returns to the second carbon atom, turning the whole molecule into another radical. This begins the polymer chain. Figure 12 shows how the orbitals of an ethylene monomer interact with a radical initiator.[9]
Figure 11: Phenyl initiator from benzoyl peroxide (BPO) attacks a styrene molecule to start the polymer chain.Figure 12: An orbital drawing of the initiator attack on ethylene molecule, producing the start of the polyethylene chain.
Once a chain has been initiated, the chain propagates (Figure 13) until there are no more monomers (living polymerization) or until termination occurs. There may be anywhere from a few to thousands of propagation steps depending on several factors such as radical and chain reactivity, the solvent, and temperature.[10][11] The mechanism of chain propagation is as follows:
Figure 13: Propagation of polystyrene with a phenyl radical initiator.
Chain termination is inevitable in radical polymerization due to the high reactivity of radicals. Termination can occur by several different mechanisms. If longer chains are desired, the initiator concentration should be kept low; otherwise, many shorter chains will result.[2]
Combination of two active chain ends: one or both of the following processes may occur.
Combination: two chain ends simply couple together to form one long chain (Figure 14). One can determine if this mode of termination is occurring by monitoring the molecular weight of the propagating species: combination will result in doubling of molecular weight. Also, combination will result in a polymer that is C2 symmetric about the point of the combination.[9]Figure 14: Termination by the combination of two poly(vinyl chloride) (PVC) polymers.
Radical disproportionation: a hydrogen atom from one chain end is abstracted to another, producing a polymer with a terminal unsaturated group and a polymer with a terminal saturated group (Figure 15).[4]Figure 15: Termination by disproportionation of poly(methyl methacrylate).
Combination of an active chain end with an initiator radical (Figure 16).[2]Figure 16: Termination of PVC by reaction with radical initiator.
Interaction with impurities or inhibitors. Oxygen is the common inhibitor. The growing chain will react with molecular oxygen, producing an oxygen radical, which is much less reactive (Figure 17). This significantly slows down the rate of propagation. Figure 17: Inhibition of polystyrene propagation due to reaction of polymer with molecular oxygen.Nitrobenzene, butylated hydroxyl toluene, and diphenyl picryl hydrazyl (DPPH, Figure 18) are a few other inhibitors. The latter is an especially effective inhibitor because of the resonance stabilization of the radical.[2]Figure 18: Inhibition of polymer chain, R, by DPPH.
Contrary to the other modes of termination, chain transfer results in the destruction of only one radical, but also the creation of another radical. Often, however, this newly created radical is not capable of further propagation. Similar to disproportionation, all chain-transfer mechanisms also involve the abstraction of a hydrogen or other atom. There are several types of chain-transfer mechanisms.[2]
To solvent: a hydrogen atom is abstracted from a solvent molecule, resulting in the formation of radical on the solvent molecules, which will not propagate further (Figure 19). Figure 19: Chain transfer from polystyrene to solvent. The effectiveness of chain transfer involving solvent molecules depends on the amount of solvent present (more solvent leads to greater probability of transfer), the strength of the bond involved in the abstraction step (weaker bond leads to greater probability of transfer), and the stability of the solvent radical that is formed (greater stability leads to greater probability of transfer). Halogens, except fluorine, are easily transferred.[2]
To monomer: a hydrogen atom is abstracted from a monomer. While this does create a radical on the affected monomer, resonance stabilization of this radical discourages further propagation (Figure 20).[2]Figure 20: Chain transfer from polypropylene to monomer.
To initiator: a polymer chain reacts with an initiator, which terminates that polymer chain, but creates a new radical initiator (Figure 21). This initiator can then begin new polymer chains. Therefore, contrary to the other forms of chain transfer, chain transfer to the initiator does allow for further propagation. Peroxide initiators are especially sensitive to chain transfer.[2]Figure 21: Chain transfer from polypropylene to di-t-butyl peroxide initiator.
To polymer: the radical of a polymer chain abstracts a hydrogen atom from somewhere on another polymer chain (Figure 22). This terminates the growth of one polymer chain, but allows the other to branch and resume growing. This reaction step changes neither the number of polymer chains nor the number of monomers which have been polymerized, so that the number-average degree of polymerization is unaffected.[12]Figure 22: Chain transfer from polypropylene to backbone of another polypropylene.
Effects of chain transfer: The most obvious effect of chain transfer is a decrease in the polymer chain length. If the rate of transfer is much larger than the rate of propagation, then very small polymers are formed with chain lengths of 2-5 repeating units (telomerization).[13] The Mayo equation estimates the influence of chain transfer on chain length (xn): . Where ktr is the rate constant for chain transfer and kp is the rate constant for propagation. The Mayo equation assumes that transfer to solvent is the major termination pathway.[2][14]
Suspension polymerization: reaction mixture contains an aqueous phase, water-insoluble monomer, and initiator soluble in the monomer droplets (both the monomer and the initiator are hydrophobic).
Emulsion polymerization: similar to suspension polymerization except that the initiator is soluble in the aqueous phase rather than in the monomer droplets (the monomer is hydrophobic, and the initiator is hydrophilic). An emulsifying agent is also needed.
Other methods of radical polymerization include the following:
Template polymerization: In this process, polymer chains are allowed to grow along template macromolecules for the greater part of their lifetime. A well-chosen template can affect the rate of polymerization as well as the molar mass and microstructure of the daughter polymer. The molar mass of a daughter polymer can be up to 70 times greater than those of polymers produced in the absence of the template and can be higher in molar mass than the templates themselves. This is because of retardation of the termination for template-associated radicals and by hopping of a radical to the neighboring template after reaching the end of a template polymer.[15]
Plasma polymerization: The polymerization is initiated with plasma. A variety of organic molecules including alkenes, alkynes, and alkanes undergo polymerization to high molecular weight products under these conditions. The propagation mechanisms appear to involve both ionic and radical species. Plasma polymerization offers a potentially unique method of forming thin polymer films for uses such as thin-film capacitors, antireflection coatings, and various types of thin membranes.[1]
Sonication: The polymerization is initiated by high-intensity ultrasound. Polymerization to high molecular weight polymer is observed but the conversions are low (<15%). The polymerization is self-limiting because of the high viscosity produced even at low conversion. High viscosity hinders cavitation and radical production.[1]
Also known as living radical polymerization, controlled radical polymerization, reversible deactivation radical polymerization (RDRP) relies on completely pure reactions, preventing termination caused by impurities. Because these polymerizations stop only when there is no more monomer, polymerization can continue upon the addition of more monomer. Block copolymers can be made this way. RDRP allows for control of molecular weight and dispersity. However, this is very difficult to achieve and instead a pseudo-living polymerization occurs with only partial control of molecular weight and dispersity.[15] ATRP and RAFT are the main types of complete radical polymerization.
Stable Free Radical Polymerization (SFRP): used to synthesize linear or branched polymers with narrow molecular weight distributions and reactive end groups on each polymer chain. The process has also been used to create block co-polymers with unique properties. Conversion rates are about 100% using this process but require temperatures of about 135 °C. This process is most commonly used with acrylates, styrenes, and dienes. The reaction scheme in Figure 23 illustrates the SFRP process.[19]Figure 23: Reaction scheme for SFRP.Figure 24: TEMPO molecule used to functionalize the chain ends. Because the chain end is functionalized with the TEMPO molecule (Figure 24), premature termination by coupling is reduced. As with all living polymerizations, the polymer chain grows until all of the monomer is consumed.[19]
In typical chain growth polymerizations, the reaction rates for initiation, propagation and termination can be described as follows:
where f is the efficiency of the initiator and kd, kp, and kt are the constants for initiator dissociation, chain propagation and termination, respectively. [I] [M] and [M•] are the concentrations of the initiator, monomer and the active growing chain.
Under the steady-state approximation, the concentration of the active growing chains remains constant, i.e. the rates of initiation and of termination are equal. The concentration of active chain can be derived and expressed in terms of the other known species in the system.
