Grafting from Fluoropolymers Using ATRP: What is Missing?

Abstract

Grafting polymers from fluoropolymer backbones is very advantageous to modify the properties of these often non‐functional materials. It is particularly useful in the field of ultrafiltration membranes to tune the hydrophilicity of PVDF membranes and to add other properties such as antifouling for example. One of the main techniques used to effect such grafting is ATRP. A large number of papers are published every year on this topic. However, careful examination of this body of work reveals that the evidence provided to support the covalent bonding of polymer chains on the fluoropolymers are often limited and rather unconvincing. This is of course due to the difficult detection of the rare connecting covalent bonds. Nevertheless, this detection problem is not the only issue. Grafting from fluoropolymers using ATRP relies on the homolytic cleavage of C−F bond, notorious for their high stability and BDE. Moreover, fluoropolymers based on VDF are prone to dehydrofluorination by reaction with bases and nucleophiles, which leads to the formation of unsaturations within the polymer backbone. This reaction is often completely ignored even though ATRP ligands can trigger this elimination. These two very often overlooked reactivity features seriously question the use of ATRP as an efficient grafting technique for fluoropolymers.

Synthesis of PVDF-Based Graft Copolymeric Antifouling Membranes Showing Affinity-Driven Immobilization of Nucleobases

“…Interestingly, new signals observed at 197.6 eV (for Cl, 2p 3/2 ) and 199.5 eV (for Cl, 2p 1/2 ) and reconvolution of the C­(1s) producing signal at 288.3 eV of treated PVDF were possibly due to the transitions of C­(1s) of the (> C FCl) units . Thus, it might be argued at this stage that the PVDF backbone upon treatment with the present catalyst complex results in formation of some (>CClF) units due to halide exchange (discussed later), and initiation of polymerization should occur predominantly through the cleavage of these relatively weaker (C–Cl) bonds besides much stronger (C–F) bonds.…”

“…The plots have shown a linear increase in the molecular weight for the three individual constituent polymers as well as random copolymeric graft chains signifying their controlled growth. The ATRP initiation from the PVDF backbone has been a matter of long standing debate in view of the very high (C–F) bond strength, possibilities of dehydrofluorination under the ATRP conditions, which might result in ill-defined polymer structures during the “grafting from” approach of synthesis . In order to further investigate these possibilities, XPS analysis of pristine PVDF and PVDF obtained after treatment with the catalyst complex in NMP, under similar experimental condition during graft copolymerization (excepting the monomers), was carried out and presented in Figure S2.…”

“…A possible explanation behind the appreciable degree of control observed during grafting may be the faster deactivation of the propagating radicals by the relatively less-halidophilic cupric chloride-based complex present as contamination with the CuCl-based catalyst complex (the stock CuCl has been found to contain 4.62% of CuCl 2 , as presented in the Supporting Information S4). This should result in formation of Cl-capped growing graft chains (after initial stages of activation being over by the cleavage of C–F bonds of PVDF backbone) allowing relatively faster repetitive activation–deactivation cycles in the subsequent steps, resulting in controlled growth of the grafted chains.…”

“…The ATRP initiation from the PVDF backbone has been a matter of long standing debate in view of the very high (C−F) bond strength, possibilities of dehydrofluorination under the ATRP conditions, which might result in ill-defined polymer structures during the "grafting from" approach of synthesis. 35 In order to further investigate these possibilities, XPS analysis of pristine PVDF and PVDF obtained after treatment with the catalyst complex in NMP, under similar experimental condition during graft copolymerization (excepting the monomers), was carried out and presented in Figure S2. The absence of the C(1s) signal at binding energy ∼284.6 eV (corresponding to C�C) 36 units.…”

Facile grafting modification of main-chain-type semi-fluorinated alternating fluoropolymersviasimultaneous CuAAC reaction and ATRP in one pot at ambient temperature

“…[21][22][23][24][25][26][27] Atom transfer radical polymerization (ATRP) as one of the RDRP methods is a powerful technique in polymer chemistry [28][29][30] that is often employed to synthesize fluorinated graft copolymers. [31][32][33][34] For example, chlorine on the main-chain-type polychlorotrifluoroethylene (PCTFE) backbone is utilized as an initiator and various side chains can be introduced into the PCTFE backbone via ATRP. 31,32,[35][36][37][38] In addition, some researchers also synthesized main-chain-type fluorinated graft copolymers from VDF-containing fluorinated polymers using C-F bonds as the initiating sites, 22,34,39,40 but the initiating efficiency was very low due to the low reactivity of the secondary fluorine atoms 41 and careful examination to support the successful grafting is missing.…”