Structure of Polyisobutene and Butyl Rubber

Suggestions that PIB may have structure (II) as opposed to structure (I) have been refuted by infra-red studies. These show clearly the split methyl absorption at 7-2 and 7 3 /u.m, characteristic of a gem-dimethyl group. Proton magnetic resonance spectra also indicate a CH3/CH2 proton ratio of about 3, as would be expected from structure (I). 

Whilst PIB crystallizes readily on stretching it is very reluctant to do so on cooling. Some crystallization is however observed if the polymer is held at -33°C for six months. 

Butyl rubber, with a diene monomer content of 0-5-5% shows, in general, the structural characteristics expected of a marginally modified PIB. One noteworthy difference is that low temperature crystallization appears virtually totally inhibited. A sample of butyl rubber containing as little as 0-5 mole% unsaturation failed to crystallize after a year at -33°C (Buckley, 1%5) although it is on record (Kell et al., 1958) that a very low unsaturation butyl rubber did show signs of crystallization after 18 months at -30°C. 

Whilst the detailed structure of butyl rubber has not been fully elucidated ozonolysis experiments (Rehner, 1944) have indicated that the isoprene enters into the structure in the 1,4 position. The polymer is free from branching provided that the degree of unsaturation is low. 

Commercial polymers have a molecular weight (My) in the range 300000 to 500000 while polymers with My of the order of 10 have been prepared experimentally. As the unsaturation is increased, towards 5%, cure rates are faster and the vulcanizates have better heat resistance and higher modulus but lower ozone resistance. 

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