In this case, the rate of chain propagation can be further described using a function of the initiator and monomer concentrations[20][21]
The kinetic chain length v is a measure of the average number of monomer units reacting with an active center during its lifetime and is related to the molecular weight through the mechanism of the termination. Without chain transfer, the kinetic chain length is only a function of propagation rate and initiation rate.[22]
Assuming no chain-transfer effect occurs in the reaction, the number average degree of polymerization Pn can be correlated with the kinetic chain length. In the case of termination by disproportionation, one polymer molecule is produced per every kinetic chain:
Termination by combination leads to one polymer molecule per two kinetic chains:[20]
Any mixture of both these mechanisms can be described by using the value δ, the contribution of disproportionation to the overall termination process:
If chain transfer is considered, the kinetic chain length is not affected by the transfer process because the growing free-radical center generated by the initiation step stays alive after any chain-transfer event, although multiple polymer chains are produced. However, the number average degree of polymerization decreases as the chain transfers, since the growing chains are terminated by the chain-transfer events. Taking into account the chain-transfer reaction towards solvent S, initiator I, polymer P, and added chain-transfer agent T. The equation of Pn will be modified as follows:[23]
It is usual to define chain-transfer constants C for the different molecules
In chain growth polymerization, the position of the equilibrium between polymer and monomers can be determined by the thermodynamics of the polymerization. The Gibbs free energy (ΔGp) of the polymerization is commonly used to quantify the tendency of a polymeric reaction. The polymerization will be favored if ΔGp < 0; if ΔGp > 0, the polymer will undergo depolymerization. According to the thermodynamic equation ΔG = ΔH – TΔS, a negative enthalpy and an increasing entropy will shift the equilibrium towards polymerization.
In general, the polymerization is an exothermic process, i.e. negative enthalpy change, since addition of a monomer to the growing polymer chain involves the conversion of π bonds into σ bonds, or a ring–opening reaction that releases the ring tension in a cyclic monomer. Meanwhile, during polymerization, a large amount of small molecules are associated, losing rotation and translational degrees of freedom. As a result, the entropy decreases in the system, ΔSp < 0 for nearly all polymerization processes. Since depolymerization is almost always entropically favored, the ΔHp must then be sufficiently negative to compensate for the unfavorable entropic term. Only then will polymerization be thermodynamically favored by the resulting negative ΔGp.
In practice, polymerization is favored at low temperatures: TΔSp is small. Depolymerization is favored at high temperatures: TΔSp is large. As the temperature increases, ΔGp become less negative. At a certain temperature, the polymerization reaches equilibrium (rate of polymerization = rate of depolymerization). This temperature is called the ceiling temperature (Tc). ΔGp = 0.[24]
The stereochemistry of polymerization is concerned with the difference in atom connectivity and spatial orientation in polymers that has the same chemical composition.
Hermann Staudinger studied the stereoisomerism in chain polymerization of vinyl monomers in the late 1920s, and it took another two decades for people to fully appreciate the idea that each of the propagation steps in the polymer growth could give rise to stereoisomerism. The major milestone in the stereochemistry was established by Ziegler and Natta and their coworkers in 1950s, as they developed metal based catalyst to synthesize stereoregular polymers. The reason why the stereochemistry of the polymer is of particular interest is because the physical behavior of a polymer depends not only on the general chemical composition but also on the more subtle differences in microstructure.[25]Atactic polymers consist of a random arrangement of stereochemistry and are amorphous (noncrystalline), soft materials with lower physical strength. The corresponding isotactic (like substituents all on the same side) and syndiotactic (like substituents of alternate repeating units on the same side) polymers are usually obtained as highly crystalline materials. It is easier for the stereoregular polymers to pack into a crystal lattice since they are more ordered and the resulting crystallinity leads to higher physical strength and increased solvent and chemical resistance as well as differences in other properties that depend on crystallinity. The prime example of the industrial utility of stereoregular polymers is polypropene. Isotactic polypropene is a high-melting (165 °C), strong, crystalline polymer, which is used as both a plastic and fiber. Atactic polypropene is an amorphous material with an oily to waxy soft appearance that finds use in asphalt blends and formulations for lubricants, sealants, and adhesives, but the volumes are minuscule compared to that of isotactic polypropene.
When a monomer adds to a radical chain end, there are two factors to consider regarding its stereochemistry: 1) the interaction between the terminal chain carbon and the approaching monomer molecule and 2) the configuration of the penultimate repeating unit in the polymer chain.[4] The terminal carbon atom has sp2 hybridization and is planar. Consider the polymerization of the monomer CH2=CXY. There are two ways that a monomer molecule can approach the terminal carbon: the mirror approach (with like substituents on the same side) or the non-mirror approach (like substituents on opposite sides). If free rotation does not occur before the next monomer adds, the mirror approach will always lead to an isotactic polymer and the non-mirror approach will always lead to a syndiotactic polymer (Figure 25).[4]
Figure 25: (Top) formation of isotactic polymer; (bottom) formation of syndiotactic polymer.
However, if interactions between the substituents of the penultimate repeating unit and the terminal carbon atom are significant, then conformational factors could cause the monomer to add to the polymer in a way that minimizes steric or electrostatic interaction (Figure 26).[4]
Figure 26: Penultimate unit interactions cause monomer to add in a way that minimizes steric hindrance between substituent groups. (P represents polymer chain.)
Traditionally, the reactivity of monomers and radicals are assessed by the means of copolymerization data. The Q–e scheme, the most widely used tool for the semi-quantitative prediction of monomer reactivity ratios, was first proposed by Alfrey and Price in 1947.[26] The scheme takes into account the intrinsic thermodynamic stability and polar effects in the transition state. A given radical and a monomer are considered to have intrinsic reactivities Pi and Qj, respectively.[27] The polar effects in the transition state, the supposed permanent electric charge carried by that entity (radical or molecule), is quantified by the factor e, which is a constant for a given monomer, and has the same value for the radical derived from that specific monomer. For addition of monomer 2 to a growing polymer chain whose active end is the radical of monomer 1, the rate constant, k12, is postulated to be related to the four relevant reactivity parameters by
The monomer reactivity ratio for the addition of monomers 1 and 2 to this chain is given by[27][28]
For the copolymerization of a given pair of monomers, the two experimental reactivity ratios r1 and r2 permit the evaluation of (Q1/Q2) and (e1 – e2). Values for each monomer can then be assigned relative to a reference monomer, usually chosen as styrene with the arbitrary values Q = 1.0 and e = –0.8.[28]
Free radical polymerization has found applications including the manufacture of polystyrene, thermoplasticblock copolymer elastomers,[29] cardiovascular stents,[30] chemical surfactants[31] and lubricants. Block copolymers are used for a wide variety of applications including adhesives, footwear and toys.
Free radical polymerization allows the functionalization of carbon nanotubes.[32] CNTs intrinsic electronic properties lead them to form large aggregates in solution, precluding useful applications. Adding small chemical groups to the walls of CNT can eliminate this propensity and tune the response to the surrounding environment. The use of polymers instead of smaller molecules can modify CNT properties (and conversely, nanotubes can modify polymer mechanical and electronic properties).[29] For example, researchers coated carbon nanotubes with polystyrene by first polymerizing polystyrene via chain radical polymerization and subsequently mixing it at 130 °C with carbon nanotubes to generate radicals and graft them onto the walls of carbon nanotubes (Figure 27).[33] Chain growth polymerization ("grafting to") synthesizes a polymer with predetermined properties. Purification of the polymer can be used to obtain a more uniform length distribution before grafting. Conversely, “grafting from”, with radical polymerization techniques such as atom transfer radical polymerization (ATRP) or nitroxide-mediated polymerization (NMP), allows rapid growth of high molecular weight polymers.
Figure 27: Grafting of a polystyrene free radical onto a single-walled carbon nanotube.
Radical polymerization also aids synthesis of nanocompositehydrogels.[34] These gels are made of water-swellable nano-scale clay (especially those classed as smectites) enveloped by a network polymer. Aqueous dispersions of clay are treated with an initiator and a catalyst and the organic monomer, generally an acrylamide. Polymers grow off the initiators that are in turn bound to the clay. Due to recombination and disproportionation reactions, growing polymer chains bind to one another, forming a strong, cross-linked network polymer, with clay particles acting as branching points for multiple polymer chain segments.[35] Free radical polymerization used in this context allows the synthesis of polymers from a wide variety of substrates (the chemistries of suitable clays vary). Termination reactions unique to chain growth polymerization produce a material with flexibility, mechanical strength and biocompatibility.
Figure 28: General synthesis procedure for a nanocomposite hydrogel.
