<html><head><meta name="color-scheme" content="light dark"></head><body><pre style="word-wrap: break-word; white-space: pre-wrap;">\makeatletter
\@ifundefined{HCode}
{\documentclass[screen,CRCHIM,Unicode,biblatex,published]{cedram}
\addbibresource{crchim20240358.bib}
\newenvironment{noXML}{}{}
\def\tsup#1{$^{{#1}}$}
\def\tsub#1{$_{{#1}}$}
\def\thead{\noalign{\relax}\hline}
\def\tbody{\noalign{\relax}}
\def\endthead{\noalign{\relax}\hline}
\RequirePackage{etoolbox}
\usepackage{upgreek}
\def\hyphen{\text{-}}
\def\jobid{crchim20240358}
%\graphicspath{{/tmp/\jobid_figs/web/}}
\graphicspath{{./figures/}}
\newcounter{runlevel}
\let\MakeYrStrItalic\relax
\csdef{Seqnsplit}{\\}
\def\refinput#1{}
\def\back#1{}
\def\0{\phantom{0}}
\def\ubreak{\break}
\newenvironment{inftab}{}{}
\usepackage{multirow} 
\def\nrow#1{\@tempcnta #1\relax%
\advance\@tempcnta by 1\relax%
\xdef\lenrow{\the\@tempcnta}}
\def\morerows#1#2{\nrow{#1}\multirow{\lenrow}{*}{#2}}
\DOI{10.5802/crchim.336}
\datereceived{2024-05-02}
\daterevised{2024-09-04}
\dateaccepted{2024-09-04}
\ItHasTeXPublished
}
{\documentclass[crchim]{article}
\def\bibliographystyle#1{}
\def\bibliography#1{}
\let\refinput\input
\def\CDRdoi{10.5802/crchim.336}
\let\newline\break
\let\ubreak\relax
\makeatletter
}
\makeatother

\usepackage{upgreek}

%\copyeditor{RK}

\dateposted{2025-03-21}
\begin{document}

\begin{noXML}

\CDRsetmeta{articletype}{clarification}

\title{Suitable radicals and monomers to obtain innovative fluorinated
polymers based on vinylidene fluoride and their applications}

\alttitle{Radicaux et monom\`eres adapt\'es pour obtenir des
polym\`eres fluor\'es innovants \`a base de fluorure de vinylid\`ene et
leurs applications}

\author{\firstname{Abdelatif} \lastname{Manseri}\CDRorcid{0000-0001-8008-4766}}
\address{Institute Charles Gerhardt, University of Montpellier, 
CNRS, ENSCM, Montpellier, France}
\email[A. Manseri]{Abdelatif.Manseri@enscm.fr}

\author{\firstname{Bruno} \lastname{Ameduri}\CDRorcid{0000-0003-4217-6664}\IsCorresp}
\addressSameAs{1}{Institute Charles Gerhardt, University of Montpellier, 
CNRS, ENSCM, Montpellier, France}
\email[B. Ameduri]{bruno.ameduri@enscm.fr}

\keywords{\kwd{2-trifluoromethacrylic acid}
\kwd{Batteries}
\kwd{Fluoropolymers}
\kwd{Radical polymerization}
\kwd{Trifluoromethyl radical}
\kwd{Vinylidene fluoride}}

\altkeywords{\kwd{Acide 2-trifluoromethacrylique}
\kwd{Batteries}
\kwd{Polym\'{e}risation radicalaire}
\kwd{Polym\`{e}res fluor\'{e}s}
\kwd{Radical trifluorom\'{e}thyle}
\kwd{Fluorure de vinylid\`{e}ne}}

\thanks{Arkema, French PEPR programme}

\begin{abstract}
Thanks to their exceptionally useful properties, fluoropolymers are
irreplaceable materials involved in many High-Tech applications, such
as aerospace, automotive, internet of things, optics and electronic
industries. They are usually synthesized by radical (co)polymerization
of fluoroalkenes under high pressure. While many radical initiators
have been used, perfluoro-3-ethyl-2,4-dimethyl-3-pentyl persistent
radical (PPFR) releases a trifluoromethyl radical, as well as
sodium trifluoromethyl sulfinate, initiating the
homopolymerization of vinylidene fluoride (VDF) or the copolymerization
of VDF with a wide range of fluoroolefins. Among the latter,
functional 2-trifluoromethacrylates (MAF-funcs) have shown characteristics 
leading to various materials such  as anticorrosion coatings, gel
polymer electrolytes for Lithium-ion batteries, and polymer exchange
membranes for fuel cell. Finally, facing per- and polyfluoroalkyl substances (PFAS) issues,
mineralizations of polyvinylidene fluoride (PVDF) and other VDF-containing copolymers were
recently studied in subcritical water to generate fluoride anions, as
an original source of CaF\textsubscript{2}, thus closing the loop of the 
fluorine\unskip\break
chemistry.
\end{abstract}

