Experimental analysis of the grafting products of diethyl maleate onto linear and branched polyethylenes

Abstract

This paper is focused on establishing the grafting products of diethylmaleate onto ethylene–α-olefin copolymers of varying comonomer contents. Several commercial ethylene/α-olefin copolymers ranging in comonomer contents from 0.35 to 3.70mol% were used, as well as a hydrogenated polybutadiene (HPB) with a 1,2 unit content of 13.9mol%. The polymers were functionalized with DEM in solution. The experimental techniques employed to verify the grafting and to ascertain the exact positioning of the insertion with respect to the branching points were: Fourier transform infra red spectroscopy (FTIR), carbon 13 nuclear magnetic resonance (13C NMR) and Distortionless enhancement polarization transfer NMR (13C NMR-DEPT). Thermal fractionation was performed by Differential Scanning Calorimetry (DSC) employing the successive self-nucleation and annealing technique (SSA).The results obtained show that the DEM insertion onto polyethylene chains occurs in secondary carbons of the main chain regardless of the copolymer branch type and content. The reasons for this behavior may be related to a statistical factor being involved in the peroxide radical attack since there is an excess of secondary carbons as compared to tertiary ones per PE chain. Also, steric effects produced by the size of the DEM molecules are probably involved in the DEM insertion step. Our 13C NMR-DEPT results suggest that DEM not only avoids the tertiary carbons where the branches are located but also, in polyethylenes with less than 4mol% branch content, it prefers to insert in the secondary carbons that are at least 5 carbon atoms away from the branch point. In those cases where unsaturations are present in PE chain ends, FTIR shows that they can also be depleted by DEM grafting. However, in the HPB case where no unsaturations are present and the branching content is very high, the insertion still occurs only in the secondary carbons of the main chain. The thermal fractionation performed by SSA corroborated the aforementioned results since the fractions with the longest linear chains were always the first to be depleted by the grafting reactions.