^ abcdefgOdian, George (2004). Principles of Polymerization (4th ed.). New York: Wiley-Interscience. ISBN978-0-471-27400-1.
^ abcdefghijklmnopqrsCowie, J. M. G.; Arrighi, Valeria (2008). Polymers: Chemistry and Physics of Modern Materials (3rd ed.). Scotland: CRC Press. ISBN978-0-8493-9813-1.
^ abHageman, H. J. (1985). "Photoinitiators for Free Radical Polymerization". Progress in Organic Coatings. 13 (2): 123–150. doi:10.1016/0033-0655(85)80021-2.
^ abcdeStevens, Malcolm P. (1999). Polymer Chemistry: An Introduction. New York: Oxford University Press. ISBN978-0-19-512444-6.
^ abIslamova, R. M.; Puzin, Y. I.; Kraikin, V. A.; Fatykhov, A. A.; Dzhemilev, U. M. (2006). "Controlling the Polymerization of Methyl Methacrylate with Ternary Initiating Systems". Russian Journal of Applied Chemistry. 79 (9): 1509–1513. doi:10.1134/S1070427206090229. S2CID94433190.
^ abIslamova, R. M.; Puzin, Y. I.; Fatykhov, A. A.; Monakov, Y. B. (2006). "A Ternary Initiating System for Free Radical Polymerization of Methyl Methacrylate". Polymer Science, Series B. 48 (3): 130–133. doi:10.1134/S156009040605006X.
^Fried, Joel R. Polymer Science & Technology (2nd ed., Prentice-Hall 2003) p.36 ISBN0-13-018168-4
^Pojman, John A.; Jason Willis; Dionne Fortenberry; Victor Ilyashenko; Akhtar M. Khan (1995). "Factors affecting propagating fronts of addition polymerization: Velocity, front curvature, temperature profile, conversion, and molecular weight distribution". Journal of Polymer Science Part A: Polymer Chemistry. 33 (4): 643–652. Bibcode:1995JPoSA..33..643P. doi:10.1002/pola.1995.080330406.
^Rudin, Alfred The Elements of Polymer Science and Engineering (Academic Press 1982) p.220 ISBN0-12-601680-1
^Rudin, Alfred The Elements of Polymer Science and Engineering (Academic Press 1982) p.212 ISBN0-12-601680-1
^The Mayo equation for chain transfer should not be confused with the Mayo–Lewis equation for copolymers.
^ abColombani, Daniel (1997). "Chain-Growth Control in Free Radical Polymerization". Progress in Polymer Science. 22 (8): 1649–1720. doi:10.1016/S0079-6700(97)00022-1.
^Kato, M; Kamigaito, M; Sawamoto, M; Higashimura, T (1995). "Polymerization of Methyl Methacrylate with the Carbon Tetrachloride / Dichlorotris-(triphenylphosphine)ruthenium(II) / Methylaluminum Bis(2,6-di-tert-butylphenoxide) Initiating System: Possibility of Living Radical Polymerization". Macromolecules. 28 (5): 1721–1723. Bibcode:1995MaMol..28.1721K. doi:10.1021/ma00109a056.
^ abAllcock H.R., Lampe F.W. and Mark J.E. Contemporary Polymer Chemistry (3rd ed., Pearson Prentice-Hall 2003) p.364 ISBN0-13-065056-0
^ abRudin, Alfred The Elements of Polymer Science and Engineering (Academic Press 1982) p.289 ISBN0-12-601680-1
^ abBraunecker, W. A.; K. Matyjaszewski (2007). "Controlled/living radical polymerization: Features, developments, and perspectives". Progress in Polymer Science. 32 (1): 93–146. doi:10.1016/j.progpolymsci.2006.11.002.
^Richard, R. E.; M. Schwarz; S. Ranade; A. K. Chan; K. Matyjaszewski; B. Sumerlin (2005). "Evaluation of acrylate-based block copolymers prepared by atom transfer radical polymerization as matrices for paclitaxel delivery from coronary stents". Biomacromolecules. 6 (6): 3410–3418. doi:10.1021/bm050464v. PMID16283773.
^Burguiere, C.; S. Pascual; B. Coutin; A. Polton; M. Tardi; B. Charleux; K. Matyjaszewski; J. P. Vairon (2000). "Amphiphilic block copolymers prepared via controlled radical polymerization as surfactants for emulsion polymerization". Macromolecular Symposia. 150: 39–44. doi:10.1002/1521-3900(200002)150:1<39::AID-MASY39>3.0.CO;2-D.
^Homenick, C. M.; G. Lawson; A. Adronov (2007). "Polymer grafting of carbon nanotubes using living free-radical polymerization". Polymer Reviews. 47 (2): 265–270. doi:10.1080/15583720701271237. S2CID96213227.
Radical polymerization is a chain-growth polymerization process in which free radicals serve as the reactive intermediates, enabling the addition of monomers—typically those containing carbon-carbon double bonds—to a growing polymer chain.[1] The mechanism involves four primary steps: initiation, where radicals are generated from initiators such as peroxides or azo compounds via heat, light, or redox reactions; propagation, the rapid addition of monomers to the radical chain end; termination, through radical coupling or disproportionation to halt chain growth; and chain transfer, which redistributes radicals to other molecules, influencing molecular weight.[1] This versatile method tolerates a wide range of functional groups and solvents, making it suitable for bulk, solution, suspension, and emulsion polymerizations.[2]As the most widely used industrial polymerization technique, radical polymerization accounts for approximately 50% of global polymer production, yielding key materials such as low-density polyethylene (LDPE), polystyrene (PS), and poly(vinyl chloride) (PVC).[1] Its cost-effectiveness and ability to process diverse vinyl and acrylic monomers have driven applications in plastics, coatings, adhesives, and rubbers, including specialized products like acrylic rubber and polytetrafluoroethylene (PTFE).[1] In recent advances, controlled radical polymerization techniques, such as atom transfer radical polymerization (ATRP) and reversible addition-fragmentation chain transfer (RAFT), enable precise control over polymer architecture, molecular weight, and polydispersity, expanding uses into biomedical hydrogels, nanomaterials, and advanced functional materials.[3]
Introduction
Definition and Scope
Radical polymerization is a chain-growth polymerization process in which the kinetic-chain carriers are free radicals, typically carbon-centered radicals at the growing chain end.[4] In this method, polymerization proceeds through the successive addition of these radicals to vinyl monomers, such as those containing carbon-carbon double bonds, leading to the formation of a polymer chain.[5] The process is characterized by a slow rate of initiation, a fast rate of propagation, and a rapid rate of termination, enabling high-speed reactions that account for approximately half of industrial polymer production.[1]Key features of radical polymerization include its tolerance for a wide range of functional groups and solvents, including water, which allows for versatile applications without stringent purification requirements.[5] It is particularly applicable to monomers like styrene (yielding polystyrene), methyl methacrylate (for polymethyl methacrylate), acrylonitrile, vinyl acetate, and vinyl chloride (producing polyvinyl chloride).[5] The reaction is highly exothermic, with enthalpy changes typically ranging from 8 to 20 kcal/mol per monomer addition, necessitating careful heat management in industrial settings.[5] Due to the planar nature of the radical intermediate, which provides no inherent stereocontrol, the resulting polymers are generally atactic, lacking regular stereochemistry along the chain.[6]In comparison to ionic polymerizations, such as anionic or cationic methods, radical polymerization offers greater simplicity and robustness, as it does not require strict anhydrous conditions or sensitivity to impurities.[1] While ionic approaches can achieve higher control over molecular weight and tacticity—often producing stereoregular polymers—radical polymerization exhibits higher exothermicity and typically yields less ordered atactic structures.[6] This trade-off contributes to its widespread industrial adoption for commodity plastics despite limitations in precision.[1]
Historical Development
The earliest documented observation of radical polymerization occurred in 1835 when French chemist Henri Victor Regnault exposed vinyl chloride gas to sunlight and noted the formation of a white, solid polymeric material at the bottom of the container.[7] This accidental discovery marked the first recognition of vinyl chloride's tendency to polymerize under light, though Regnault did not pursue its implications further.[8]In the 1920s, German chemist Hermann Staudinger advanced the understanding of polymerization through his seminal work on chain reactions, proposing in his 1920 paper "Über Polymerisation" that high-molecular-weight substances form via the sequential addition of monomer units into long covalent chains rather than mere associations of small molecules.[9] Staudinger's chain reaction model laid the groundwork for viewing polymerization as a controlled linkage process, influencing subsequent research on reaction mechanisms.[10]The 1930s brought key milestones in elucidating the radical nature of the process, with American chemist Paul J. Flory describing the kinetics of vinyl polymerization in 1937 as a chain reaction involving free radicals, where initiation, propagation, and termination steps govern the overall rate.[11] Concurrently, German chemist Gerhard V. Schulz contributed to this framework through his 1939 studies on the radical mechanism, confirming bimolecular termination and providing experimental validation for Flory's theoretical predictions.[11] These insights solidified the free radical chain mechanism as the dominant paradigm for understanding polymerization kinetics.During World War II in the 1940s, the urgent need for synthetic rubber spurred the development of emulsion polymerization techniques, particularly for styrene-butadiene rubber (SBR), which was produced on an industrial scale in the United States starting in 1942 to replace natural rubber supplies disrupted by wartime blockades.[12] This method, refined from earlier German Buna-S processes, enabled high-yield production of durable elastomers via aqueous emulsions, demonstrating radical polymerization's scalability for strategic materials.[13]Industrial adoption of radical polymerization expanded in the 1940s with batch processes for commodity plastics like polystyrene and polyvinyl chloride (PVC), which were conducted in stirred autoclaves to manage exothermic reactions and achieve consistent molecular weights.[8] By the 1970s, the shift to continuous reactors, such as tubular and stirred-tank systems, improved efficiency and product uniformity for large-scale operations, reducing labor and energy costs in PVC and polyethylene production.[14]The post-1980s era saw a pivot toward controlled radical methods to achieve precise polymer architectures, exemplified by the introduction of atom transfer radical polymerization (ATRP) in 1995 by Krzysztof Matyjaszewski and colleagues, who demonstrated living polymerization using transition metal catalysts to mediate halogen transfer and minimize termination.[15] Building on this, in 1998, John Chiefari and coworkers at CSIRO developed reversible addition-fragmentation chain transfer (RAFT) polymerization, employing thiocarbonylthio compounds as chain transfer agents to enable controlled growth with low polydispersity across diverse monomers. These innovations marked a transition from conventional free radical processes to more versatile techniques for advanced materials.