\begin{altabstract}
Gr\^{a}ce \`{a} leurs propri\'{e}t\'{e}s exceptionnelles, les
polym\`{e}res fluor\'{e}s sont des mat\'{e}riaux irrempla\c{c}ables
impliqu\'{e}s dans de nombreuses applications High-Tech, telles que les
industries a\'erospatiale, automobile, internet, optique et
\'{e}lectronique. Ces polym\`{e}res de sp\'{e}cialit\'{e} sont
g\'{e}n\'{e}ralement synth\'{e}tis\'{e}s par (co)polym\'{e}risation
radicalaire d'alc\`{e}nes fluor\'{e}s sous haute pression. Tandis que
des amorceurs radicalaires ont \'{e}t\'{e} utilis\'{e}s avec
succ\`{e}s, le radical persistant
perfluoro-3-\'ethyl-2,4-dim\'ethyl-3-pentyle persistent radical (PPFR) est
susceptible de g\'{e}n\'{e}rer un radical trifluorom\'{e}thyle tout
comme le trifluorom\'ethyle sulfinate de sodium, permettant d'amorcer
l'homopolym\'{e}risation du fluorure de vinylid\`{e}ne (VDF) ou la
copolym\'{e}risation du VDF avec une large gamme de fluorool\'efines.
Parmi elles, les 2-trifluorom\'ethacrylates fonctionnels (MAF-funcs) ont
montr\'{e} leur efficacit\'{e} pour conduire \`{a} de nouveaux
mat\'{e}riaux tels que des rev\^{e}tements anticorrosion, des
\'{e}lectrolytes polym\`{e}res pour batteries aux ions Lithium, et des
membranes \'{e}changeuses d'ions pour piles \`{a} combustible.
Finalement, face aux dangers des PFAS, la min\'{e}ralisation du PVDF et
d'autres copolym\`{e}res \`{a} base de VDF a \'{e}t\'{e} r\'{e}cemment
\'{e}tudi\'{e}e en milieu eau sub-critique water pour g\'{e}n\'{e}rer
des anions fluorures, comme source originale de CaF\textsubscript{2}, permettant
ainsi de fermer le cycle de la chimie du fluor.
\end{altabstract}

\maketitle

\twocolumngrid

\end{noXML}

\let\rmmu\upmu

\section{Introduction}\label{sec1}
Fluoropolymers are niche macromolecules regarded as irreplaceable
materials, endowed with exceptional chemical, thermal, electrical,
optical, and UV resistance properties. Hence, they have found many
applications in high-tech domains, such as aerospace, automotive,
internet of things, optics and electronic  industries~\cite{1,2,3,4,5a,5b}.
Many specific items such as high performance elastomers, stable cables
in photonics (core and cladding in optical fibers), chemically resistant membranes 
for desalination or in ion exchange for fuel cells, gaskets, binders for batteries in lithium (or sodium) ion
batteries, lubricants for wind mills, for O-rings and diaphragms,
protective paints, coatings and backsheets for photovoltaic 
panels~\cite{1,2,3,4,5a,5b}. The aims of this mini-review are: (i)~elucidating 
efficient radicals can be used to initiate (co)polymerization of
one of the most manufactured monomer, vinylidene fluoride (VDF), 
(ii)~proposing a comonomer as an efficient VDF partner and 
(iii)~presenting applications expected from the resulting copolymers.

\begin{scheme*}
\includegraphics{sc01}
\vspace*{.5pc}
\caption{\label{sch1}Non-exhaustive strategies to generate a
trifluoromethyl radical.}
\end{scheme*}

\begin{scheme*}
\includegraphics{sc02}
\vspace*{.5pc}
\caption{\label{sch2}Mechanism of oxidation of trifluoromethyl sulfinate, leading 
to the generation of ${}^{\bullet}$CF$_{3}$ radicals~\cite{8}.}
{\vspace*{-2pt}}
\end{scheme*}

\section{Use of fluorinated radicals to initiate the polymerization of
fluoroolefins}\label{sec2} 
Fluoropolymers are classically prepared by radical (co)polymerization
of fluoroalkenes under high pressure~\cite{1,2,3,4,5a,5b}. While many
radical initiators have been  utilized~\cite{6}, fluorinated ones have
already demonstrated their high efficiency to initiate such a reaction.
A distinct 
example is the trifluoromethyl radical which can be released from various 
reactants  (Scheme~\ref{sch1}). This halogenated radical and its addition to 
fluoroalkenes was comprehensively studied by Tedder and Walton~\cite{7}.

