by Kevin Kulbaba, Dana Adkinson, Greg Davidson and Sarah Elliott, Arlanxeo
Butyl rubber (IIR) is formed from the cationically initiated copolymerization of isobutylene and isoprene at reaction temperatures below -90°C. Commercially available grades have between 0.5 to 2.5 mol % of isoprene monomer available along the polymer backbone for curing or further functionalization reactions. The unique properties of IIR are due to the isobutylene units, which are sterically hindered by the presence of two bulky methyl groups positioned on alternating carbon units along the backbone; see, for example, R.H. Boyd et al. (ref. 1) and P.V. Krishana Pant (ref. 2). To accommodate the bulky methyl groups, the polymer backbone must elongate and twist, causing the butyl rubber chains to adopt a more densely packed structure. This distortion and increased packing density in butyl rubber results in excellent permeation resistance to gases and moisture. In addition, the extremely low level of unsaturation of IIR ensures excellent thermal, oxidative and chemical stability.