Hydrolysable groups which may be introduced using end-blocking agents include alkoxy groups and acetoxy groups and ???SiOH3, ???(Ra)SiOH2, ???(Ra)2SiOH, ???RaSi(ORb)2, ???Si(ORb)3, ???Ra 2SiORb or ???Ra 2 Si???Rc??? SiRd p(ORb)3-p where each Ra independently represents a monovalent hydrocarbyl group, for example, an alkyl group, in particular having from 1 to 8 carbon atoms, (and is preferably met