Polymethylpentene (PMP), also known as poly(4-methyl-1-pentene). It is used for gas-permeable packaging, autoclavable medical and laboratory equipment, microwave components, and cookware. It is commonly called TPX, which is a trademark of Mitsui Chemicals.

Polymethylpentene is a 4-methyl-1-pentene-derived linear isotactic polyolefin and is made by Ziegler–Natta type catalysis. The commercially available grades are usually copolymers. It can be extruded and moulded (by injection moulding or blow moulding).

Polymethylpentene melts at ≈ 235 °C. It has a relatively low density (0.84 g/cm³) among plastics and is transparent. It has low moisture absorption, and exceptional acoustical and electrical properties.^{[citation needed]} Its properties are reasonably similar to those of other polyolefins, although it is more brittle and more gas permeable. The polymer also has a high thermal stability, excellent dielectric characteristics and a high chemical resistance. The crystalline phase has a lower density than the amorphous phase.

In comparison to other materials being used for operating in THz range, TPX shows excellent optical properties with a wavelength independent refractive index of 1.460±0.005 between visible light and 100~GHz.^{[citation needed]} While having a very good transmission in the THz area, TPX also shows a very wide transmission range spreading from UV to THz.