Poly(vinylidene fluoride) (PVDF) samples from homogeneous phase polymerizations in supercritical CO2 and subsequent expansion to ambient conditions were analyzed with respect to polymer end groups, crystallinity, type of polymorphs, and morphology. Upon expansion the polymer was obtained as white particles. Scanning electron microscopy showed that DTBP (di-tert-butyl peroxide) derived polymer end groups lead to stack-type particles whereas sponge- or rose-type particles were obtained in case of chain transfer agent fragments as end groups. FTIR (Fourier-transform infrared spectroscopy) and WAXD (wide-angle X-ray diffraction) indicated that the type of polymorph, α or β crystal phase, is significantly affected by the type of end group. In addition, the crystallinity of the material, as determined via differential scanning calorimetry, is affected by the end groups and polymer molecular weights. For example, with M n around 6700 g·mol−1 crystallinity ranges from 29% for DTBP-derived endgroups to a maximum of 64% for end groups originating from perfluorinated hexyl iodide used as chain transfer agent during polymerization.

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For the first time, stabilizer‐free vinylidene fluoride (VDF) homopolymerizations were carried out in homogenous phase with supercritical CO2 using the conventional initiator di‐tert butyl peroxide (DTBP). In‐line FT‐NIR spectroscopy showed that complete monomer conversion may be obtained. Molecular weights were determined via size‐exclusion chromatography and polymer endgroup analysis by 1H‐NMR spectroscopy. The number average molecular weights were below 104 g mol−1 and polydispersities ranged from 3.1 to 5.7 depending on DTBP and VDF concentration. For allowing isothermal reaction, high CO2 contents ranging from 61 to 83 wt % were used. The high‐temperature and high‐pressure conditions required for homogeneous polymerization did not alter the amount of defects in VDF chaining. Scanning electron microscopy indicated that regular stack‐type particles are obtained upon expansion of the homogeneous polymerization mixture. © 2007 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 45: 5626–5635, 2007

Summary: Vinylidene fluoride (VDF) polymerizations were carried out in homogeneous phase with supercritical carbon dioxide (scCO 2 ) at 120 8C and 1500 bar. To control molecular weight perfluorohexyl iodide was used. Molecular weight analysis by size-exclusion chromatography indicates that polymers of low polydispersities ranging from 1.5 to 1.2 at the highest iodide concentration of 0.25 mol Á L À1 were obtained. In addition, polymer molecular weight increases linearly with reaction time, indicating that living conditions were established. The ''livingness'' is based on the labile C-I bond. The weakness of the C-I bond is associated with a fast decomposition of the original hexyl iodide, thus, contributing to initiation and to a significant increase in the initiation rate in the initial phase of the polymerization.

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