Initiation of radical polymerizations of main chain fluorinated monomers including vinylidene fluoride (CH2=CF2, VDF) demands very reactive primary alkyl radicals and high temperatures. We have previously shown that visible light photolysis of certain metal carbonyl dimers ((CO)5Mt-Mt(CO)5, Mt = Mn, Re) produces Mt(CO)5● radicals, which while they do not add to alkenes, they abstract halides and initiate from a variety of alkyl, semi- and perfluoroalkyl R-X halides even at room temperature. Conversely, we have also shown that the photolysis of analogous group 14 dimers R’3Q-QR’3 (R’ = alkyl, aryl, Q = Sn, Ge, Pb) affords a mixture of R● and QR3● radicals which do add to VDF and other monomers, and are universal initiators for free radical polymerizations (FRP), even in the absence of R-X. Since such QR’3● radicals (e.g. Bu3Sn●) are well-known strong radical halide abstractors, their assessment as R-X activators in VDF initiation is warranted. We are describing herein the evaluation of a variety of regular, semi- and perfluorinated chlorides, bromides and iodides as chain transfer agents (CTAs) in the radical polymerization of VDF mediated by the photolysis of Me3Sn-SnMe3 and Bu3Sn-SnBu3. In-depth 1H/19F NMR and GPC investigations indicate that with increasing the R-X chain transfer ability, initiation proceeds via the competition of radicals derived from R’6Sn2 (R’●, R’3Sn●), solvent, and R-X, according to the R-X bond dissociation energy, i.e. its chain transfer constant and polymerizations consequently shift from free- to controlled radical. Accordingly, tin and solvent-derived initiation fragments are seen for poor CTAs based on alkyl halides such as CH3-(CH2)5-Cl/Br/I, -(CH2-CHCl-)n-, Cl-CH2CH2-Cl, CH2Cl2/Br2, etc, whereas better CTAs (CCl3Cl/Br, Cl/Br-(CF2)8-Cl/Br) additionally present both the R initiator fragments and the Cl/Br PVDF chain ends, but the polymerization remains uncontrolled. However, perfluorinated RF-I type CTAs (CF3-(CF2)3-I, I-(CF2)6-I etc.) do enable a controlled radical polymerization (CRP) via the iodine degenerative transfer (IDT) mechanism which additionally suppressed tin and solvent initiations.

Free and controlled radical VDF polymerization with alkyltin dimers and (fluoro)alkyl halides

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