Mg. Mikhael et al., COMPETITION BETWEEN DIELS-ALDER CYCLOADDITION AND SPONTANEOUS COPOLYMERIZATION OF 1-METHOXY-1,3-BUTADIENE WITH ELECTROPHILIC OLEFINS, Macromolecules, 26(16), 1993, pp. 4100-4104
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.