Polymer stabilizers (British English: polymer stabilisers) are chemical additives which may be added to polymeric materials to inhibit or retard their degradation. Mainly they protect plastic and rubber products against heat, oxidation, and UV light. The biggest quantity of stabilizers is used for polyvinyl chloride (PVC), as the production and processing of this type of plastic would not be possible without stabilizing chemicals. Common polymer degradation processes include oxidation, UV-damage, thermal degradation, ozonolysis, combinations thereof such as photo-oxidation, as well as reactions with catalyst residues, dyes, or impurities. All of these degrade the polymer at a chemical level, via chain scission, uncontrolled recombination and cross-linking, which adversely affects many key properties such as strength, malleability, appearance and colour.

Stabilizers are used at all stages of the polymer life-cycle. They allow plastic items to be produced faster and with fewer defects, extend their useful lifespan, and facilitate their recycling. However they also continue to stabilise waste plastic, causing it to remain in the environment for longer. Many different types of plastic exist and each may be vulnerable to several types of degradation, which usually results in several different stabilisers being used in combination. Even for objects made from the same type of plastic, different applications may have different stabilisation requirements. Regulatory considerations, such as food contact approval are also present. Environmentally friendly stabilizers for bioplastics should be made from bio-based materials, e.g. epoxidized soybean oil, and cause hardly any odor or VOC emissions. A wide range of stabilizers is therefore needed.

The market for antioxidant stabilisers alone was estimated at US$1.69 billion for 2017, with the total market for all polymer stabilizers expected to reach US$6.5 billion by 2033. In 2023, almost half of all polymer stabilizers sold worldwide were based on calcium, followed by lead (25.1 %), tin (15.4 %), liquid mixed metals (LMM) and other types.

Antioxidants inhibit autoxidation that occurs when polymers reacts with atmospheric oxygen. Aerobic degradation occurs gradually at room temperature, but almost all polymers are at risk of thermal-oxidation when they are processed at high temperatures. The molding or casting of plastics (e.g. injection molding) require them to be above their melting point or glass transition temperature (~200-300 °C). Under these conditions reactions with oxygen occur much more rapidly. Once initiated, autoxidation can be autocatalytic. As such, even though efforts are usually made to reduce oxygen levels, total exclusion is often not achievable and even exceedingly low concentrations of oxygen can be sufficient to initiate degradation. Sensitivity to oxidation varies significantly depending on the polymer in question; without stabilizers polypropylene and unsaturated polymers such as rubber will slowly degrade at room temperature where as polystyrene can be stable even at high temperatures. Antioxidants are of great importance during the process stage, with long-term stability at ambient temperature increasingly being supplied by hindered amine light stabilizers (HALs). Antioxidants are often referred to as being primary or secondary depending on their mechanism of action.

Primary antioxidants (also known as chain-breaking antioxidants) act as radical scavengers and remove peroxy radicals (ROO•), as well as to a lesser extent alkoxy radicals (RO•), hydroxyl radicals (HO•) and alkyl radicals (R•). Oxidation begins with the formation of alkyl radicals, which are formed when the high temperatures and high shear stress experienced during processing snaps the polymer chains in a homolytic manner. These alkyl radicals react very rapidly with molecular oxygen (rate constants ≈ 10⁷–10⁹ mol^–1 s^–1) to give peroxy radicals, which in turn abstract hydrogen from a fresh section of polymer in a chain propagation step to give new alkyl radicals. The overall process is exceedingly complex and will vary between polymers but the first few steps are shown below in general:

Due to its rapid reaction with oxygen the scavenging of the initial alkyl radical (R•) is difficult and can only be achieved using specialised antioxidants the majority of primary antioxidants react instead with the longer lasting peroxy radicals (ROO•). Hydrogen abstraction is usually the rate determining step in the polymer degradation and the peroxy radicals can be scavenged by hydrogen donation from an alternative source, namely the primary antioxidant. This converts them into an organic hydroperoxide (ROOH). The most important commercial stabilizers for this are hindered phenols such as BHT or analogues thereof and secondary aromatic amines such as alkylated-diphenylamine. Amines are typically more effective, but cause pronounced discoloration, which is often undesirable (i.e., in food packaging, clothing). The overall reaction with phenols is shown below:

The end products of these reactions are typically quinone methides, which may also impart unwanted colour. Modern phenolic antioxidants have complex molecular structures, often including a propionate-group at the para position of the phenol (i.e. they are ortho-alkylated analogues of phloretic acid). The quinone methides of these can rearrange once to give a hydroxycinnamate, regenerating the phenolic antioxidant group and allowing further radicals to be scavenged. Ultimately however, primary antioxidants are sacrificial and once they are fully consumed the polymer will begin to degrade.

Secondary antioxidants act to remove organic hydroperoxides (ROOH) formed by the action of primary antioxidants. Hydroperoxides are less reactive than radical species but can initiate fresh radical reactions: