The homopolymerization of vinylidene fluoride (VF2) in supercritical carbon dioxide (scCO₂) has been carried out by both precipitation^1-7 and dispersion^8-12 polymerization in both continuous^{1, 2, 5, 6} and batch^7-12 processes. The molecular weight distributions (MWDs) of the synthesized PVDF exhibited some features that were not captured by the average molecular weights. In particular, the MWDs of PVDF showed bimodal distributions under certain reaction conditions.

Typically, broad and bimodal MWDs are obtained by polymer blending, i.e. mixing of high and low molecular weight fractions. Polymer blending usually requires a multi-step process, where the low and high molecular weight fractions are synthesized independently and then mixed together. On the other hand, CO₂-based polymerization technique is a single step process. In addition, the broad polydispersity indices (PDI) and the bimodality contribute to improved flow characteristics and processing behavior^{13, 14}. Therefore, the production of bimodal MWDs is of significant commercial interest. However, in order to synthesize polymer with the desired properties, it is very important to understand the origin of the bimodal MWD, and to be able to control the relative amounts and molecular weights of the two fractions.

One possibility is that the bimodality is a result of polymerization in both the fluid- and the polymer-rich phases giving rise to two MWD modes. This hypothesis was first presented by Saraf et al.⁶ and a model describing polymerization in these two phases was developed by Morbidelli and coworkers⁴. In order to obtain a deeper understanding of where VF2 polymerization occurs, we studied the continuous copolymerization of VF2 with hexafluoropropylene (HFP) for both low-HFP-content¹⁵ and high-HFP-content¹⁶ copolymers in scCO₂. An important feature of this copolymer is that its solubility in scCO₂ increases with increasing HFP content. Depending on the copolymer composition, the molecular weight, and the reaction pressure, either a homogeneous (solution) or a heterogeneous (precipitation) polymerization can be observed. The reaction kinetics and molecular weight were independent of the mode of polymerization, i.e. homogeneous or heterogeneous. In fact, the experimental data for both the polymerization rate and polymer molecular weight agreed reasonably well with conventional solution polymerization kinetics. In particular, there was no effect of the polymer volume fraction in the reactor on either the normalized rate of polymerization or the normalized molecular weight. This suggests that the carbon-dioxide-rich fluid phase is the main locus of polymerization for these fluoropolymers, even when the polymer precipitates during the reaction, and that the precipitation polymerization of PVDF occurs mainly in the fluid phase. This conclusion is consistent with the very limited solubility of VF2 monomer in PVDF in presence of CO₂^{6, 17, 18}.

Here, we present a detailed kinetic model that can account for the bimodality and broad MWD of PVDF when the fluid phase is the main locus of polymerization. The model takes into account the change of the termination reaction from kinetic control at short chain lengths to diffusion control at longer chain lengths. The change of the termination scheme resultes in two populations of macroradicals that are responsible for the bimodality observed in the continuous polymerization. The model also includes the chain transfer to polymer reaction, which is responsible for the breadth in the observed MWDs of the synthesized PVDF. The model is successful in accounting for the change of modality with reaction conditions such as monomer concentration, average residence time at low and high monomer concentrations, and reaction temperature. In addition, the model can capture the occurrence of gelation, which is responsible for an inoperability region that was confirmed in the polymerization experiments^{5, 19}. References

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