Effect of ZnCl2 on the Spontaneous Copolymerization of Acrylonitrile with Substituted 1,3-Dienes

Abstract

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 isoprene (IP) were investigated in 1,2-dichloroethane in the presence of various amounts of zinc chloride at different temperatures. ZnC1, complexes with the ester group of MA and increases the MA's electron-poor character and therefore its tendency towards reactions with electron-rich dienes. In the absence of ZnCl,, no reaction occurs between the investigated dienes and MA under the used reaction conditions. Both alternating copolymers and [4+2] cycloadducts formed spontaneously in the presence of ZnC1, even at room temperature.The structure ofthe diene and its nucleophilicity control the product distribution.For the DMB and IP systems, raising the reaction temperature and increasing the ZnC1, concentration enhance the overall reaction rate and in most cases also favor the cycloaddition over the copolymerization. IP is less reactive than DMB. With MPD only copolymer is formed because the two terminal methyl groups retard the concerted [4+2] cycloaddition.These results are in agreement with a postulated mechanism involving competition between the formation of a n-allyl2-hexene-1,6-diradical, capable of initiating free radical copolymerization, and concerted [4+2] Diels-Alder cycloaddition.

Photoinduced Copolymerization and Cycloaddition of 2,3-Dimethyl-1,3-butadiene and Acrylonitrile

“…10 The thermal reactions lead to both cycloadduct and copolymer by parallel competitive second-order reactions. The photochemical results, in contrast, involve competitive reactions of AN and DMB with excited DMB.…”

A paradigm for the mechanisms and products of spontaneous polymerizations

“…In totally analogous fashion, on reacting 2,3‐dimethylbutadiene with acrylonitrile in the presence of zinc chloride as the Lewis acid, free radical copolymerization competed with concerted cycloadditions; higher rates were observed with than without Lewis acid 74. These reactions were also investigated using 1‐methylbutadiene, 1,1‐dimethylbutadiene, and indene as the dienes, acrylonitrile and methyl acrylate as the electrophilic olefin, and lithium perchlorate or zinc chloride as the Lewis acid (refs 75,76…”

Effect of ZnCl2 on the spontaneous copolymerization of methyl acrylate with substituted 1,3‐dienes

“…This might be due to the larger electron disparity in the case of DMB/MA-ZnC12, which favors the cycloaddition reaction [6,7]. No cycloaddition was observed for MPD because of steric hindrance at its terminal carbon atom.…”

“…(Table 3) Based on its degree of substitution (two methyl groups) we can assume that MPD is about as electron-rich as DMB. It is well known that two terminal methyl groups hinder the [4+2] cycloaddition [7,17]. In the absence of ZnC12 no reaction occurred with MA after 24 h at room temperature, but at 85°C a yield of 4.6% copolymer was isolated.…”