COMPETITION BETWEEN CYCLOADDITION AND SPONTANEOUS COPOLYMERIZATION OF 4-METHYL-1,3-PENTADIENE WITH ELECTROPHILIC OLEFINS

Competition between cycloadditions and spontaneous copolymerizations in the reactions of 4-methyl-1,3-pentadiene (MPD; 1,1-dimethylbutadiene) with seven electrophilic olefins was investigated at two temperatures (25 and 70-degrees-C) and in two solvents of different polarities (acetonitrile and 1,2-dichloroethane). Tetracyanoethylene (TCNE), the most electrophilic olefin, reacted with MPD to produce a mixture of [2 + 2] and [4 + 2] cycloadducts, and the ratio of these isomers depended mainly on solvent polarity. No polymerization was observed in this system. Both methyl beta,beta-dicyanoacrylate (MDA) and dimethyl cyanofumarate (DCF) reacted with MPD to produce mixtures of [4 + 2] and [2 + 2] cycloadducts and alternating copolymers. Reactions with fumaronitrile (FN), maleic anhydride (MAnh), and acrylonitrile (AN) with MPD produced only alternating copolymers and no cycloadducts. Trimethyl ethylenetricarboxylate (TrE), the least electrophilic investigated olefin, did not react at all with MPD even at 70-degrees-C. All the obtained copolymers are of high molecular weight (10(4)-10(5)). The suggested mechanism involves the formation of a 2-hexene 1,6-diradical formed by reaction of the s-trans diene with the electrophilic olefin, which initiates the diene-olefin alternating copolymerization. This polar diradical is also the proposed intermediate in the [2 + 2] cycloaddition reaction. The diene in its s-cis conformation undergoes concerted Diels-Alder cycloaddition with the electrophilic olefins.