Mechanism of the spontaneous copolymerization of alkyl-1,3-dienes with acrylonitrile accompanying their Diels-Alder reaction.

Abstract

The Bond Forming Initiation Theory, proposed for spontaneous polymerization accompanying (2+2) reactions, was successfully applied to (4+2) systems. Polymerizable weak electron-rich alkyl-1,3-dienes and acrylonitrile, a weak electron-poor olefin copolymerized spontaneously while undergoing their Diels-Alder reaction, providing evidence for a diradical intermediate. These spontaneous reactions of alkyl-1,3-dienes show great dependence on the conformation of the 1,3-dienes: cis-fixed 1,3-dienes give only (4+2) cycloadduct in the reactions with acrylonitrile; trans-locked 1,3-diene forms exclusive copolymer when reacted with acrylonitrile; for those 1,3-dienes in which rotation of the middle single bond is not limited, the copolymerization competes with (4+2) cycloaddition. Extensive investigations were done for the reactions of 2,3-dimethyl-1,3-butadiene and acrylonitrile, as well as the reactions of isoprene and acrylonitrile. The copolymerizations were found to be inhibited completely by trace amounts of free radical scavengers and the propagation reactivity ratios are in good agreement with those deliberately initiated. It was observed that high temperature favors the cycloaddition and lower temperature favors the copolymerization, while the influence of concentration is not very significant. Neither copolymerization nor cycloaddition was affected by solvent polarity and both yields reached maxima at 1:1 initial comonomer feed ratio. The alternating copolymerization of 2,3-dimethyl-1,3-butadiene with acrylonitrile, as well as verbenene with acrylonitrile, follows the second order reaction and the kinetic scheme derived for them was consistent with the experiments. Two separate mechanisms are proposed for the (4+2) cycloaddition and the spontaneous copolymerization of alkyl-1,3-dienes and acrylonitrile: The 1,3-dienes with cis conformation undergo Diels-Alder cycloaddition with acrylonitrile via a concerted pathway, while 1,3-dienes with trans or gauche conformations undergo copolymerization with acrylonitrile initiated by a diradical intermediate which is generated through interaction between a weak donor alkyl-1,3-diene and a weak acceptor acrylonitrile.