Understanding the Controlled Polymerization of Methyl Methacrylate with Low Concentrations of 9-(4-Vinylbenzyl)-9H-carbazole Comonomer by Nitroxide-Mediated Polymerization: The Pivotal Role of Reactivity Ratios
Résumé
Previously, nitroxide-mediated controlled copolymerization (NMP) of methyl methacrylate (MMA) using BlocBuilder unimolecular initiator was found to require much less controlling comonomer when using 9-(4-vinylbenzyl)-9H-carbazole (VBK) compared to styrene (S) as a comonomer (minimum ?1 mol % VBK versus 4.4 mol % S). Here, we explored why this was the case. Initially, the use of dimethylformamide (DMF) solvent in the copolymerization of MMA/S was studied as the MMA/VBK copolymerizations were done in DMF. The results confirmed that the increased effectiveness of VBK as a controlling comonomer was not due to the solvent or other experimental conditions. Second, the propagation rate constant kP,VBK and various ?kP?MMA/VBK for MMA/VBK copolymerizations were determined using pulsed laser polymerization?size exclusion chromatography (PLP-SEC), and the dissociation rate constants kd,VBK for the VBK-BlocBuilder adduct and PVBK chains were determined using electron paramagnetic resonance (EPR) spectroscopy, showing that kd for VBK was very similar to S and ?kP?MMA/VBK was very similar to kP,MMA. Finally, modeling of the system using PREDICI was done and illustrated that the difference in reactivity ratios between MMA/S and MMA/VBK was ultimately one of the major reasons for the increased control of VBK versus S. This study showed that propagation rate of the copolymerization and equilibrium constant for the dormant/active species are not the only parameters that govern controlled NMP, and the effect of reactivity ratios between the methacrylate and the controlling comonomer must also be considered for the controlled nitroxide-mediated copolymerization of methacrylate-rich mixtures.