Fundamental Mechanism
Initiation
Initiation is the initial step in radical polymerization, wherein primary radicals are generated from an initiator and subsequently add to a monomer molecule, forming the first propagating chain carrier (or active center). This process establishes the number of polymer chains and influences the overall polymerization rate and molecular weight distribution. The efficiency of initiation depends on the successful escape of primary radicals from the initiator's decomposition site to interact with the monomer, avoiding recombination or other deactivation pathways.[16]Several methods exist for generating initiating radicals, categorized by the energy source or chemical pathway employed. Thermal initiation commonly uses organic peroxides, such as benzoyl peroxide (BPO), which decompose at temperatures around 80–100°C to produce radicals via homolytic bond scission. Photochemical initiation involves photoinitiators like benzoin ethers, which cleave under ultraviolet (UV) light to generate radicals suitable for polymerization at ambient conditions. Redox initiation employs pairs such as persulfates (e.g., ammonium persulfate) with reducing agents like ferrous ions or amines, enabling radical formation at lower temperatures (often below 50°C) through electron transfer mechanisms. Radiation-induced initiation, typically using gamma rays or electron beams, directly ionizes the monomer or solvent to produce radicals without added chemical initiators, offering precise control in specialized applications.[17][18][19][20]The decomposition of initiators, particularly peroxides, proceeds through homolytic cleavage of the weak O–O bond (bond dissociation energy ≈ 150 kJ/mol), yielding two alkoxy or aryloxy radicals that can further fragment or abstract hydrogen. This unimolecular process follows first-order kinetics, with the rate constant described by the Arrhenius equation:kd=Aexp(−RTEa)where kd is the decomposition rate constant, A is the pre-exponential factor, Ea is the activation energy (typically 100–170 kJ/mol for peroxides), R is the gas constant, and T is the absolute temperature. Initiator efficiency (f), defined as the fraction of primary radicals that successfully initiate polymer chains rather than recombining or undergoing side reactions, ranges from 0.3 to 0.8. Factors reducing f include the cage effect—wherein geminate radicals recombine within the solvent cage before diffusing apart—and competing reactions like induced decomposition by propagating radicals. Higher monomer concentrations and lower viscosities generally improve f by facilitating radical escape.[21][22][23]
Propagation
In the propagation step of radical polymerization, a growing polymer chain radical adds to the double bond of a monomer molecule, forming a new bond and generating a new radical at the end of the extended chain. This process, represented as \ceMn∙+CH2=CHX−>Mn+1∙, where \ceMn∙ is the chain radical and X is the substituent on the monomer, repeats rapidly, allowing the chain to grow by hundreds or thousands of monomer units while maintaining a head-to-tail regioregularity.[24]Each propagation addition is highly exothermic, with the enthalpy change ΔHp typically around -70 to -80 kJ/mol per monomer unit; for styrene, this value is approximately -71 kJ/mol, contributing significantly to the overall heat release in the polymerization process.[25][26]The rate of propagation is governed by the propagation rate constant kp, which generally falls in the range of 102 to 104 L/mol·s depending on the monomer and temperature, and becomes diffusion-controlled at high conversions where chain mobility decreases.[24] The rate equation for propagation is Rp=kp[\ceM∙][\ceM], where [\ceM∙] is the concentration of propagating radicals and [\ceM] is the monomer concentration, highlighting the second-order dependence on these species.[24]Factors influencing the propagation rate primarily stem from monomer structure, such as the electronic nature of substituents that affect radical stability and transition state energies; for instance, electron-rich monomers like acrylates exhibit higher kp values compared to electron-deficient ones due to favorable polar interactions in the addition step. Additionally, the reactivity of the propagating radical remains largely unchanged along the chain length because each new radical end possesses similar stability to the previous one, avoiding variations in rate with chain size.[27][28]
Termination
Termination in radical polymerization primarily occurs through bimolecular reactions between two propagating radicals, effectively quenchingchain growth without generating new radicals.[28]The two dominant mechanisms are combination (coupling), in which the radicals form a new carbon-carbon bond to yield a single polymer molecule, and disproportionation, involving hydrogen atom abstraction that produces one chain with a saturated end and another with an unsaturated end.[29]These processes follow second-order kinetics with respect to radical concentration, expressed by the rate equation:Rt=2kt[M∙]2where Rt is the rate of termination, kt is the termination rate constant (typically 107 to 109 L mol−1 s−1), and [M∙] denotes the concentration of propagating radicals.[30][31]The relative prevalence of combination versus disproportionation varies by monomer structure; in styrene polymerization, combination predominates (approximately 80-90%), whereas in methacrylates such as methyl methacrylate, disproportionation is favored (about 70-75% at ambient temperatures).[32][33]This mechanistic preference directly impacts polymer end-group functionality, with combination yielding difunctional chains suitable for further reactions and disproportionation introducing variability in chain-end chemistry.[34]Overall, termination depletes the population of active chains, thereby controlling the extent of polymerization and contributing to the molecular weight distribution, often resulting in polydispersity indices greater than 1.5 in conventional systems.[28]
Chain Transfer
In radical polymerization, chain transfer occurs when a propagating polymer radical abstracts an atom, typically hydrogen, from a chain transfer agent (CTA), thereby terminating the growth of the current chain while generating a new radical capable of initiating another chain. This process relocates the radical activity without net loss of radicals, distinguishing it from termination, which destroys radicals altogether. The mechanism is represented as:\ceRn∙+S−H−>Rn−H+S∙where Rn∙ is the propagating radical, S−H is the transfer agent, Rn−H is the dead polymer chain, and S∙ is the new radical. The rate of chain transfer is given by Rtr=ktr[Rn∙][S], where ktr is the rate constant for transfer and [S] is the concentration of the transfer agent.[35]Chain transfer agents include the monomer itself, solvents, or deliberately added compounds. Transfer to monomer often involves abstraction of labile hydrogens, such as the allylic hydrogen in α-methylstyrene, which exhibits a transfer constant Ctr=ktr/kp (where kp is the propagation rate constant) of approximately 0.041 at 50°C. Solvents like chloroform serve as CTAs through hydrogen donation, with Ctr≈3.4×10−4 for styrene polymerization. Added CTAs, such as thiols (e.g., n-dodecanethiol), are highly effective due to the weak S-H bond, yielding Ctr values ranging from 10 to 20 in many systems, and up to 21 for certain thiols (e.g., 1-butanethiol) in styrene polymerization at 60°C. These constants quantify the relative likelihood of transfer versus propagation, with values typically much less than 1 for monomers and solvents but significantly higher for efficient CTAs.[35][36]The primary effect of chain transfer is to limit molecular weight by increasing the number of polymer chains formed per initiating radical, as each transfer event caps one chain and starts another. This introduces specific end-groups from the CTA (e.g., thiol-derived groups) and can promote branching if transfer occurs to polymer chains via hydrogen abstraction from the backbone. In systems with high transfer rates, such as those using thiols, molecular weights are intentionally kept low, often below 10,000 g/mol, to produce oligomers or telechelic polymers. Transfer competes with propagation and termination but primarily influences chain length distribution without substantially affecting the overall polymerization rate.[35]High chain transfer is exploited in telomerization, a process where excess CTA (telogen) reacts with monomer (taxogen) to yield low-molecular-weight telomers with controlled functionality, such as in the production of fluorinated surfactants from vinylidene fluoride and mercaptans. This intentional use of efficient CTAs like chloroform or thiols enables precise control over chain length, typically achieving degrees of polymerization of 1–20, while maintaining the radical mechanism's versatility.[37]
Polymerization Techniques
Conventional Free Radical Methods
