Reaction of polymer radicals with chromium(II) acetate

Abstract

The polymerization of styrene initiated by azobisisobutyronitrile (AIBN) in the presence of Cr²⁺ has been studied in dimethylformamide. Cr²⁺ behaves as a retarder for the polymerization of styrene: p·+ Cr²⁺ [graphic omitted] dead polymer + Cr³⁺. The rate constants (k_x) at 40, 50, 60 and 70°C for the reaction of polystyryl radical (p·) with Cr²⁺ are 42.8, 74.6, 130 and 227 dm³ mol^–1 s^–1, respectively. From these results, k_x can be expressed as 7.15 × 10¹¹ exp(– 61.4 kJ/RT). Further, the effect of additives such as some amines and acids on the termination reaction of the polystyryl radical with Cr²⁺ has been investigated. The termination effect of Cr²⁺ is enhanced by formation of an ethanolamine chelate complex; hence α-α′-bipyridyl and o-phenanthroline complexes of Cr²⁺ inhibit polymerization under the same conditions. In contrast to amines, acetic acid reduces the value of k_x, while mineral acids such as hydrochloric and sulphuric acids accelerate the rate of the termination reaction. The latter is explained by an electron transfer bridge with the anion being placed between the free radical and the metal. The polymerization of polar monomers such as acrylonitrile and methyl methacrylate by AIBN is inhibited by Cr²⁺ without additives.

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