A composition of matter comprising vinyl ended copolymer chains having a number average molecular weight (M.sub.n) of about 1500 to 75,000, the number average molecular weight being determined by gel permeation chromatography (GPC) at 145.degree. C., a ratio of vinyl groups to total olefin groups according to the formula ##EQU1## where a and b have the following sets of valuer: when a=-0.24, b=0.8; when a=-0.20, b=0.8; when a=-0.18, b=0.83; when a=-0.15, b=0.83; and when a=-0.10, b=0.85; and whe...