Conventional free radical polymerization methods encompass bulk, solution, emulsion, and suspension techniques, which rely on thermal or photochemical initiation without mechanisms for radical deactivation, leading to polymers with broad molecular weight distributions.[38] These approaches are widely used industrially due to their simplicity and scalability, though they often require careful management of heat and viscosity to prevent uncontrolled reactions.[39]Bulk polymerization involves heating pure monomer with a free radical initiator, such as benzoyl peroxide for styrene, to produce high-purity polymers without solvent residues.[40] This method offers advantages like straightforward process design and maximal monomer concentration for high reaction rates, but it suffers from rapid viscosity increase as conversion progresses, complicating mixing and heat dissipation.[39] A key challenge is the Trommsdorff-Norrish effect (also known as autoacceleration or gel effect), where rising viscosity reduces termination rates more than propagation, causing sudden rate acceleration, potential thermal runaway, and irregular polymer properties; this was first observed in the bulk polymerization of methyl methacrylate.[41] Bulk methods are commonly applied to monomers like methyl methacrylate for cast sheets, but are limited to low conversions (typically below 30-50%) to mitigate these issues.[39]Solution polymerization dissolves the monomer and initiator in an organic solvent, such as benzene for styrene to form polystyrene, enabling better control over reaction conditions.[42] The solvent reduces viscosity, facilitating stirring and efficient heat removal through reflux or evaporation, which is particularly beneficial for exothermic polymerizations.[43] However, it dilutes the monomer concentration, lowering the overall rate and polymer yield per volume, and necessitates downstream solvent recovery, which can be energy-intensive and may introduce impurities if chain transfer to solvent occurs—benzene, for instance, exhibits moderate chain transfer activity.[44] This technique is favored for producing soluble polymers like polystyrene in applications requiring uniform molecular weights, though solvent choice must minimize degradation of bulk properties during removal.[42]Emulsion polymerization disperses water-insoluble monomer droplets (e.g., styrene or vinyl chloride) in water using surfactants above the critical micelle concentration, with water-soluble initiators like persulfates generating radicals that enter micelles to initiate polymerization, forming stable latex particles.[38]Micellar nucleation predominates, where oligoradicals enter surfactant micelles swollen with monomer, leading to compartmentalization that suppresses termination and yields high molecular weight polymers (often >10^6 g/mol) at rapid rates.[45] The Smith-Ewart theory describes the kinetics, predicting the number of particles proportional to surfactant concentration (N_p ∝ [S]^{0.6}) and outlining three cases based on average radicals per particle (ñ): Case II (ñ = 0.5) applies to many systems, explaining the linear rate with conversion in Interval II.[38] This method produces latexes for products like polyvinyl chloride (PVC) via batch processes and styrene-butadiene rubber (SBR) through semibatch feeding to control copolymer composition, offering advantages in heat dissipation and easy product isolation but requiring surfactant removal.[38]Suspension polymerization suspends monomer droplets (typically 0.1-2 mm) in water with mechanical agitation and stabilizers like polyvinyl alcohol, where oil-soluble initiators polymerize within droplets to form solid beads without micellar involvement.[46] Stabilizers prevent coalescence, yielding spherical particles larger than those from emulsion (10-1000 μm vs. 50-500 nm), which simplifies recovery via filtration and filtration without emulsion breaking.[46] For polystyrene production using styrene/divinylbenzene, this process provides uniform beads for ion-exchange resins or chromatography, with advantages including effective heat transfer through the aqueous phase and no residual surfactants, though it demands precise control of agitation to maintain droplet size distribution.[46] Compared to emulsion, suspension yields coarser products but avoids latex stability issues, making it suitable for bead-based applications.[46]
Controlled Radical Polymerization
Controlled radical polymerization encompasses a class of techniques designed to confer "living" or controlled characteristics upon free radical polymerization processes. These methods operate through the establishment of a rapid and reversible equilibrium between a low concentration of active propagating radicals and a large excess of dormant polymer chains, which temporarily sequesters the radicals to suppress irreversible termination reactions. This dynamic equilibrium minimizes side reactions, enabling the production of polymers with predictable molecular weights and narrow polydispersity indices (PDI) typically ranging from 1.1 to 1.5, a significant improvement over conventional free radical polymerization where PDI values often exceed 2.[47][48]The primary techniques in controlled radical polymerization include nitroxide-mediated polymerization (NMP), atom transfer radical polymerization (ATRP), and reversible addition-fragmentation chain transfer (RAFT) polymerization. NMP, pioneered in the 1980s with early demonstrations of nitroxide trapping in radical systems, achieved practical control in the early 1990s using stable nitroxides such as 2,2,6,6-tetramethylpiperidin-1-oxyl (TEMPO) as mediators; these species reversibly couple with carbon-centered radicals to form alkoxyamines, effectively deactivating the chains.[49]ATRP, introduced in 1995, employs a transition metal catalyst, typically copper(I) complexes with bipyridine or similar ligands, in conjunction with alkyl halide initiators to facilitate reversible halogen atom transfer between the metal center and the propagating radical, maintaining the dormant species as alkyl halides.[15]RAFT polymerization, developed in 1998, utilizes thiocarbonylthio compounds as chain transfer agents (CTAs) to mediate control across a broad range of monomers without requiring metal catalysts.[50]Mechanistically, these techniques rely on a general dormant/active equilibrium that supports controlled propagation. In the active state, a propagating radical Pn∙ adds monomer (M) to form Pm∙, which then reversibly deactivates to an inactive species, such as an alkoxyamine in NMP, an alkyl halide in ATRP, or an intermediate adduct radical in RAFT. For RAFT specifically, the process involves an addition-fragmentation cycle: the propagating radical adds to the CTA (R-S-C(=S)-Z) to form an intermediate radical, which fragments to release a new propagating radical (R^\bullet) and a thiocarbonylthio-macromolecule; this macro-CTA then participates in further reversible transfers, ensuring equal chain growth opportunities.[50][51]These methods offer distinct advantages, including the ability to synthesize well-defined block copolymers through sequential monomer addition, leveraging the retained dormant chain ends for reactivation, and the preservation of end-group functionality for post-polymerization modifications such as grafting or conjugation.[52] However, RAFT polymerization can introduce challenges, as the sulfur-containing end-groups often impart a yellow-to-red color and a characteristic odor to the resulting polymers, necessitating additional purification steps for certain applications.[53]
Kinetics and Molecular Weight Control
Rate Laws and Mechanisms
The kinetics of radical polymerization are governed by the rates of initiation, propagation, and termination steps, leading to specific rate laws that describe the overall polymerization rate Rp, defined as the rate of monomer consumption −dtd[M]. Under typical conditions, the concentration of propagating radicals [M•] is low and varies rapidly, necessitating the steady-state approximation, which assumes that the rate of radical formation equals the rate of radical destruction, such that dtd[M•]=0. This approximation yields the radical concentration as [M•]=(2ktRi)1/2, where Ri is the initiation rate, kt is the termination rate constant, and the factor of 2 accounts for the bimolecular nature of termination.[54]The propagation rate is then Rp=kp[M•][M], where kp is the propagation rate constant and [M] is the monomer concentration. Substituting the steady-state expression for [M•] gives the fundamental rate law Rp=kp(2ktRi)1/2[M]. This shows that Rp is first-order in [M] and half-order in Ri, reflecting the square-root dependence on radical concentration.[54]For thermal initiation with a decomposable initiator, the initiation rate is Ri=2fkd[I], where f is the initiator efficiency (typically 0.3–0.8, accounting for radicals lost to side reactions), kd is the initiator decomposition rate constant, and [I] is the initiator concentration. The decomposition rate kd follows Arrhenius behavior, increasing exponentially with temperature, which accelerates initiation and thus Rp. Combining this with the propagation rate law yields the overall expression Rp=(2kt)1/2kp(fkd[I])1/2[M], assuming negligible chain transfer. This half-order dependence on [I] is a hallmark of radical polymerization kinetics.[54][55]In photopolymerization, initiation occurs via photoinitiator absorption of light, making Ri proportional to light intensity I0, typically Ri=2fϕϵ[PI]I0, where ϕ is the quantum yield, ϵ is the molar absorptivity, and [PI] is the photoinitiator concentration; higher I0 thus increases Rp via enhanced radical generation, though excessive intensity can promote side reactions.[56]Deviations from the ideal rate law arise at higher conversions due to physical effects. Autoacceleration, or the Trommsdorff-Norrish effect, manifests as a sudden increase in Rp because rising polymer concentration elevates solution viscosity, creating local microenvironments with higher effective [M] near reaction sites. More critically, the gel effect reduces kt as radical termination becomes diffusion-limited in the viscous medium, amplifying [M•] and thus Rp beyond the steady-state prediction; this can lead to uncontrolled exotherms in bulk polymerizations.[57]
