100 g of a straight-chain compound having a viscosity of 27,300 cSt at 25??? C. blocked with a methyldiacetoxysilyl group at both ends of the molecular chain, represented by the following formula: ##STR46## wherein a is 2 on average, Rf is a divalent group represented by the formula: ##STR47## wherein m and n are an integer such that m+n is 38 on average, and R2 is a divalent group represented by