wherein L is an optional linking group and is independently a straight or branched C1-C30 alkyl group, a C3-C30 cycloalkyl group, a C5-C30 aryl group, an ether group, a C1-C20 ester group, an amide group, a siloxanyl, an arylsiloxanyl or a fluorosiloxanyl; R1 independently represents one or more inertly substituted groups and X is independently a group which links the inertly substituted group and