1544 Communications to the Edito Macromolecules,Vol.38,No.5,2005 Without stabilizer Vith PDMS-n With PDMS- C:With Graft stabilizer Sum C With Gr nd c ced at th c.Effect of F-g-PMVE-MA on Molecular Weight e Polymer.Monomer concen Observationdicated that the as of th molec tand the the ads to an ineffec ive agitat meronentratiomthe mo amount of p PMVE-MA er(0.6 with the me 11 petat0bohonoe2edab0m0ewe ca the (ntry6in Table 1).These product he F VDF produc roducts le without stabilizer pr t(entries 1 an the then termi PMVE-MA()did n 1 and T cate that the polym r obtained at low m nomer the was o cular weight better mo (15 w the The reduction of the explain the decrease of the aproc wit hology at highmo the ntrie 4 and 6 (carried out at differ t m product content in the autoclave constant at ca.33 g/L concentrations,terminated at the same monomer con c. Effect of F-g-PMVE-MA on Molecular Weight and Morphology of the Polymer. Monomer concen￾tration is a key factor for VDF polymerization in scCO2; it has strong effect on both molecular weight and the morphology of the PVDF product.27 With increasing monomer concentration, the molecular weight of prod￾ucts increased (entries 1 and 5 in Table 1). A small amount of F-g-PMVE-MA stabilizer (0.6 wt % with respect to VDF monomer) led to high molecular weight PVDF at both low (entry 2 in Table 1) and high mono￾mer concentration (entry 6 in Table 1). These products have much higher molecular weight compared to the products made without stabilizer present (entries 1 and 5 in Table 1). However, increasing the amount of F-g-PMVE-MA (2 wt %) did not increase the molecular weight (entries 3 and 7 in Table 1) as was demonstrated with PDMS-ma.27 By contrast, the molecular weight decreased. This is likely a result of the chain transfer to F-g-PMVE-MA during the polymerization. This in￾dicates that F-g-PMVE-MA acts as both stabilizer and chain transfer agent during VDF polymerizations. As described in the Experimental Section, the experi￾ments were terminated when the pressure inside the autoclave fell by 300 psi. This was to keep the final product content in the autoclave constant at ca. 33 g/L. Observations in the view cell indicated that the auto￾clave was full of polymer above this point because of the very low apparent density of the polymer, which in turn leads to an ineffective agitation in the autoclave.30 Monomer conversions for the experiments conducted at low monomer concentration (entries 1-3) were termi￾nated at ca. 15 wt %; at high monomer concentrations (entries 5-8), the monomer conversions were termi￾nated at ca. 5 wt %. To study the effect of monomer conversion on the molecular weight and morphology of the PVDF product, the polymerization (entry 4 in Table 1) was carried out at the same reaction conditions as the experiment entry 2 in Table 1 and then terminated at a monomer conversion ca. 5 wt %. The results indi￾cate that the polymer obtained at low monomer conver￾sion (5 wt %) has slightly higher molecular weight and better morphology than that obtained at high monomer conversion (15 wt %). The reduction of the monomer concentration as the polymerization proceeds could explain the decrease of the molecular weight with increasing monomer conversion. The slight loss of morphology at high monomer conversion might be due to the limitation of the stabilization. The results for entries 4 and 6 (carried out at different monomer concentrations, terminated at the same monomer con￾Figure 2. Effect of fluorinated graft poly(methyl vinyl ether-alt-maleic anhydride) (F-g-PMVE-MA) stabilizer on polymer morphology. Images A, B, and C were obtained at a high magnification (×4000). Images A′, B′, and C′ were obtained at a low magnification (×50) for three PVDF samples A, B, and C, respectively. These three samples were produced at the high monomer concentration (9.8 M) in scCO2 with 0.6 wt % of poly(dimethylsiloxane) monomethylacrylate (B, B′, entry 8 in Table 1) or F-g￾PMVE-MA (C, C′, entry 6 in Table 1) or without any stabilizers (A, A′, entry 5 in Table 1). A clear effect of stabilizer was observed. 1544 Communications to the Editor Macromolecules, Vol. 38, No. 5, 2005