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 chaingrowth 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 Mn of 13200 Da.