This study concerns a new group of low-density polyethylenes (LDPEs)-unsaturated LDPE. The new LDPE is a copolymer between ethylene and 1,9-decadiene and was polymerized in a commerical high-pressure tubular reactor. The diene copolymerized with one double bond, leaving the other unreacted as a pendant side group. This yielded a copolymer containing a higher number of vinyl groups than ordinary LDPE. Fractionation of the copolymer and determination of the number of unsaturated structures in the different fractions by Fourier transform infrared spectroscopy revealed that the diene is homogeneously incorporated along the molar-mass distribution curve. It is also possible to obtain copolymers with a varying vinyl content, without drastic changes in molar mass or molar-mass distribution, by a controlled addition of 1,9-decadiene to the reactor. (C) 2003 Wiley Periodicals, Inc.