EFFECT OF ZNCL2 ON THE SPONTANEOUS COPOLYMERIZATION OF METHYL ACRYLATE WITH SUBSTITUTED 1,3-DIENES

The reactions of the mildly electron-poor olefin methyl acrylate (MA) with three electron-rich substituted hydrocarbon 1,3-dienes, namely 2, 3-dimethyl-1,3-butadiene (DMB), 4-methyl-1,3-pentadiene (MPD) and isop rene (IP) were investigated in 1,2-dichloroethane in the presence of v arious amounts of zinc chloride at different temp eratures. ZnCl2 comp lexes with the eater group of MA and increases the MA's electron-poor character and therefore its tendency towards reactions with electron-r ich dienes. In the absence of ZnCl2, no reaction occurs between the in vestigated dienes and MA under the used reaction conditions. Both alte rnating copolymers and [4+2] cycloadducts formed spontaneously in the presence of ZnCl2 even at room temperature. The structure of the diene and its nucleophilicity control the product distribution. For the DMB and IP systems, raising the reaction temperature and increasing the Z nCl2 concentration enhance the overall reaction rate and in most cases also favor the cycloaddition over the copolymerization. IP is less re active than DMB. With MPD only copolymer is formed because the two ter minal methyl groups retard the concerted [4+2] cycloaddition. These re sults are in agreement with a postulated mechanism involving competiti on between the formation of a pi-allyl 2-hexene-1,6-diradical, capable of initiating free radical copolymerization, and concerted [4+2] Diel s-Alder cycloaddition.