Chain-growth versus step-growth mechanisms for the Suzuki-Heck polymerisation of fluorenyldibromides with potassium vinyl trifluoroborate.

The mechanism of the Suzuki-Heck (SuHe) polymerisation of 2,7-dibromo-9,9-di(n-dodecyl)fluorene (1) with potassium vinyl trifluoroborate (PVTB) for the synthesis of poly(fluorenylene vinylene)s (PFVs) has been investigated. In the first stage, a palladium-catalysed chain-growth AA/B(C)-type polycondensation occurs, as evidenced by the linear trend observed when plotting the molecular weights of the polymer formed against the consumption of the monomer. The chain-growth stage takes place until complete consumption of 1 and allows one to envisage the alternating addition of PVTB (by a Suzuki step) and 1 (by a Heck step) to the growing chain. Such alternating addition seems to proceed through a peculiar catalyst transfer during which the metal is constantly bound to the growing chain, confirmed by MALDI end-group analysis. On prolonging the reaction after the disappearance of 1, a second stage takes place, leading to higher molecular weight polymers, which are formed by the step-growth condensation of the fragments generated in the first stage. The molecular weights of the final PFVs depend on the PVTB/1 feed ratio. Thus, by using a PVTB/1 molar ratio of 1.1, the final PFV was characterised by an M(n) of 39600 Da, whereas when using a PVTB/1 molar ratio of 2.0, the final PFV was characterised by an M(n) of 13200 Da.