AbstractHigh molecular weight block copolymers having the structure [(CH3)2SiO]a[(CH3)2‐Si‐p‐C6H4‐Si(CH3)2O](b)n, can be conveniently prepared from the copolymerization of hydroxy end‐blocked dimethylpolysiloxanes andp‐bis(dimethylhydroxysilyl)benzene by use of essentially nonequilibrating catalysts, such as tetramethylguanidine di‐2‐ethylhexoate orn‐hexylamine 2‐ethylhexoate. The values ofaandbabove may be altered by employing hydroxy‐ended dimethylpolysiloxane of different degrees of polymerization or by preforming tetramethyl‐p‐silphenylene‐siloxane units. If the length of a segment is maintained at a constant value in the copolymers decreasing the mole ratioa/bresults in higher tensile strengths and higher melting points which are manifestations of increased degrees of crystallinity. If the mole ratioa/bis held constant, increasing the length of the a and b segments leads to higher melting points and higher tensile strengths again reflecting an increase in the degree of crystallinity. High molecular weight copolymers [η] ranging from 1.0 to 2.1, possessing unstressed (unvulcanized) gum tensiles of 1000–2700 psi and elongations of 500–1000% can be readily o