Among reactants, trifluoromethyl sulfinate (CF$_{3}$SO$_{2}$Na)
enables generation of $^{\circ}$CF$_{3}$ radical in the presence of an
oxidant (Scheme~\ref{sch2}) but has scarcely been used to initiate the
radical polymerization of  fluoroolefins~\cite{8}. Typically, in aqueous 
polymerization processes, persulfates are suitably used as oxidants 
to favor the (co)polymerization(s) of VDF with hexafluoropropylene (HFP) and/or 
perfluoromethyl vinylether (PMVE).

One of the main insights gleaned from this strategy is that the
resulting fluoropolymers (e.g., PVDF~\cite{8}) displays a higher
thermal stability than those synthesized from conventional persulfates 
(Figure~\ref{fig1}) since CF$_{3}$ end group slows down the
``unzipping'' depolymerization of such fluoropolymers.\looseness=1

\begin{scheme*}
\includegraphics{sc03}
\vspace*{.5pc}
\caption{\label{sch3}Perfluoro-3-ethyl-2,4-dimethyl-3-pentyl persistent
radical (PPFR) generates a trifluoromethyl radical capable of initiating 
radical polymerization of VDF~\cite{9,10}.}
\end{scheme*}

\begin{figure}
\includegraphics{fig01}
\caption{\label{fig1}TGA thermograms of PVDF achieved in various
initiations: ammonium persulfate (APS only, dotted line) and potassium
trifluorosulfinate with  CF$_{3}$SO$_{2}$K: APS (0.9: 1.0), full line)
under nitrogen (reproduced with permission from \textit{Amer. Chem. 
Soc.}~\cite{8}).}
\end{figure}

As displayed in Scheme~\ref{sch1}, various fluorinated reactants can
release trifluoromethyl radical, which can also be generated at
$T&gt;80~\text{\textdegree}\mathrm{C}$ from
perfluoro-{\ubreak}3-ethyl-2,4-dimethyl-3-pentyl persistent radical (PPFR)
at stable room temperature (Scheme~\ref{sch3})~\cite{9,10}, or
from by using CF$_{3}$I as a chain transfer agent involved either in
telomerization~\cite{11} or in iodine transfer (co)polymerization of 
fluoroolefins~\cite{12}. This strategy was also extended to copolymerization 
of VDF with a wide range of fluoroolefins (Scheme~\ref{sch4})~\cite{13}.

\begin{scheme*}
\includegraphics{sc04}
\vspace*{.5pc}
\caption{\label{sch4}Radical copolymerization of VDF with various
comonomers initiated by PPFR~\cite{13}.}
\end{scheme*}

Following these findings, the simultaneous use of PPFR and iodinated
chain transfer agents (CTAs) can enable the obtaining of $\omega$-iodo and
telechelic diiodo vinylidene fluoride (VDF)-based 
(co)polymers~\cite{14,15,16} (Scheme~\ref{sch5}). Different CTAs, such
as trifluoroiodomethane and 1,4-diiodoperfluorobutane, were used
with/without \textit{tert}-butyl 2-trifluoromethacrylate (MAF-TBE). All
of the obtained (co)polymers were characterized by ${}^{1}$H and 
${}^{19}$F NMR spectroscopies and their thermal properties were assessed. 

\begin{scheme*}
\includegraphics{sc05}
\vspace*{.5pc}
\caption{\label{sch5}Strategic routes to obtain PVDF-I and
poly(VDF-\textit{co}-MAF-TBE)-I from PPFR, a persistent radical at RT, 
which releases a ${}^{\bullet}$CF$_{3}$ radical from  60~\textdegree C. 
(PVDF, PPFR and MAF-TBE stand for polyvinylidene fluoride, perfluoro-3-ethyl-2,4-dimethyl-3-pentyl
persistent radical and \textit{tert}-butyl 2-trifluoromethacrylate, 
respectively)~\cite{12}.}
\vspace*{-4pt}
\end{scheme*}

A mechanistic investigation of these reactions evidenced the competitive
presence of CF$_{3}$ end group in CF$_{3}$-PVDF-I originating from the CTA
and showed a negligible amount of  CF$_{3}$-PVDF oligomer dead chains
produced by the direct initiation from ${}^{\bullet}$CF$_{3}$ radical
(released by PPFR) onto VDF. Very low chain defects of chaining (i.e.,
reversed --CH$_{2}$CF$_{2}$--CF$_{2}$CH$_{2}$-- and
--CF$_{2}$CH$_{2}$--CH$_{2}$CF$_{2}$-- in  VDF--VDF dyads, even the
absence of latter types) led to high melting points 
(161--173~\textdegree C) of the resulting  polymers~\cite{12}.

