Rational design of tapered multiblock copolymers for thermoplastic elastomers

Thermoplastic elastomers (TPEs) combine the features of vulcanized thermoset rubbers and thermoplastic materials in their phase-separated microdomain structure. As a consequence soft, flexible and resilient materials are obtained, which can be high-speed processed from the melt state. In the last decades, a variety of polymerization strategies has been proven successful to synthesize block copolymers for TPE materials on an industrial scale. Motivated by the outstanding properties of natural rubber (cis-1,4-polyisoprene), the alkyllithium initiated anionic polymerization of isoprene and butadiene plays a key role for the flexible block of TPEs. The synthesis of ABA-type triblock copolymers based on styrene and 1,3-dienes leads to phase-segregated systems which do not require chemical crosslinking. The living character of the carbanionic chain end was utilized in numerous studies to synthesize complex, defined comonomer sequences by multi-step synthesis. Systematic variation of numerous parameters, e.g., block size and sequence allowed to correlate the resulting mechanical and morphological properties with polymer structure. In this review the focus is placed on multiblock structures and gradient copolymers, addressing key parameters of the molecular architecture to enable a general concept for the design of TPE materials with tailor-made properties. The choice of monomers, also bio-based diene structures such as β-myrcene or β-farnesene is another parameter. The major focus is put on the direct, i.e. statistical anionic copolymerization kinetics as the method of choice to synthesize rather complex multiblock sequences in a one-pot reaction. On-line spectroscopic methods are presented that enable to monitor the monomer consumption during the copolymerization, which directly translates to the comonomer incorporation and gradient formation. To enable precise insight into the comonomer composition of the formed chains, an overview of the theory of copolymerization and the determination of reactivity ratios is given. Kinetic Monte Carlo simulation (kMC) is a versatile tool. Based on experimentally determined kinetic rate constants, the copolymerization can be performed in silico. This enables access to relevant parameters, as for example the conversion as a function of the time as well as the composition and monomer sequence in individual chains, which allow rational design and evaluation of synthetic experiments.