Synthesis of macrocyclic polystyrene-polydimethylsiloxane block copolymers

Abstract

A series of macrocyclic polystyrene (PS)-polydimethylsiloxane (PDMS) block copolymers and similar block copolymers was synthesized by sequential polymerization of styrene and hexamethyl cyclotrisiloxane (D3) initiated by a difunctional anionic initiator in THF at −78° followed by coupling with Cl2SiMe2 in very dilute (10−5 – 10−6 M) solutions. Total molecular weights ranged from about 2–85 × 103. The formation of monodisperse macrocyclic block copolymers was indicated by the lower (15–30%) hydrodynamic volume of the rings compared to that of the linear block copolymers. Carbon-13 and 29Si NMR likewise supported the absence of linear polymer in the macrocyclic block copolymer. The behavior of second virial coefficient A2 of the rings and the linears versus temperature was examined by static light scattering in cyclohexane. Below 20° the A2 for the linear polymer goes negative while that for the cycle remains positive. Dynamic light scattering (DLS) as a function of temperature also reflects that the cyclic polymers remain well solvated even down to 12°C. The DLS autocorrelation functions for the linear triblock however demonstrate the onset of aggregation and phase separation as the temperature is reduced below 20°C.