Anionic Copolymerization Enables the Scalable Synthesis of Alternating (AB)_<i>n</i> Multiblock Copolymers with High Molecular Weight in <i>n</i>/2 Steps

Abstract

Based on the highly disparate reactivities of isoprene (I, <i>r</i>_I = 25.4) and 4-methylstyrene (4MS, <i>r</i>_4MS = 0.007) in the anionic copolymerization in nonpolar media, a general strategy for the rapid and scalable synthesis of tapered multiblock copolymers with an extremely steep gradient has been developed. A repetitive addition strategy of a mixture of isoprene and 4MS leads to a tapered diblock in each case, giving access to linear alternating multiblock copolymers of the (AB)_<i>n</i> type with up to 10 blocks. All multiblock copolymers showed narrow molecular weight distributions (dispersity <i>Đ</i> = 1.04-1.12). High molecular weights in the range of 80 to 400 kg mol^-1 were achieved. Due to the incompatibility of PI and P4MS segments, the multiblock copolymers exhibit nanophase separation, manifested by separate glass transitions for both constituents. Stress-strain measurements revealed extraordinary toughness and elongations up to 1150% strain at break, even at a 50/50 molar ratio I/4MS (i.e., 37 wt% isoprene). Our synthesis permits access to a wide range of tapered multiblock copolymer architectures with rigid (P4MS, high glass transition, <i>T</i>_g) and flexible (low <i>T</i>_g) chains, in <i>n</i>/2 steps, while keeping overall dispersity low.