Abstract
The synthesis and the characterization of fluoroelastomers, made of vinylidene fluoride (VDF) and perfluoromethyl vinyl ether (PMVE), poly(VDF-co-PMVE) copolymers, by iodine transfer copolymerization are presented. These statistic poly(VDF-co-PMVE) copolymers were prepared in the presence of a perfluoromonoiodo functional chain transfer agent (CTA), C6F13I, or telechelic diiodoperfluoroalkane CTAs, IC6F12I and IC4F8I, using an emulsion process without any surfactant. Quasi-optimal conditions were found to lead to poly(VDF-co-PMVE) copolymers in satisfactory yields. The copolymer compositions, the molecular weights and end-groups of these random-type copolymers were assessed by means of 19F NMR spectroscopy. Model molecules containing only one PMVE unit were synthesized by the radical addition of different CTAs (ICl, ICF2I, CF3I) onto PMVE to help in the assignments by 19F NMR spectroscopy of the characteristic signals of the end group, i.e. -CF2CF(OCF3)I and -CF(OCF3)CF2I. Indeed, only -CH2CF2-I (major amount) and -CF2CH2-I end-groups of these poly(VDF-co-PMVE) copolymers were noted showing the high reactivity of PMVE-I in the iodine transfer polymerization of VDF. The CTA concentration enabled one to control the molecular weights of the resulting telechelic diiodo-poly(VDF-co-PMVE) copolymers ranging from 1,000 to 18,000 g mol-1 (assessed by 19F NMR), produced for the first time. It was also noted that the higher the molar masses, the higher the amount of -CF2CH2-I end-group. The controlled behavior of that polymerization was also evidenced by the absence of the reversed -CH2CF2-CF2CH2- VDF dyads, narrow polydispersity indices (<1.75), and by the linear “molar mass versus monomer conversions” relationship. Finally, the thermal properties, i.e. the glass transition temperatures (Tg) and degradation temperatures, of these copolymers were assessed and were found to depend on the molecular weights and on the monomer composition of the copolymers. These fluoroelastomers had Tgs ranging from -63 to -35 oC according to their molecular weights and the contents of both comonomers, with decomposition temperatures greater than 250 oC under air.