This characterisation and crosslinking study has been carried out on a new low density polyethylene, a poly(ethylene-co-divinyisiloxane), copolymerised via a free radical mechanism in a high pressure reactor. This copolymer enabled a study of the role of vinyl groups in the peroxide crosslinking mechanism without the interference of allylic hydrogens; such interference has been present in previous studies. The copolymers contain varying amounts of siloxane vinyl groups, which depend on the concentration of divinylsiloxane in the monomer mix added in the reactor. Peroxide crosslinking experiments show that the siloxane vinyl groups are rapidly consumed, demonstrating that allylic hydrogens are not needed for the reaction of vinyl groups. The effects of a high concentration of long chain branches are also discussed. (C) 2004 Elsevier Ltd. All rights reserved.