Mechanism of Ethylene Polymerization by Trialkylaluminum—Lewis Base—Peroxide Catalyst

Abstract

Studies were conducted on the mechanism of ethylene polymerization by the ternary catalyst system consisting of certain specific combinations of trialkylaluminum, Lewis base, and peroxide. Comparison of the polymerization product by the ternary system of triethylaluminum (AlEt3)—γ-butyrolactone (BL)—tert-butyl peroxyisobutyrate with those products brought about by two low-temperature free-radical initiators suggested that two different active sites were formed in the ternary system, i.e., one responsible for the formation of solid polyethylene, and the site of the usual free-radical polymerization responsible for oily oligomers. The ESR analysis employing a bifunctional spin-trapping agent, 2,4,6-tri-tert-butylnitrosobenzene, disclosed that the sterically hindered alkyl radical which can be regarded as being in the coordinated state was formed along with the usual free alkyl radical from the trialkylaluminum. Participation was suggested on the part of the coordinated alkyl radical, as we propose to call it, in the initiation reaction for the solid polyethylene, since cyclopentene added to the ternary system showed only the coordinated ethyl radical quantitatively transformed into the cyclopentyl radical. IR and NMR measurements of AlEt3—BL complex in the presence of cyclopentene suggested possible activation of ethylene monomer by the complex in the polymerization system. The analysis for the ternary system both in the absence and presence of 4-methyl-1-pentene showed a possibility of the coordinated radical taking part in the propagation reaction with the coordinated state retained. Further direct support was obtained upon introduction of ethylene monomer to the ternary system containing trimethylaluminum when the coordinated methyl radical was found to be transformed into the coordinated methylene growing-end radical. These facts suggest that the formation of the solid polyethylene should rest solely on the mechanism of the coordinated radical polymerization, i.e., the mechanism in which the coordinated alkyl radical formed from trialkylaluminum attacks the ethylene molecules which are in the activated state causing the initiation reaction followed by the propagation reaction, with the coordinated state retained throughout.