NEW DEVELOPMENTS IN BOND-FORMING INITIATION - LEWIS-ACIDS ACCELERATE SPONTANEOUS INITIATION OF COPOLYMERIZATIONS OF OLEFINS AND DIENES

Lewis acids have long been known to influence the free radical propaga tion of donor-acceptor olefin monomer pairs, changing random copolymer s into strictly alternating ones, by complexing to the lone pair elect rons of the acceptor monomer, and enhancing its electron-poor characte r. This work reports that this same interaction enhances the rates of the spontaneous initiation in these systems. In agreement with earlier literature, we found that styrene-acrylonitrile copolymerizations pro ceed readily in the presence of ZnCl2 at 40 degrees where no copolymer ization occurs in the absence of the Lewis acid. p-Methoxystyrene and isobutyl vinyl ether with AN gave more complicated results, due to com petitive complexation of oxygen lone pairs with ZnCl2. Reaction of mor e electrophilic olefins with these donor monomers gave steadily increa sing rates and a shift in product from spontaneous copolymer to invers e Diels-Alder adduct (in the case of ester substituents) to cyclobutan e. As for substituted diene monomers as the donor component, reactions of isoprene and 2,3-dimethylbutadiene with AN are accelerated in the presence of ZnCl2 and the Lewis acid affects the competition between t he [4+2] cycloaddition and the copolymerization. All of these results are interpreted in the context of formation of Lewis-acid complexed in itiating diradicals by bond-formation.