\section{Radical copolymerization of vinylidene fluoride (VDF) with
2-trifluoromethacrylates}\label{sec3} 

\subsection{Introduction}\label{sec3.1}
VDF has been copolymerized with a wide range of fluorinated or
non-halogenated  monomers~\cite{17} (Scheme~\ref{sch6}), Q--e Alfrey
and Price parameters~\cite{18,19} to fine-tune the efficiency of
the reaction.

\begin{scheme*}
\vspace*{-2pt}
\includegraphics{sc06}
\vspace*{.5pc}
\caption{\label{sch6}Conventional or controlled (RDRP) radical
copolymerization of VDF with functional comonomers and use of the 
resulting copolymers for diverse applications~\cite{17}.}
\vspace*{-2pt}
\end{scheme*}

\subsection{Copolymerization of VDF with\newline 2-trifluoromethacrylic
acid}\label{sec3.2}
Among functional comonomers, 2-trifluoro\-{\ubreak}methacrylic 
acid (MAF), discovered
by Ojima et~al.\ in the early 80s (Scheme~\ref{sch7})~\cite{20} and
later manufactured by the Tosoh Finechem Corp. (ex-Tosoh F-Tech), has
shown an efficiency leading to a wide range of  materials~\cite{21,22}
including polymer electrolytes for Lithium-ion batteries~\cite{23} and
polymer exchange membranes for fuel cells~\cite{24a,24b}. 

\begin{scheme*}
\includegraphics{sc07}
\vspace*{.5pc}
\caption{\label{sch7}Synthesis of 2-trifluoromethacrylic acid (MAF)
from bromination of 3,3,3-trifluoropropene and further
steps~\cite{20}.}
\vspace*{-4pt}
\end{scheme*}

Among the first series, functional 2-trifluoro\-{\ubreak}methacrylates (MAF-funcs)
have been shown to be suitable partners to VDF~\cite{21,22,23,24a,24b,25}.
Interestingly, though this monomer cannot homopropagate under radical
polymerization, \textit{tert}-butyl 2-trifluoromethacrylate (MAF-TBE)
is quite efficient in copolymerization with VDF leading to
quasi-alternating copolymers, as displayed in Scheme~\ref{sch8} that
mainly highlights crosspropagation steps. Their reactivity ratios were
calculated to be: $r_{\mathrm{VDF}}= 0.040$ and
$r_{\mathrm{MAF}\hyphen\mathrm{TBE}}= 0.036$ at 57~\textdegree
C~\cite{25}.

\begin{scheme*}
\includegraphics{sc08}
\vspace*{.5pc}
\caption{\label{sch8}Mechanism of alternating radical copolymerization
of VDF with MAF-TBE (reproduced with permission from RSC~\cite{25}).}
\end{scheme*}

A wide variety of MAF-func monomers were synthesized from the
condensation of MAF acid chloride onto functional alcohol 
(Figure~\ref{fig2})~\cite{21,23,26,27,28,29,30}.

\begin{figure*}
\includegraphics{fig02}
\caption{\label{fig2}Overall strategies to synthesize novel functional
2-trifluoromethyl monomers from MAF and their radical copolymerization with VDF (left). Steel 
plates coated with poly(VDF-\textit{co}-MAF Phosphonate) copolymer at the beginning
of the experiment (A), after 1~h (B), and after 18~h (C). (D)~Uncoated
steel plate as reference sample after 1~h (right).}
\end{figure*}

\begin{scheme*}
\includegraphics{sc09}
\vspace*{.5pc}
\caption{\label{sch9}Synthesis of dimethyl phosphonate
2-trifluoromethyl acrylate (MAF-DMP) by condensation of MAF acroyl
chloride onto 2-hydroxy dimethyl phosphonate~\cite{28}.}
\end{scheme*}

As an example, dimethyl phosphonate 2-trifluoromethyl acrylate
(MAF-DMP) was  prepared in two steps. First, paraformaldehyde was
reacted with dimethyl phosphite to afford 2-hydroxydimethyl
phosphonate~\cite{28}.
Then, the latter was condensed onto MAF acryloyl chloride to yield such
a MAF-DMP (Scheme~\ref{sch9}), the kinetics of copolymerization of
which led to the 
\mbox{reactivity} ratios ($r_{\mathrm{VDF}}= 0.76 \pm 0.34$
and $r_{\mathrm{MAF}\hyphen\mathrm{DMP}} = 0$ at 74~\textdegree
C~\cite{28}, determined using Mayo-Lewis law from the data
listed in the diagram of composition 
\mbox{(Figure~\ref{fig3})}.

\begin{figure}
\includegraphics{fig03}
\vspace*{-3pt}
\caption{\label{fig3}Diagram of composition during radical
copolymerization of VDF with MAF-DMP at 74~\textdegree C~\cite{28}.}
\vspace*{-3pt}
\end{figure}

The application of such original poly(VDF-\textit{co}-MAF-DMP)
copolymers deals with anticorrosion coatings of steel 
(Figure~\ref{fig2})~\cite{28}. The presence of phosphonic acid (after
hydrolysis of dimethyl phosphonate function in the copolymers) enables
a strong adhesion onto steel surface, the PVDF chain, making the
protection.

