COMPETITION BETWEEN DIELS-ALDER CYCLOADDITION AND SPONTANEOUS COPOLYMERIZATION OF 1-METHOXY-1,3-BUTADIENE WITH ELECTROPHILIC OLEFINS

Concurrent Diels-Alder cycloaddition and spontaneous copolymerization occurred in the reactions of 1-methoxy-1,3-butadiene(MBD) with six ele ctrophilic olefins at 25-degrees-C. Reaction of fumaronitrile with MBD produced high molecular weight alternating copolymers and cycloadduct mixtures, even in dilute solutions. MBD reacted with maleic anhydride in bulk to produce mixtures of cycloadducts and low molecular weight alternating copolymers, while in solution only [4+2] cycloadducts were obtained. When the more electrophilic trisubstituted olefins trimethy l ethylenetricarboxylate, dimethyl cyanofumarate, and methyl beta,beta -dicyanoacrylate reacted with MBD, copolymerization competed with the concerted cycloaddition. Increasing electrophilicity of the olefin inc reased the yield of cycloadducts at the expense of that of copolymers. The less electrophilic acrylonitrile did not react with MBD at room t emperature, while at 70-degrees-C oligomers are formed. The proposed i nitiation mechanism in each case involves a 2-hexene-1,6-diyldiradical intermediate formed from the s-trans diene and the electrophilic olef in, which initiates the diene-olefin alternating copolymerization. The diene in the s-cis conformation cycloadds in concerted fashion to the electrophilic olefins, giving Diels-Alder adducts with retained stere ochemistry.