ipso-Arylative polymerization as a route to π-conjugated polymers: synthesis of poly(3-hexylthiophene)

Abstract

Palladium-catalyzed ipso-arylative polymerization of diphenylcarbinol-substituted thiophene derivatives proceeds to reasonably high molecular weights, but is limited by side reactions.

CH Activation as a Shortcut to Conjugated Polymer Synthesis

“…As a final note, some recent studies have explored the possibility of organic leaving groups for the synthesis of conjugated polymers. In analogue conditions to polymerization by CH activation, cross‐coupling involving brominated and carbinol‐substituted heteroarenes has been achieved in high efficiency to yield conjugated copolymers and homopolymers ( Figures and ) . In this reaction, rather than a CH bond being cleaved via a concerted metalation‐deprotonation mechanism, a palladium alkoxide intermediate is formed, from which the CC coupling product is generated via the extrusion of the ketone followed by reductive elimination.…”

“…The nature of the carbinol group was found to be crucial for the success of the polymerization. In a study of the synthesis of P3HT using this method, diphenylcarbinol (which is lost as benzophenone) led to the desired polymer in high molecular weight and yield, whereas a dimethylcarbinol leaving group (which is eliminated as acetone) contributed to only oligomers in poor yield (Figure ) . Small molecule results revealed that the loss of the carbinol group was the weight‐limiting side‐reaction.…”

ipso‐Arylative Ring‐Opening Polymerization as a Route to Electron‐Deficient Conjugated Polymers

“…In a previous study, we investigated the ipso-arylative crosscoupling polymerization of 5-bromo-4-hexylthiophen-2yl)diphenylmethanol (3) to prepare poly(3-hexylthiophene) (P3HT) (Scheme 1). [15] While P3HT with Mn values of up to 20 kg/mol could be prepared, molecular weights were ultimately limited by end group loss over the course of the polymerization and dispersity values were typical of a step growth polymerization (Đ 1.4-2.2). [15] An additional limitation, common to all ipso-arylation reactions, was the generation of one equivalent of ketone (benzophenone) for each monomer coupling step.…”

“…ROP of monomer 1 was carried out by ipso-arylative crosscoupling under conditions that were previously found to be effective for the polymerization of 5-bromo-4-hexylthiophen-2yl)diphenylmethanol (Scheme 1). [15] Monomer 1 (0.1 M) was heated with Pd(OAc)2 (3 mM), PCy3 (6 mM), and Cs2CO3 (0.15 M) in o-xylene at 140 °C for 48 h. Over the course of the polymerization, formation of an insoluble red powder was observed. After sequential washing of the isolated solid with acetone and THF, the fraction of P1 that was isolated by Soxhlet extraction with CHCl3 was partially dissolved in THF, and found by SEC to have a moderate molecular weight (Mn = 10 kg/mol vs PS standards) and a dispersity typical of a step-growth polymerization (Đ = 2.0).…”

“…Higher catalyst loading (5 mM) resulted in a lower molecular weight (P2b, Mn = 4.9 kg/mol) and a lower yield (67%) (Table 1), which suggests that the catalyst is playing a role in the loss of active end-groups, as has been observed previously for ipsoarylative polymerization of monomer 3. [15] Higher temperature (170 °C) with lower [2] (0.1 M) resulted in slightly lower molecular weight (P2c, Mn = 7 kg/mol) and lower yield (58%) than P2a, suggesting the occurrence of catalyst decomposition or other side reactions.…”