least one preferably at least two alkenyl groups in a molecule examples of the substituted or unsubstituted monovalent hydrocarbon group represented by r 1 include alkyl groups such as methyl ethyl propyl isopropyl isobutyl tert butyl and butyl cycloalkyl groups such as cyclohexyl aryl groups such as phenyl and tolyl aralkyl groups such as benzyl and substituted forms of the foregoing groups in which some or all of the hydrogen atoms bonded to carbon atoms are substituted with halogen atoms hydroxyl groups cyano groups or the like such as 3 3 3 trifluoropropyl hydroxypropyl and cyanoethyl of these methyl and phenyl are most preferred it is more preferred that methyl and phenyl account for at least 50 mol and 1 to 20 mol of the entire r 1 groups respectively x is selected from alkenyl containing monovalent organic groups preferably of 2 to 12 carbon atoms for example alkenyl groups which may be separated by an ether bond u2014 o u2014 or ester bond u2014 c u2550 o u2014 o u2014 examples include vinyl allyl propenyl isopropenyl hexenyl octenyl meth acryloyloxypropyl meth acryloyloxyethyl meth acryloyloxymethyl cyclohexenylethyl and vinyloxypropyl the amount of alkenyl containing monovalent organic groups is preferably 0 001 to 10 mol more preferably 0 01 to 5 mol based on the amount of x and r 1 combined the organopolysiloxane a may be oily or gum like it preferably has a viscosity at 25 u00b0 c of at least 50 mpa u00b7 s more preferably at least 100 mpa u00b7 s the upper limit of viscosity is not critical although the viscosity as measured in.