Using the same strategy, an original fluorinated monomer
bearing a cyclocarbonate function, (2-oxo-1,3-dioxolan-4-yl)methyl
2-(trifluoromethyl)acrylate (MAF-cyCB), was synthesized in 70\% overall
yield from glycerol 1,2-carbonate  (Scheme~\ref{sch10})~\cite{29}.

\begin{scheme*}
\includegraphics{sc10}
\vspace*{.5pc}
\caption{\label{sch10}Synthesis of (2-oxo-1,3-dioxolan-4-yl)methyl
2-(trifluoromethyl)acrylate (MAF-cyCB) from the condensation of
glycerol 1,2-carbonate with MAF-acryloyl chloride~\cite{29}.}
\end{scheme*}

Thus, a functional monomer was successfully copolymerized with
VDF under radical initiation  (Scheme~\ref{sch11}).

\begin{scheme*}
\includegraphics{sc11}
\vspace*{.5pc}
\caption{\label{sch11}Radical copolymerization of VDF with
(2-oxo-1,3-dioxolan-4-yl)methyl 2-(trifluoromethyl){\ubreak}acrylate (MAF-cyCB)
initiated by \textit{tert}-amyl peroxy-2-ethylhexanoate 
(TAPE)~\cite{23,29}.}
\end{scheme*}

Then, the resulting copolymers were chemically modified by
3-aminopropyl triethoxysilane from the nucleophilic substitution of
amino function that opens the cyclocarbonate yielding a PVDF bearing
triethoxysilane side groups reinforced by a urethane link 
(Scheme~\ref{sch12})~\cite{29}. Then, hydrolysis of these trifunctional
groups under acidic conditions (to avoid any dehydrofluorination of VDF
units) followed by condensation led to a crosslinked network.

\begin{scheme*}
\includegraphics{sc12}
\vspace*{.5pc}
\caption{\label{sch12}Chemical modification of
poly(VDF-\textit{co}-MAF-CyCB) copolymers by ring opening of
cyclocarbonate with an aminopropyl  triethoxysilane~\cite{29}.}
\end{scheme*}

In addition, these poly(VDF-\textit{co}-MAF-cyCB) copolymers were used
as relevant polymer gel electrolytes dissolving Lithium perchlorate for
Lithium ion batteries~\cite{23} (Figure~\ref{fig4}), the
electrochemical properties (conductivities and stability in
cyclic voltammetry (Figure~\ref{fig5}) of which were determined.

\begin{figure*}
\vspace*{4pt}
\includegraphics{fig04}
\vspace*{4pt}
\caption{\label{fig4}Single polymer electrolyte based on
poly(VDF-\textit{co}-MAF-cyCB) copolymers and LiClO$_{4}$ salt for
which the Li$^{+}$ cation transport may occur {via} the
amorphous state (reproduced with permission from RSC~\cite{23}).}
\vspace*{4pt}
\end{figure*}

\begin{figure*}
\includegraphics{fig05}
\vspace*{4pt}
\caption{\label{fig5}Temperature dependence of the ionic conductivity
(left) and linear sweep voltammetry curves of polymer electrolyte with
$\mathrm{cyCB}/\mathrm{Li}^{+}=5$. The scan rate is 0.5~mV/s (right)
for the investigated SPEs using VTF model (reproduced with permission
from RSC~\cite{23}).}
\vspace*{4pt}
\end{figure*}

A similar strategy was also applied to PVDF bearing oligoPEG side 
groups~\cite{27} (Scheme~\ref{sch13}), whose ionic conductivities are
illustrated in  Figure~\ref{fig6}.

\begin{scheme*}
\includegraphics{sc13}
\vspace*{.5pc}
\caption{\label{sch13}Synthesis of oligo(ethylene oxide)
2-trifluoromethyl acrylate and its radical copolymerization with 
VDF~\cite{27}.}
\end{scheme*}

\begin{figure}
\vspace*{6pt}
\includegraphics{fig06}
\vspace*{6pt}
\caption{\label{fig6}Ionic conductivities of
PVDF-$g$-oligo(ethylene oxide) copolymers filled with Li salt
and nanosilica (reproduced with permission from RSC~\cite{27}).}
\end{figure}

In addition, another MAF-func could be prepared by changing
oligo(ethylene oxide) moiety with a polyfluoroether in an original
comomoner able to copolymerize with VDF as novel solid polymer 
electrolyte (Scheme~\ref{sch14})~\cite{30}.

\begin{scheme*}
\includegraphics{sc14}
\vspace*{.5pc}
\caption{\label{sch14}Radical copolymerization of VDF with MAF derivative
bearing polyfluoroethers~\cite{30}.}
\vspace*{-3pt}
\end{scheme*}

