A Critical Review of Model Studies in Cationic Polymerization

Recently two ponderous papers appeared by Kriz and Marek which purported to be model studies of the cationic polymerization of isobutylene (11,12). In the first phase of their study (11) the authors chose 1,1 -dineopentyl ethylene, a trimer of isobutylene, as their model compound and worked with AlBr3, HBr (or DBr) in n-heptane at —70° (11). They mixed these reagents by the following sequence nC + HBr + trimer+AlBr3. The products were analyzed by gas chromatography, NMR (60 MHz), and mass spectroscopy. Useful, hard data of this research are as follows 1.1,1 -dineopentylethylene rapidly isomerizes 

The reaction is essentially over in a few seconds. 3. In case DBr is used, multiple deuterium incorporation occurs, the position analysis of which is extremely difficult. 4. Slow isomerization occurs in the presence of AlBr3, alone, in the absence of HBr or H20. 

In their subsequent paper (12) the Czechoslovak authors extended their research over other isobutylene trimers and investigated the reaction which occurs in the system individual isobutylene trimers+HBr (or DBr) + AlBr3 in n-heptane at — 70°. The following trimers have been 

Structure III can exist in two geometrically different (a cisoid and a transoid) configurations, however, the exact nature of the isomer has not been specified. 

The main findings can be paraphrased as follows 1. Positional (double bond migration) and skeletal isomerizations occur with all trimers. 2. In addition to multiple isomerizations, all trimers give rise to some unidentified tetramers, pentamers, and hexamers, the most important contribution coming from the pentamers. 3. Extensive deuterium exchange occurs between olefins. 

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The first part of this paper is a critical review of model studies in cationic polymerization. In the second part we describe and discuss our investigations exploring the effect of a variety of alkylaluminum/alkyl halide initiating systems under a variety of conditions on the competitive reactions of the Keimedy-Gillham scheme (9). This scheme represents a comprehensive set of model reactions developed for the study of competitive reactions in cationic olefin polymerizatioa It involves the cationation of a nonpolymerizable (steric-hindrance) olefin under simulated polymerization conditions and the complete analysis of reaction products which in turn reflect initiation, propagation, termination and transfer. 

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