Molecular Weight and Distribution
In radical polymerization, the number-average degree of polymerization (DPn), which represents the average number of monomer units per polymerchain, is governed by the relative rates of chain growth and chain-stopping processes. Specifically, DPn is given by the ratio of the propagation rate (Rp) to the combined rates of termination (Rt) and chain transfer (Rtr): DPn=Rt+RtrRp.[58] This expression arises because each propagation step adds one monomer unit to a growing chain, while termination or transfer events halt that chain's growth, determining its final length.Substituting the kinetic expressions for these rates yields a more detailed form: Rp=kp[M][M∙], Rt=2kt[M∙]2, and Rtr=∑ktr[S][M∙], where kp is the propagation rate constant, [M] the monomer concentration, [M∙] the concentration of propagating radicals, kt the termination rate constant, and the sum encompasses chain transfer rate constants ktr and concentrations [S] of transfer agents (such as monomer, solvent, or added agents). Thus, DPn≈2kt[M∙]+∑ktr[S]kp[M].[58] Polymerization conditions, including initiator concentration (which influences [M∙]), monomer concentration, and temperature (affecting rate constants), directly control DPn by modulating these terms; higher [M] or lower [M∙] typically increases chain length.The polydispersity index (PDI, defined as Mw/Mn) in conventional radical polymerization typically ranges from 1.5 to 2, reflecting the breadth of the molecular weight distribution due to the stochastic nature of termination events.[59] This value arises from the equal probability of chain growth versus stopping at each step, leading to a most probable (Flory-Schulz) distribution where the weight fraction of chains with degree of polymerization x is wx=x(1−p)x−1px−1 (with p the probability of propagation); for high conversion, PDI approaches 1.5 if termination occurs exclusively by combination or 2 by disproportionation.[60] In contrast, controlled radical methods achieve lower PDI values (often 1.1–1.5) by minimizing termination and enabling more uniform chain growth.[61]When chain transfer dominates over termination (Rtr≫Rt), the molecular weight is primarily controlled by transfer agents, as described by the Mayo equation: DPn1=DPn01+Ctr[M][S], where DPn0 is the degree of polymerization without transfer, and Ctr=ktr/kp is the transfer constant.[62] This linear relationship allows precise tuning of chain length by adjusting [S], such as adding thiols (with high Ctr≈10−2 to 1) to limit DPn in industrial processes like polystyrene production.[63] The Flory-Schulz distribution persists in transfer-dominated systems, maintaining the characteristic PDI of 2, but with a shifted average length.[60]
Thermodynamics and Process Considerations
Thermodynamic Principles
Radical polymerization is governed by the thermodynamic favorability of the propagation step, which is highly exothermic with an enthalpy change (ΔH) typically ranging from -20 to -100 kJ/mol per monomer unit.[40] This exothermicity stems from the net energy release during bond reorganization, where the C=C double bond in the monomer (bond energy ≈ 610 kJ/mol) is effectively broken, and two C-C σ bonds (total ≈ 710 kJ/mol) are formed in the growing polymer chain.[64] The process is accompanied by a negative entropy change (ΔS ≈ -100 to -120 J mol⁻¹ K⁻¹), arising from the loss of translational and rotational degrees of freedom as discrete monomer molecules are constrained within the polymer chain.[64]The ceiling temperature (T_c) represents the equilibrium point where the forward propagation rate equals the reverse depropagation rate, rendering net polymerization zero at that monomer concentration.[64] This temperature is derived from the condition ΔG = 0, yielding Tc=ΔSΔH
where ΔH and ΔS are the enthalpy and entropy changes for propagation.[64] For instance, bulk styrene exhibits a T_c of 310°C, while methacrylates generally have T_c values around 220°C.[64] Above T_c or at low monomer concentrations, depropagation dominates, limiting polymer formation.Although propagation is thermodynamically irreversible under standard conditions (e.g., room temperature), reversibility becomes relevant at elevated temperatures approaching T_c or for monomers with inherent strain, such as α-methylstyrene (T_c ≈ 60°C).[64] In these cases, the equilibrium monomer concentration [M]_e increases, potentially leading to depolymerization if the system is heated sufficiently.[64]
Heat Transfer and Reactor Design
Radical polymerization reactions are highly exothermic, with typical enthalpies of polymerization ranging from -30 to -100 kJ/mol, releasing significant heat that can lead to thermal runaway if not properly managed.[28] This exothermicity poses substantial challenges in process control, as uncontrolled heat accumulation can cause rapid temperature increases, potentially exceeding the boiling point of the reaction mixture and resulting in pressure buildup or vessel rupture.[65] Autoacceleration, also known as the gel or Trommsdorff-Norrish effect, further exacerbates local heating by increasing the polymerization rate at higher conversions due to diffusion-limited termination of radicals in the viscous medium, creating hotspots that intensify heat transfer difficulties.[66]Effective cooling strategies are essential to dissipate the heat generated during radical polymerization. In batch reactors, jacket cooling with circulating coolant provides direct heat removal from the reactor walls, maintaining isothermal or near-isothermal conditions for small-scale operations.[67] For continuous processes, tubular flow reactors facilitate heat management through high surface-to-volume ratios and the introduction of cold monomer feeds at multiple points, which absorb and distribute heat along the reactor length.[68] In solution polymerization, inert diluents such as hydrocarbons or alcohols serve as heat sinks, lowering the reaction viscosity and enhancing convective heat transfer to cooling surfaces, thereby preventing localized overheating.[69]Reactor configurations are selected based on production scale, control needs, and heat management requirements. Batch reactors offer simplicity and flexibility for small-scale or specialty polymer production, allowing straightforward charging and discharge while relying on jacket cooling for heat removal.[68] Semi-batch reactors improve temperature control by enabling controlled monomer addition, which limits the instantaneous heat release and mitigates exotherm peaks, commonly used in processes requiring precise composition gradients.[70] Continuous stirred-tank reactors (CSTRs) achieve steady-state operation with uniform mixing, supporting large-scale production by balancing continuous feed and withdrawal to stabilize temperature and conversion.[71]Safety in radical polymerization design hinges on predicting and limiting potential temperature excursions. The adiabatic temperature rise, ΔT_ad, which estimates the maximum temperature increase under no heat removal, is calculated as ΔT_ad = (-ΔH [M]_0) / (C_p ρ), where ΔH is the enthalpy of polymerization, [M]_0 the initial monomer concentration, C_p the heat capacity, and ρ the density; this metric guides reactor sizing and cooling capacity to prevent runaway. Monomer feed strategies, such as starved-feed in semi-batch modes, further enhance safety by restricting the monomer inventory at any time, thereby capping the heat release rate and keeping ΔT below critical thresholds.[72]
Polymer Structure and Reactivity
Stereochemistry
