xtension of the polymer upon exposure to actinic radiation, said polymer being of the formula ##STR19## wherein x is an integer of 0 or 1, A is one of several specified groups, such as ##STR20## B is one of several specified groups, such as ##STR21## or mixtures thereof, and n is an integer representing the number of repeating monomer units, wherein said photosensitivity-imparting substituents are