Anionic solution copolymerization of α-methylstyrene with conjugated dienes above the ceiling temperature of α-methylstyrene

The successful copolymerization of α-methylstyrene with conjugated dienes such as 1,3-butadiene, isoprene and/or styrene was achieved by anionic solution polymerization in hexane using formulated catalyst systems at the ceiling temperature of α-methylstyrene. This polymerization is unique, since α-methylstyrene cannot homopolymerize above its equilibrium ceiling temperature of 60 °C. Anionic copolymerization of 1,3-butadiene and α-methylstyrene was first performed in hexane at 65 °C using 2/2/1 molar ratio of three different types of catalyst systems; cesium 2-ethylhexoxide (CsOR)/dibutylmagnesium (Mg(Bu)2)/N,N,N′,N′-tetramethylethylene diamine (TMEDA), CsOR/TMEDA/n-butyl lithium (n-BuLi), and potassium amylate (KOAm)/TMEDA/Mg(Bu)2. Polymers produced from the CsOR-based system showed faster incorporation rates of α-methylstyrene than the KOAm-based system, resulting in the composition of α-methylstyrene as high as 50% compared to a maximum of approximately 25% for the KOAm. Very little or no α-methylstyrene blocks were present in the polymer chains. Copolymerizations have also been successfully carried out using CsOR/TMEDA/n-butyl lithium (n-BuLi), although other n-BuLi containing systems were not successful. Other monomers such as isoprene and styrene can be used with the CsOR/TMEDA/Mg(Bu)2 catalyst system to produce randomly α-methylstyrene incorporated multiblock diene copolymers. In α-methylstyrene/styrene/butadiene and α-methylstyrene/isoprene/butadiene/styrene polymerizations, the styrene reacted significantly faster than 1,3-butadiene and α-methylstyrene, while isoprene reacted much slower. This would indicate that a styrene block grows at the beginning of the polymerization, followed by random incorporation of α-methylstyrene, and then a tapered isoprene tail grows at the end of the polymerization.