Regioregular, head-to-tail coupled poly(3-alkylthiophenes) made easy by the GRIM method: Investigation of the reaction and the origin of regioselectivity

An investigation of the new synthetic method to synthesize regioregular, he ad-to-tail coupled poly(3-alkylthiophenes)using magnesium-halogen exchange (Grignard metathesis) called the GRIM method is described. Treatment of 2,5 -dibromo-3-alkylthiophenes with a variety of alkyl and vinyl Grignard reage nts resulted in two metalated, regiochemical isomers, namely, 2-bromo-3-alk yl-5-bromomagnesiothiophene and 2-bromomagnesio-3-alkyl-5-bromothiophene in an 85:15 ratio. This ratio appears to be independent of reaction time, tem perature, and Grignard reagent employed. Introduction of a catalytic amount of Ni(dppp)Cl-2 to this isomeric mixture afforded poly(3-alkylthiophene) t hat contained greater than 95% KT-HT couplings (typically 98% HT couplings were seen). The high degree of regioregularity found in the polymer can be explained by a combination of kinetic and thermodynamic effects arising fro m steric and electronic effects found in the catalytic reaction. A series o f reaction investigations led to a general explanation of the origin of reg ioregularity in polythiophene polymerization reactions. These reactions inc luded kinetic studies and competition experiments.