Polymerization Chemistry and Manufacture

Polyisobutene (PIB) and butyl rubber are unusual in that they are polymerized commercially by the process of cationic polymerization. As in the case of free radical and other polymerizations involving a double bond the process involves a chain reaction but in this case the active species is a carbonium ion. The chemistry of cationic polymerization is complex and outside the scope of this book but has been the subject of a number of reviews (Tsukamoto and Vogl, 1971 Plesch, 1963). There are however a number of salient features which should be noted. 

Polymerization is facilitated by a Lewis Acid such as BF3 and AICI3. 

Such a polymerization, when carried out at very low temperatures, e.g. -lOO C, is extremely rapid and conversion times are only of the order of a second. 

The lower the reaction temperature the higher the molecular weight. However the linear ascending relationship between log(molecular weight) and reciprocal temperature shows a break at about - lOO C and a further reduction in temperature does not lead to any substantial increase in molecular weight. 

A co-catalyst, such as water, appears to play a crucial role in the reaction. 

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