Hk. Hall et al., NEW DEVELOPMENTS IN BOND-FORMING INITIATION - LEWIS-ACIDS ACCELERATE SPONTANEOUS INITIATION OF COPOLYMERIZATIONS OF OLEFINS AND DIENES, Macromolecular symposia, 95, 1995, pp. 233-242
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.