Stereochemistry of alternating isobutene–maleic anhydride, isobutene–dimethyl fumarate, and isobutene–dimethyl maleate copolymers

Abstract

AbstractThe stereochemical composition of the free radical alternating isobutene–maleic anhydride (IB/MA), isobutene–dimethyl fumarate (IB/DMF), and isobutene–dimethyl maleate (IB/DMM) copolymers was investigated by proton magnetic resonance. In contrast to the singlet gem‐dimethyl resonance found in polyisobutene or in the alternating isobutene/acrylonitrile copolymer, the gem‐dimethyl resonance of IB/MA, hydrolyzed IB/MA, and esterified IB/MA is a quadruplet with peaks of approximately equal intensity. The multiplicity of the spectra is consistent with the presence of equal amounts of threo‐di‐isotactic and threo‐di‐syndiotactic triads, disproving previous claims that such copolymers are predominantly threo‐di‐isotactic. The spectrum of the analogous IB/DMF indicates that the copolymer is composed entirely of erythro‐di‐ isotactic and erythro‐di‐syndiotactic triads. This result is consistent with the exclusive trans opening of the dimethyl fumarate double bond and provides the first example for the stereospecific double bond opening of a noncyclic monomer in free radical polymerization. In contrast, the spectrum of IB/DMM shows that the dimethyl maleate double bond opens approximately 93% cis and 7% trans during copolymerization. Since the stereochemical composition of IB/DMF and IB/DMM is not the same, it is concluded that the radicals formed from dimethyl maleate and/or dimethyl fumarate do not equilibrate freely among all the possible configurations before isobutene addition.