UC Santa Barbara

Asymmetric miktoarm star polymers, comprising a single A arm coupled to a multitude of B or AB′ arms, produce unique material properties. In particular, morphologies at significantly elevated volume fraction of the A material can be obtained. For instance, the performance of thermoplastic elastomers (TPEs) stands to significantly improve from increasing the volume fraction of A while maintaining an elastic matrix. However, existing synthetic strategies are beleaguered by complicated reaction schemes restricted in both the monomer scope and yield. Bottlebrushes copolymers, on the other hand, are easily synthetically accessible using Grubbs third-generation catalyst via the ring-opening metathesis co-polymerization (ROMP) of macromonomers. The well controlled synthesis of bottlebrush copolymers at low backbone degrees of polymerization (n) was demonstrated. Self-assembly in the bulk was studied as a function of molecular composition, arm stoichiometry, and n. Insights generated from scattering experiments and self-consistent field theory simulations indicate these materials behave as disperse miktoarm stars at low n with a transition to brush-like conformations as n increases. The star-to-bottlebrush transition is quantifiable for both statistical and diblock sequences by unique signatures in the experimental scaling of domain spacing and simulated distribution of backbone/side-chain density within lamellar unit cells. These findings represent a conceptual framework that simplifies the synthesis of miktoarm star polymers.These conclusions were used to develop a second-generation synthetic approach, coined “μSTAR”, miktoarm synthesis by termination after ring-opening metathesis polymerization, wherein asymmetry is introduced by first homopolymerizing a macromonomer followed by in situ enyne-mediated termination to install a single mikto-arm with exceptional efficiency. This modular μSTAR platform cleanly generates ABn and A(BA′)n miktoarm star polymers with unprecedented versatility in the selection of A and B chemistries as demonstrated using many common polymer building blocks. The average number of B or BA′ arms (n) is easily controlled by the equivalents of Grubbs catalyst. While these materials are characterized by dispersity in n that arises from the statistics of polymerization, they self-assemble into mesophases at elevated volume fractions of the A material that are identical to those predicted for precise miktoarm stars. In particular phases at significantly elevated volume fraction of the A material were obtained. The μSTAR technique provides a significant boost in design flexibility and synthetic simplicity while retaining the salient phase behavior of precise miktoarm star materials. The usefulness of μSTAR for generating high performing materials was demonstrated by synthesizing sustainable and high molecular weight asymmetry miktoarm star polymers and examining their performance as TPEs. Stiff, tough, and elastic TPEs at greater than 50% volume fraction were tested. The versatility and efficiency of μSTAR at generating a library of stars facilitated the elucidation of structure-property relationships in this unique class of materials. Comparisons to literature examples of thermoplastic elastomers show that these sustainable stars offer advantages over other sustainable linear as well as petroleum derived TPEs.