Besides conventional radical copolymerization of VDF with MAF-func
derivatives, the reversible deactivation radical copolymerization
(RDRP) of MAF with VDF has also been studied, either {via}
iodine transfer copolymerization in presence of 
\mbox{1-iodoperfluoroalkanes}~\cite{31} or by RAFT in presence of 
linear~\cite{32} or cyclic~\cite{33} xanthates, the linear one acting
as an emulsifier of polymer blend to obtain original membrane for fuel
cell  (PEMFC)~\cite{34}. Fuentes-Exposito et~al.~\cite{35a,35b} acheived
surfactant-free PVDF latexes by RAFT-mediated emulsion polymerization
of VDF in presence of commercially available poly(ethylene glycol)
chains borne by a linear xanthate (PEG-Xa). The particle size was
significantly smaller when the authors used PEG-Xa, evidencing the
positive effect on particle stabilization of the involvement of
xanthate chain end in the free radical process.

\vspace*{-2pt}

\subsection{Radical terpolymerization of VDF with MAF and another
fluorinated comonomer}\label{sec3.3}

\subsubsection{Hexafluoropropylene (HFP) as a
termonomer}\label{sec3.3.1}
The insertion of MAF units into copolymers based on non-functional
comonomers (as VDF, HFP, tetrafluoroethylene (TFE),
chlorotrifluoroethylene (CTFE), etc) also brings a functional group
that commercially available copolymers do not have (e.g.,
poly(VDF-\textit{co}-HFP) as Kynar$^{\mbox{\textregistered}}$,
Viton$^{\mbox{\textregistered}}$ Daiel$^{\mbox{\textregistered}}$, 
Solef$^{\mbox{\textregistered}}$ and 
Technoflon$^{\mbox{\textregistered}}$, marketed by Arkema, Chemours,
Daikin, Solvay and Syensqo,  respectively)~\cite{36}. 
Scheme~\ref{sch15} illustrates how a simple radical 
terpolymerization~\cite{37} yields these copolymers bearing carboxylic
side groups.

\begin{scheme}
\includegraphics{sc15}
\vspace*{.5pc}
\caption{\label{sch15}Radical terpolymerization of VDF with MAF and
HFP~\cite{37}.}
\end{scheme}

\subsubsection{Trifluoroethylene (TrFE) as a
termonomer}\label{sec3.3.2} 
Copolymers based on VDF and trifluoroethylene (TrFE) display piezo- or
ferroelectric  properties~\cite{38,39}. The use of 1\% MAF
(Scheme~\ref{sch16}) enables satisfactory adhesion properties to stick
golden electrodes on the poly(VDF-\textit{ter}-TrFE-\textit{ter}-MAF)
terpolymer films for possible electroactive materials~\cite{40}. 

\begin{scheme}
\includegraphics{sc16}
\vspace*{.5pc}
\caption{\label{sch16}Conventional radical terpolymerization of VDF,
trifluoroethylene (TrFE) and 2-trifluoromethacrylic acid (MAF) to
obtain electroactive devices. In the peroxydicarbonate initiator, R
stands for \textit{tert}-butyl(cyclohexyl)~\cite{40}.}
\vspace*{-10pt} 
\end{scheme}

The resulting device displays some ferroelectric properties as
evidenced by Figures~\ref{fig7} and~\ref{fig8}. DSC thermogram of the
resulting film exhibits Curie  $(T_{\mathrm{c}})$ and melting
$(T_{\mathrm{m}})$ temperatures  (Figure~\ref{fig7}), $T_{\mathrm{c}}$
being the temperature at which its spontaneous polarization undergoes
an induced polarization and vice versa (i.e., the transition
temperature at which a ferroelectric product becomes paraelectric
material and 
\mbox{vice-versa}).

\begin{figure}
\vspace*{-2pt}
\includegraphics{fig07}
\vspace*{-4pt}
\caption{\label{fig7}DSC thermograms of
poly(VDF$_{64}$-\textit{ter}-TrFE$_{35}$-\textit{ter}-MAF$_{1}$)
terpolymer (reproduced with permission from RSC~\cite{38}).}
\vspace*{-2pt}
\end{figure}

\begin{figure*}
\vspace*{-2pt}
\includegraphics{fig08}
\vspace*{-2pt}
\caption{\label{fig8}Continuous D--E loops of a 
poly(VDF$_{69}$-\textit{ter}-TrFE$_{31}$-\textit{ter}-MAF$_{1}$)
terpolymer (inset is a photograph of the flexible gold electrode coated
with a  20~$\rmmu$m thick film) (reproduced with permission from\break 
RSC~\cite{40}).}
\vspace*{-2pt}
\end{figure*}