In radical polymerization, the propagating radical intermediate is typically planar at the carbon-centered radical site, enabling rapid rotation and minimal steric bias during monomer addition. This lack of stereoselectivity results in polymers with predominantly atactic stereochemistry, where the configuration of adjacent chiral centers is randomly distributed along the chain.[73]Tacticity describes the stereochemical arrangement of repeat units in a polymer chain and is classified into three primary types: isotactic, where all adjacent stereocenters have the same relative configuration (meso diads, m); syndiotactic, featuring alternating configurations (racemic diads, r); and atactic, characterized by a random mixture of m and r diads. Conventional free radical polymerization of typical vinyl monomers, such as styrene (yielding polystyrene, PS) and methyl methacrylate (yielding poly(methyl methacrylate), PMMA), produces largely atactic polymers due to the absence of external stereocontrol. For PS, the meso diad probability (P_m) is approximately 0.36 across a wide temperature range, indicating a slight syndiotactic bias (P_r ≈ 0.64) but overall random microstructure. In contrast, PMMA exhibits a stronger inherent syndiotactic preference, with P_m ≈ 0.17 at 30 °C (P_r ≈ 0.83), though still considered atactic relative to highly stereoregular forms produced by other methods.[74]Exceptions to this random stereochemistry arise under specific conditions that subtly influence radical approach or conformation. Lowering the polymerization temperature can enhance syndiotacticity in monomers like vinyl acetate or methyl methacrylate by restricting rotational freedom and amplifying steric interactions. In alternating copolymers formed via radical polymerization, such as styrene-maleic anhydride, the obligatory 1:1 monomer sequencing imposes constraints on the propagating radical.[75][74]The tacticity of radical polymers is commonly assessed using nuclear magnetic resonance (NMR) spectroscopy, particularly ^{1}H and ^{13}C NMR, which resolve triad sequences—mm (isotactic), mr (heterotactic), and rr (syndiotactic)—based on chemical shift differences in the backbone or side-chain protons. For instance, in PMMA, the methylene ^{13}C signals at approximately 54-55 ppm distinguish these triads, allowing quantification of P_m and P_r from peak intensities via the relations P_m = [mm] / ([mm] + [mr]/2 + [rr]), and deviations from Bernoullian statistics indicate penultimate or antepenultimate effects. This analysis is crucial because tacticity profoundly impacts polymer properties: atactic configurations typically yield amorphous materials with lower glass transition temperatures and no crystallinity (e.g., atactic polypropylene melts near 0 °C), whereas even modest syndiotactic content enhances chain packing, solubility, and mechanical strength compared to fully isotactic crystalline forms.[76][73][74]
Copolymerization Reactivity
In radical copolymerization, the reactivity ratios r1 and r2 quantify the relative reactivities of propagating radicals toward the two monomers. Specifically, r1=k11/k12, where k11 is the rate constant for addition of monomer 1 to a radical chain ending in monomer 1, and k12 is the rate constant for addition of monomer 2 to the same radical; r2=k22/k21 is defined analogously.[77] These parameters indicate the preference of each radical type for homopropagation versus cross-propagation during chain growth.[77]The nature of the copolymer sequence distribution depends on the values of r1 and r2. An ideal copolymerization occurs when r1=r2=1, resulting in random monomer incorporation proportional to the feed composition without bias toward blocks or alternation.[77] Alternating tendencies arise when both r1<1 and r2<1, favoring cross-addition due to lower homopropagation rates. An azeotropic composition, where the instantaneous copolymer composition matches the monomer feed and remains constant throughout polymerization, exists when r1r2=1.[78]The instantaneous copolymer composition is described by the Mayo-Lewis copolymer equation:d[M2]d[M1]=1/r2+[M1]/[M2]r1[M1]/[M2]+1This differential equation predicts how the polymer chain composition drifts with conversion based on initial feed ratios and reactivity ratios. For example, in the free radical copolymerization of styrene (monomer 1) and methyl acrylate (monomer 2) in bulk at 60°C, r1=0.4 and r2=6.0, leading to gradient copolymers enriched in acrylate units early in the reaction and shifting toward styrene-rich sequences at higher conversions.[74]To predict reactivity ratios for dissimilar monomers without extensive experimentation, the Alfrey-Price Q-e scheme assigns parameters reflecting resonance stabilization (Q) and polar effects (e) to each monomer and radical. The scheme expresses r1=(Q1/Q2)exp[−e1(e1−e2)] and r2=(Q2/Q1)exp[−e2(e2−e1)], with styrene as the reference monomer having Q=1.0 and e=−0.8. This empirical model accounts for electronic influences on addition rates, enabling estimation of copolymer behavior for new monomer pairs.[74]
Applications and Industrial Use
Key Polymers and Processes
Poly(styrene) (PS) is one of the most widely produced polymers via radical polymerization, typically conducted through bulk or suspension processes using initiators such as benzoyl peroxide (BPO). In bulk polymerization, the reaction occurs without solvent, relying solely on the monomer, initiator, and optional chain-transfer agents, which allows for high monomer concentration but requires careful control to manage the exothermic reaction. Suspension polymerization disperses styrene droplets in water with stabilizers, enabling easier heat dissipation and producing bead-like particles suitable for further processing. PS finds extensive use in expanded foam for insulation and packaging due to its lightweight and insulating properties, as well as in rigid packaging applications for its clarity and moldability.[1][79]Poly(methyl methacrylate) (PMMA) is synthesized by radical polymerization in solution or bulk methods, employing peroxide initiators like benzoyl peroxide to achieve high molecular weights. The bulk process involves direct polymerization of the monomer, while solution polymerization uses solvents to moderate viscosity and facilitate heat transfer. The resulting atactic structure of PMMA, characterized by irregular stereochemistry from the free radical mechanism, renders it amorphous and contributes to its exceptional optical clarity, with transmittance exceeding 90% in the visible spectrum, making it ideal for lenses and glazing.[80][81]Poly(vinyl chloride) (PVC) is predominantly manufactured through suspension polymerization, where vinyl chloride monomer is dispersed in water with suspending agents and polymerized using redox initiators, such as combinations of organic peroxides and reducing agents, to generate radicals at moderate temperatures around 50–60°C. This process yields porous resin particles that are easily separated and processed into various forms. However, PVC is prone to dehydrochlorination during polymerization and subsequent processing, where elimination of HCl leads to chain unsaturation, discoloration, and potential autocatalytic degradation, necessitating stabilizers and precise temperature control to mitigate these risks.[82][83]Polyacrylonitrile (PAN) is produced via aqueous suspension radical polymerization for applications in textile fibers, where the process involves dispersing acrylonitrile in water with stabilizers and initiators like persulfates to form a polymer that is dissolved in solvents and wet-spun into fibers with high tensile strength.[84]Low-density polyethylene (LDPE) is obtained through high-pressure radical polymerization of ethylene, pioneered by Imperial Chemical Industries (ICI) in the 1930s at pressures of 1000–3000 bar and temperatures of 150–300°C using peroxide initiators, resulting in branched chains that impart flexibility.[85]Styrene-butadiene rubber (SBR) is synthesized primarily by emulsion polymerization, involving the copolymerization of styrene and butadiene in aqueous micelles with redox initiators at low temperatures (around 5–10°C for cold emulsion) to produce a latex that is coagulated into rubber, offering good abrasion resistance and processability. This process accounts for a significant portion of the synthetic rubber market, with SBR comprising approximately 40% of global production due to its versatility in tire manufacturing.[86][87]Acrylic rubber, a copolymer of acrylates such as ethyl acrylate with cure-site monomers, is produced via emulsion radical polymerization in aqueous systems using redox initiators, resulting in elastomers with excellent oil and heat resistance for seals and gaskets in automotive applications.[1]Polytetrafluoroethylene (PTFE) is manufactured through suspension radical polymerization of tetrafluoroethylene (TFE) in water with fluorosurfactants and persulfate initiators under moderate pressure and temperature, yielding granular or fine powder resins known for their non-stick and chemical-resistant properties in coatings and seals.[2]
Commercial Significance
