Refining Synthetic Strategies in Suzuki-Miyaura Catalyst Transfer Polymerization

Innovations in synthetic method development have spurred the discovery of new breakthrough compounds and materials in a wide variety of sectors including but not limited to the pharmaceutical and plastics industries. With applications ranging from packaging materials to electronics and textiles, polymers are nearly impossible to avoid in daily life. The growing demand for functional materials is linked to the ease with which their properties can be tailored to suit specific applications. And as the market for polymers continues to grow, advances in the synthetic techniques used to produce these materials are required. 

One example of the relationship between synthetic organic chemistry and polymer science is the structural evolution of conjugated polymers, which is inextricable from the emergence of modern cross coupling techniques. The early history of these materials was dominated by Ziegler-Natta polymerization or electropolymerization, resulting in simple polymers that were mainly explored in photovoltaic applications. And yet, the highly structurally complex conjugated polymers of today are used in applications as far ranging as display technologies, neural monitoring caps, implantable microelectronics, and artificial skin and muscles. This shift toward “designer materials” was made possible by the development of more sophisticated methods that can be used to stitch together repeat units (e.g., Stille and Suzuki cross coupling). However, issues including poor reproducibility and low molecular weights continue to plague this realm of organic synthesis, confining most of these technologies to academic interest instead of potential commercialization. As the performance of this class of materials is especially sensitive to the presence of structural defects, new approaches for the reproducible precision synthesis of conjugated polymers are strongly desired. 

This thesis begins with a general overview of conjugated polymer synthesis followed by an introduction to Suzuki-Miyaura catalyst transfer polycondensation in Chapter 1. This emerging method encompasses the use of organoboron-based cross-coupling techniques to enable predictable polymer molecular weights, access to block-copolymers and other compositions, as well as pre-determined endgroups. My personal contributions to this area of research are detailed in the following chapters, in which I describe new approaches to monomer activation (Chapter 2), improving reaction efficiency (Chapter 3), catalyst selection (Chapter 4), and monomer design (Chapter 5). 

Chapter 2 describes a Rh-based method for the synthesis of these organoboron-bearing monomers from bromoiodoarenes. The reaction is high yielding, operationally simple, and boasts exceptional selectivity for activation and subsequent borylation of the C–I bond at room temperature. This protocol is complementary to other methods for borylation (i.e., C–X or C–H borylation), but is particularly well-suited to monomer synthesis for chain-growth polymerization, where excellent regioselectivity for these transformations is critical. 

Next, the typical approach to Suzuki polycondensation is streamlined in Chapter 3, where active monomers are generated in-situ using C–H borylation, then polymerized directly in a one-pot, tandem sequence using Pd N-heterocyclic carbene catalysts. This approach is remarkably effective for the rapid construction of conjugated polymers with high molecular weights. We show that this method can be used to polymerize several heterocyclic monomers, two bearing alkyl side groups and two bearing ester side chains. 

Chapter 4 discloses the use of Pd-dialkylbiarylphosphine catalysts for the controlled, chaingrowth Suzuki polycondensation of benzo[1,2,3]triazole to obtain high molecular weights (~15- 28 kg/mol) and moderate molecular weight dispersities based on the structure of the ligand used. To illustrate the versatility of this catalyst system, a block copolymer of benzotriazole (a weakly electron deficient arene) and hexylthiophene (an electron rich building block) was synthesized. The success of this highly modular commercially available class of catalysts for the chain-growth polymerization of benzotriazole and thiophene highlights their potential for future use in the controlled polymerization of other polycyclic aromatics. Preliminary results showing that these catalysts can be used to synthesize a new class of material, poly(alkoxybenzotriazoles), are disclosed in Chapter 5.