Heat-resistant silicone block polymer

A heat-resistant silicone block polymer obtained by reaction of 100 parts by weight of organosilsesquioxane having the following formula (5):

• ##STR17## wherein R.sup.1 is a lower alkyl group having 1 to 5 carbon atoms and/or an aryl group and n is an integer of from 0 to 4,000, with 5 to 400 parts by weight of a silicone compound or organosiloxane represented by the formula (6) or (7):
• ##STR18## wherein R.sup.2 is an arylene group, R.sup.3 and R.sup.4 are lower alkyl groups having 1 to 5 carbon atoms, alkenyl groups and/or aryl groups, and X is a chlorine atom, a hydroxyl group, an alkoxy group or an N,N-dialkylamino group, or ##STR19## wherein R.sup.2, R.sup.3 and R.sup.4 and X have the same meanings as defined heretofore and p is an integer of from 0 to 100, in an organic solvent and in the presence of 0.1 to 200 parts by weight of a basic catalyst per 100 parts by weight of the compound of the general formula (5).