Radical polymerization underpins a substantial portion of the global plastics industry, with production exceeding 200 million metric tons annually as of 2023, accounting for approximately 50% of all industrial polymers.[1][88] This dominance stems from its role in manufacturing key commodity plastics such as polystyrene (PS), polyvinyl chloride (PVC), low-density polyethylene (LDPE), and acrylates, which together represent over half of thermoplastic production volumes. The process's economic viability arises from its low-cost initiators, tolerance for impurities, and scalability to large-scale reactors, enabling efficient output for mass-market applications.[1]Key economic drivers include the accessibility of radical methods, which facilitate high-volume production at competitive prices; for instance, the global PS market was valued at around USD 34 billion in 2023, driven by demand in consumer goods and packaging. Similarly, PVC production reached approximately 50 million tons in 2023, with pipes and fittings comprising about 44% of its applications, particularly in construction where durable, cost-effective piping systems account for a significant share of infrastructure projects. These factors have solidified radical polymerization's position as a cornerstone of the USD 800 billion-plus polymers sector, supporting industries from automotive to electronics through reliable supply chains.[89][90]In societal terms, radical polymerization-derived materials permeate daily life, with PS foam serving as a primary insulator and protective layer in food packaging, safeguarding perishable goods during transport and storage due to its lightweight and thermal properties. Acrylate-based adhesives, produced via radical processes, enable versatile bonding in consumer products like tapes and labels, enhancing durability in packaging and assembly applications. However, environmental concerns have escalated, as PS contributes notably to microplastic pollution; fragmented PS particles persist in ecosystems, entering food chains and posing risks to marine life and human health through bioaccumulation.[91][92][93]The industry's evolution reflects broader technological and societal shifts, originating in the 1940s with wartime imperatives for synthetic rubber via emulsion radical polymerization, which addressed natural rubber shortages during World War II. By the 2020s, focus has pivoted toward recyclability, with innovations in chemical depolymerization enabling recovery of monomers from PS and PVC waste streams to mitigate plastic pollution and support circular economies, aligning production with sustainability regulations.[94][95]
Recent Developments
Advances in Initiators and Catalysts
Significant advances in initiators for radical polymerization since 2010 have focused on photoinitiators that enable mild, visible-light-mediated processes, enhancing control and reducing energy requirements. A notable example is the use of fac-Ir(ppy)₃ as a photoredox catalyst in atom transfer radical polymerization (ATRP), which allows for the controlled polymerization of methacrylates under visible light irradiation at room temperature.[96] This system operates with low catalyst loadings (ppm levels) and provides temporal control by toggling the light source, yielding polymers with narrow molecular weight distributions (Đ < 1.2) and high chain-end fidelity. Similarly, organocatalysts have emerged to eliminate metal components entirely, addressing toxicity concerns in biomedical applications. For instance, phenothiazine derivatives serve as organic photoredox catalysts in metal-free ATRP (O-ATRP), enabling the synthesis of well-defined polymers from acrylates and methacrylates in the presence of visible light, with molecular weights up to 10,000 g/mol and low dispersities.[97]Improvements in catalysts have centered on enhancing efficiency through ligand design, particularly for copper-based ATRP systems. Modified bipyridine ligands, such as 4,4'-di(5-nonyl)-2,2'-bipyridine (dNbpy), combined with reducing agents in activators regenerated by electron transfer (ARGET) ATRP, have reduced copper loadings to 50-100 ppm while maintaining control over polymerization kinetics and polymer architecture. These ligands accelerate the activation-deactivation equilibrium, allowing for the production of high-molecular-weight polymers (M_n > 100,000 g/mol) with minimal catalyst residue. Organometallic-mediated radical polymerization (OMRP) represents another precision-oriented advancement, utilizing main-group or transition-metal complexes like cobaltocene to reversibly trap radicals, achieving low polydispersities (Đ ≈ 1.1-1.3) for a range of monomers including methyl methacrylate and styrene. OMRP's tunability via metal-ligand interactions provides superior control in challenging media, such as aqueous or polar solvents.Hybrid systems integrating photochemistry with chain transfer agents have further expanded spatiotemporal control. Photoinduced reversible addition-fragmentation chain transfer (Photo-RAFT) polymerization, developed around 2015, combines visible light with dithioester chain transfer agents (CTAs) and organic photocatalysts like eosin Y, enabling oxygen-tolerant, living polymerization of acrylamides with precise spatial patterning via masked irradiation. This approach yields block copolymers with quantitative chain-end retention and dispersities below 1.2, facilitating applications in 3D printing and microstructured materials. Overall, these innovations reduce toxicity by minimizing metal use—such as in metal-free O-ATRP—and improve energy efficiency through low-intensity light sources, as demonstrated in aqueous photoATRP for synthesizing biocompatible polymers like poly(ethylene glycol) acrylates with Cu levels below 10 ppm.In 2025, further advances include stereoregular radical polymers enabling selective spin transfer over long distances for spintronic applications.[98] Verdazyl radical polymers have demonstrated high spin mixing conductance, advancing organic spintronics.[99]
Sustainable and Green Approaches
Efforts to enhance the sustainability of radical polymerization have increasingly focused on water-based systems, which minimize the use of volatile organic solvents and reduce environmental impact. Miniemulsion polymerization, a variant of emulsion techniques, disperses hydrophobic monomers in aqueous media using surfactants and co-stabilizers to form stable submicron droplets, enabling efficient polymerization with significantly lower solvent consumption compared to traditional organic-phase methods.[100] This approach has been particularly effective for producing latexes and coatings, achieving high monomer conversion rates while aligning with green chemistry principles by promoting water as the primary reaction medium.[100] Additionally, research in the 2020s has explored enzyme-mimetic initiators, such as peroxidase mimics, to catalyze reversible deactivation radical polymerization (RDRP) under mild aqueous conditions, mimicking biological radical control mechanisms to yield well-defined polymers with reduced energy input and byproduct formation.[101]The integration of bio-based monomers represents a key strategy for replacing petroleum-derived feedstocks with renewable alternatives, thereby lowering the carbon footprint of radical polymerization processes. Itaconic acid, derived from fermentation of carbohydrates, undergoes free radical polymerization to form poly(itaconic acid) and its copolymers, which exhibit biodegradability and pH-responsive properties suitable for applications like superabsorbent polymers and drug delivery systems.[102] For instance, dialkyl itaconates copolymerize with conventional monomers via radical mechanisms, demonstrating reactivity ratios that support tunable compositions for sustainable materials.[103] Similarly, terpenes such as β-myrcene and d-limonene serve as green substitutes for styrene in radical copolymerizations, yielding elastomers and porous monoliths with mechanical properties comparable to synthetic analogs while being sourced from plant essential oils.[104] These bio-based polymers degrade more readily in natural environments, addressing the persistence of traditional styrenic materials.[104]Recycling integration through radical depolymerization has emerged as a vital approach to close the loop in polymer lifecycles, enabling monomer recovery from post-consumer waste. For poly(methyl methacrylate) (PMMA), thermal radical unzipping initiates chain-end radicals that propagate depolymerization, reverting the polymer to nearly pure methyl methacrylate monomer at temperatures as low as 250°C when using controlled radical polymerization precursors.[105] This process achieves up to 92% monomer yield in bulk conditions without solvents, facilitating scalable chemical recycling that outperforms conventional pyrolysis in selectivity and energy efficiency.Energy reduction strategies in radical polymerization emphasize flow chemistry combined with LED photopolymerization, which utilize visible light for precise initiation and lower power consumption than traditional UV or thermal systems. In continuous flow reactors, LED-driven photoinduced RDRP enables rapid polymerization of acrylates and methacrylates with spatiotemporal control, reducing reaction times to hours (e.g., 90% conversion in 3 hours) compared to batch methods, with improved efficiency through efficient light penetration and heat dissipation.[106] This setup supports metal-free conditions, minimizing waste and enabling on-demand production scales. In circular economy contexts, controlled radical grafting has been applied to upcycle polystyrene waste, functionalizing it with anhydrides or bio-monomers to create value-added composites, as demonstrated in European initiatives from 2022 onward that enhance recyclate purity for packaging applications.[107]
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