\subsubsection{2H-pentafluoropropylene (PFP) as a\newline
termonomer}\label{sec3.3.3}
On another note, the terpolymerization of MAF-TBE with VDF and
2H-pentafluoropropylene (PFP)  (Scheme~\ref{sch17}) was reported with
various monomer \mbox{ratios,} enabling PFP (that does not
homopolymerize) to be reactive leading to poly(VDF-\textit{co}-PFP)
\mbox{copolymers} in low 
yields~\cite{41}. It was initiated by a mixture of two initiators
(Trigonox$^{\mbox{\textregistered}}$ 101/di\textit{tert}-butyl
peroxide) in 1,1,1,3,3-pentafluoropropane as the solvent. The
incorporation of PFP in the terpolymerization was favored by
termonomer-induced  copolymerization~\cite{42,43}. 

\begin{scheme*}
\includegraphics{sc17}
\vspace*{.5pc}
\caption{\label{sch17}Radical terpolymerization of
2H-pentafluoropropylene (PFP) with VDF and \textit{tert}-butyl
2-trifluoromethyl acrylate  (MAF-TBE)~\cite{41}.}
\vspace*{-2pt}
\end{scheme*}

A mechanistic sketch of radical terpolymerization, based on the
homopropagation of VDF and cross-propagation of PFP and HFP, is
proposed (Scheme~\ref{sch18}) that also takes into account the
recombination step of macroradical  (Scheme~\ref{sch19})~\cite{41}.

\begin{scheme*}
\includegraphics{sc18}
\vspace*{.5pc}
\caption{\label{sch18}Mechanism of the radical terpolymerization of PFP
with VDF and MAF-TBE~\cite{41}.}
\end{scheme*}

It is worth noting that the radical (co)polymeriza\-{\ubreak}tion of fluorinated
monomers is usually terminated by recombination (and not 
disproportionation)~\cite{44}. Thus, the recombination of two growing
macroradicals bearing PFP and VDF end-groups can be observed in 
Scheme~\ref{sch19}.

\begin{scheme*}
\includegraphics{sc19}
\vspace*{.5pc}
\caption{\label{sch19}Recombination of growing macroradicals based on
PFP and VDF end-units~\cite{41}.}
%\vspace*{8pt}
\end{scheme*}

\subsection{Chemical modification of\newline 
poly(VDF-\textit{co}-MAF)
copolymers}\label{sec3.4} 
Thanks to the carboxylic acid side function, poly(VDF-\textit{co}-MAF)
copolymers may undergo some grafting or chemical modification.
Actually, thermal stability of such copolymers is not as good as
expected and the higher the MAF content~\cite{24a,24b}, the poorer the
stability of these copolymers because of strong decarboxylation 
(Figure~\ref{fig9}). 

\begin{figure}
\includegraphics{fig09}
\vspace*{-4pt}
\caption{\label{fig9}TGA thermograms of poly(VDF-\textit{co}-MAF) at
various MAF molar  contents~\cite{24a,24b}.}
\vspace*{-6pt}
\end{figure}

Thus, the reduction of such acid groups was reported, the resulting
hydroxyl group undergoing a Mitsunobu reaction with para phenol
sulfonic acid (route 2,  Scheme~\ref{sch20}), hence inserting strong
acid as a possible example of fuel cell membranes~\cite{24a,24b}. As
expected, this etherification enhanced the thermal stability of the
copolymer and could be straightforward compared to an esterification in
medium yields (way 1,  Scheme~\ref{sch20}) and for which the ester link
is poorly stable in acid  media~\cite{24a,24b}. 

\begin{scheme*}
\includegraphics{sc20}
\vspace*{.5pc}
\caption{\label{sch20}Insertion of sulfonic acid function into
poly(VDF-\textit{co}-MAF) copolymers for possible fuel cell membranes
(reproduced with permission from \textit{Amer. Chem.
Soc.}~\cite{24a,24b}).}
\end{scheme*}

\section{Issues on PFAS}\label{sec4}
In recent years, per- or polyfluoroalkyl substances (PFAS) have attracted 
significant attention~\cite{45,46}. However, one must distinguish different 
categories linked to molar masses of PFAS 
(Figure~\ref{fig10})~\cite{45}. The lower ones are water soluble and
are found in creeks, rivers and oceans, which are exposed to their
toxicity, bioaccumulation and  persistency~\cite{46}. 

\begin{figure*}
\includegraphics{fig10}
\caption{\label{fig10}Distinguishing families of per- or
poly-fluoroalkyl substances (PFAS) into two domains (reproduced with
permission from MDPI~\cite{45}).}
\end{figure*}

Except persistency (that can be also an advantage in resistance to
aggressive media), these 
\mbox{issues}
are not assigned to fluoropolymers (FPs) which are
biocompatible and can be tolerated by human body~\cite{47,48,49}.
In addition, FPs fulfill the 13 criteria of polymer of low concern (PLC), in terms of 
residual monomers, molar masses, dispersities, charges, etc.~\cite{50,51} 
and really behave and contrast to low molar mass-compounds  (Figure~\ref{fig11}). 
However, 5 member states have pushed ECHA and the regulatory agencies to restrict 
them~\cite{52}, though more than 5600 answers to the consultation were
submitted~\cite{52}.

