(315a) Continuous Precipitation Polymerization of Vinylidene Fluoride In Supercritical Carbon Dioxide: Understanding and Controlling the Molecular Weight Distribution

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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