Synthetic methods leading to novel polysiloxanes and polystyrene copolymers are described. Both types of polymer could be modified by incorporation of the highly sterically demanding tris(trimethylsilyl)methyl substituent (Me3Si)(3)C. The modification increases the rigidity of polymers as shown by d.s.c. analyses. The effect is especially dramatic in the case of silyl substituted polysiloxane copolymer. in that case the steric bulk of the tris(trimethylsilyl)methyl group renders 8% of Si-H bonds in poly(methylhydrosiloxane) (PMHS) unreactive towards the hydrosilylation process. The Si-H bonds of the resultant copolymer can be utilised in crosslinking of the linear polysiloxane.