\begin{figure*}
\includegraphics{fig11}
\caption{\label{fig11}The categorization of PFAS by their molar masses
displays totally different advantages and issues (reproduced with
permission from MDPI~\cite{45}).}
\end{figure*}

Searching alternatives to fluorosurfactants has been well-reported in
the last few years~\cite{46,53} and one strategy lies on the simple
addition of water onto MAF  (Scheme~\ref{sch21})~\cite{54}. This
original and simple surfactant was successfully involved in emulsion
homopolymerization of  VDF~\cite{55} and copolymerization of VDF with
PMVE~\cite{56}.

\begin{scheme*}
\includegraphics{sc21}
\vspace*{.5pc}
\caption{\label{sch21}Addition of water into MAF to yield a fluorinated
short chain surfactant~\cite{54} in emulsion (co)polymerizations of 
VDF~\cite{55} and VDF and perfluoromethyl vinyl ether
(PMVE)~\cite{56}.}
\vspace*{-8pt}
\end{scheme*}

Though many strategies attempted recycling FPs~\cite{57},
thermal unzipping~\cite{58,59,60,61a,61b}, even achieved at the pilot
scale at 3M/Dyneon Company on PTFE, FEP and PFA~\cite{62a,62b}, seems
an appropriate option to recover TFE. The question lies on minimizing
the released low molar mass compounds that led to hazardous issues. 

Finally, mineralizations of PVDF~\cite{63} and
poly(VDF-\textit{co}-MAF)  copolymers~\cite{64} were recently studied
in subcritical water (Figure~\ref{fig12}) to generate fluoride anions,
which in the presence of Ca(OH)$_{2}$, leads to an original source of
CaF$_{2}$, thus closing the loop of the fluorine chemistry.
\vspace*{-4pt}

\begin{figure*}
\includegraphics{fig12}
\vspace*{-4pt}
\caption{\label{fig12}Mineralization of poly(VDF-\textit{co}-MAF)
copolymers in subcritical water (and oxidant in certain cases)
(reproduced with permission from \textit{Amer. Chem.
Soc.}~\cite{63,64}).}
\vspace*{-6pt}
\end{figure*}

\section{Conclusion}\label{sec5} 
Fluoropolymers (FPs) are niche macromolecules with outstanding properties, designed by
radical (co)polymerization. Their synthesis depends on
the choice of the initiators, processes
(not mentioned in this review), compared reactivity of the comonomers
by the assessment of their reactivity ratios and the functional group
that induces specific features for the searched applications of the
targeted fluorinated copolymers. Polyvinylidene fluoride (PVDF) can be used in many areas as
protective coatings, specific membranes for water purification, binders
for cathodes in batteries, and electroactive (ferro- or piezoelectric)
devices for haptics, actuators, organic transistors or sonars.
Structures and reactivities of comonomers must also be considered in
order to bring a specific property to the resulting materials. Among
comonomers, MAF is a suitable partner for VDF and also opens up to a
wide range of various reactive functional comonomers offering original
VDF-containing copolymers for various applications (anticorrosion
coatings, gel polymer electrolytes and possible binders for batteries,
fuel cell membranes to name a few). The controlled radical
(co)polymerization (or RDRP) of MAF with VDF has also been studied (ITP
or RAFT polymerizations) leading to new architectures such as block,
graft, gradient, alternated copolymers as original materials. Finally,
related to recent severe pressure and regulations linked to PFAS
issues, these fluorinated copolymers do not show any toxicity,
bioaccumulation and crossing the cellular human membrane in addition to
fulfilling the 13 PLC criteria. Hence, it can be anticipated that these
macromolecules will exit from the PFAS family following scrutiny.
Regarding recycling of VDF-containing copolymers, though several routes
exist, one relevant strategy lies in their mineralization in subcritical
water leading to fluoride anions as precursors of CaF2 as a starting
point of fluorine chemistry as a circular economy process.

\section*{Declaration of interests}
The authors do not work for, advise, own shares in, or receive funds
from any organization that could benefit from this article, and have
declared no affiliations other than their research organizations.

\section*{Funding}
Arkema and French PEPR programme are acknowledged for financial
supports.

\section*{Acknowledgments}
The authors thank all coauthors cited in the references below, as well
as Tosoh FineChemical {Corporation} (Shunan, Japan) for supplying MAF
monomer. BA thanks the French Fluorine Network (GIS).

\back{}

\printbibliography
\refinput{crchim20240358-reference.tex} 

\end{document}
</pre></body></html>