Novel silsesquioxane polymers are formed by methods which avoid the use of BBr3. The novel silsesquioxane polymers are especially useful in negative photoresist compositions and photolithographic processes. Alternatively, improved silsesquioxane polymer-containing negative photoresist compositions are obtained by using a polymer component containing a blend of silsesquioxane polymer and non-silsesquioxane polymer. The photoresist compositions provide improved dissolution characteristics enabling the use of 0.26N TMAH developer. The photoresist compositions also provide improved thermal characteristics enabling use of higher processing temperatures. The photoresist compositions are especially useful in a multilayer photolithographic processes and are capable of producing high resolution.