The present invention consists of a composition of matter which consists essentially of (A) 100 parts by weight of a siloxane copolymer consisting essentially of R3 SiO1/2 units and SiO4/2 units wherein R is a monovalent organic radical having 1-10 carbon atoms and containing at least two unsaturated groups and at least two alkoxy groups bound to silicon atoms per molecule, (B) 0-600 parts by weig