1 Introduction
Functionalized graft modification of polymers is an important means to conveniently prepare new functional polymers, which can not only change their original physical and chemical properties, but also endow materials with new functions to meet the needs of different application fields. By activating the carbon-halogen bonds on the carbon atoms of the main chain, modifying the grafted side chains (such as introducing reactive halogen atoms, mercapto groups, azide groups, etc.), or directly polymerizing "trifunctional monomers" (monomers containing double bonds, halogens and potential reactive groups) into the main chain,Various functionalized side chains can be introduced into the polymer main chain to form a graft polymer with a complex structure (block structure side chain, asymmetric Janus structure side chain, organic-inorganic hybrid structure side chain and the like) in a manner of initiating grafting polymerization from the main chain and coupling grafting onto the main chain respectively/simultaneously
[1,2][3⇓⇓~6][7⇓⇓~10].
Poly (vinylidene fluoride) (PVDF) -based polymers exhibit many special properties, such as chemical stability, thermal stability, aging resistance, electrochemical stability and fatigue resistance, due to the existence of strong carbon-fluorine (C — F) bonds (110 kcal·mol-1) in their structure. In addition, PVDF-based fluorine-containing materials are widely used in sensors, coatings, solar photovoltaic and other fields because of their low surface energy, high crystallization properties and outstanding processability. However, the large crystal size, poor hydrophilicity and poor compatibility with other polymers of unmodified PVDF-based fluoropolymers limit their further application. The introduction of functional segments into PVDF-based fluorine-containing polymers can effectively improve their properties and expand their application fields (such as high energy storage dielectric materials, separation membrane materials, polymer electrolytes, etc.).
In recent years, the functional modification of PVDF-based fluoropolymers by chemical methods has become a hot research topic. Compared with the physical blending method, the chemical grafting method has the advantages of good interfacial compatibility between the functional chain segment and the fluorine-containing main chain and uniform reaction degree. Chemical modification methods are divided into direct modification and indirect modification. Direct modification, that is, direct copolymerization of vinylidene fluoride (VDF) and designed functional groups (such as chlorotrifluoroethylene, hexafluoropropylene, etc.) To prepare functional fluorine-containing polymers. Indirect modification is to introduce functional segments/groups into the main chain of fluoropolymer by chemical means (such as thermal crosslinking method, hydrogenation method, elimination method, (living) radical polymerization grafting modification, etc.) To improve the performance of PVDF-based fluoropolymer.
The preparation of functional fluoropolymers by direct copolymerization is usually limited by the types of monomers, differences in reactivity ratios, production costs and other factors. Compared with the direct modification method, the indirect modification method has the advantages of high efficiency, convenience and economy. For example, the P (VDF-co-HFP) copolymer is crosslinked and modified by using an amphiphilic nucleating agent; Hydrogenation of P (VDF-co-CTFE) with tributyltin hydride; The functionalized monomer is grafted and copolymerized on the side chain of the fluorine-containing polymer by (living) radical polymerization, and the like. Compared with the nucleophile with stronger alkalinity used in the crosslinking modification process, the nucleophile with stronger alkalinity used in the crosslinking modification process is easy to cause the main chain of the fluoropolymer to be broken; In the hydrogenation process, the toxic tributyltin hydride reagent is used, which is not friendly to the environment and operators. Graft functionalization modification not only has the characteristics of high efficiency and safety, but also can endow PVDF based fluoropolymer with different properties according to the characteristics of grafted side chains, so it has attracted much attention.
In recent years, living radical polymerization, including reversible addition-fragmentation transfer (RAFT), atom transfer radical polymerization (ATRP), single electron transfer radical polymerization (SET-LRP) and photo-induced reversible radical deactivation polymerization (RDRP) with various mechanisms, has been vigorously developed, which has laid a solid foundation for the synthesis of polymers with precise structure and controllable molecular weight
[11⇓⇓⇓~15][16⇓⇓⇓~20][21⇓⇓⇓~25][26⇓⇓⇓~30]. Functionalized graft modification of fluoropolymers by (living) radical polymerization allows for more precise distribution of the functionalized side chains in the fluoropolymer.
The grafting modification of fluorine-containing polymers can also be divided into "grafting from" and "grafting onto". "grafting from" "refers to the graft modification by introducing free radicals into the main chain of PVDF and then initiating the (controlled) copolymerization of monomers; The "grafting onto" "is mainly to introduce active sites into the PVDF backbone through functional modification, and to graft the prepared functional segments onto the fluoropolymer backbone through click chemistry and other means
[31⇓~33]. Because the "grafting onto" method involves multi-step reactions in the implementation process, and most of the grafting side chains containing active sites are introduced through the "grafting from" method before coupling grafting, it will not be discussed in detail
[34⇓⇓⇓⇓~39].
In this paper, living radical polymerization (including ATRP, SET-LRP, organocatalyzed atom transfer radical polymerization (O-ATRP), photoinduced Cu (Ⅱ) -mediated RDRP) and high energy radiation (γ-ray, UV, electron beam) were reviewed. The methods of functional graft modification of fluorine-containing copolymers and the changes of the properties of the polymers before and after modification were discussed. The research status of functionalized graft modification of fluorine-containing polymers was summarized, and the development trend and application prospect of the modified polymers were prospected.
2 Modification of PVDF based fluoropolymer by atom transfer radical polymerization (ATRP)
ATRP is the most widely studied RDRP method, which is used to graft modification of PVDF-based fluorine-containing polymers, mainly using cuprous chloride (CuCl) and cuprous bromide (CuBr) as catalysts, 10-hexamethyltriethylenetetramine (HMTETA), 4,4 '-dimethyl-2,2' -dipyridine (DMDP), 2,2 '-bipyridine (Bpy) and 1,1,4,7,7-pentamethyldiethylenetriamine (PMDETA) as ligands.
2.1 ATRP Graft Modification of PVDF
Using PVDF as initiator,1-methyl-2-pyrrolidone(NMP)as solvent,HMTETA as ligand,and CuCl as catalyst,Min et al.Initiated 2-hydroxyethyl acrylate(HEA)containing acrylate structure to carry out ATRP,and successfully prepared hydrophilic PVDF-g-PHEA graft copolymer with a grafting rate of 22.6%
[40]。 the grafting process is shown in fig.1.Subsequently,PVDF and PVDF-g-PHEA graft copolymer were blended by non-solvent induced phase separation(NIPS)method with dimethyl sulfoxide(DMSO)as solvent and deionized water as non-solvent coagulation bath to prepare PVDF original membrane and PVDF/PVDF-g-PHEA blend membrane for ultrafiltration membrane.Compared with the original PVDF membrane,the PVDF membrane containing the PVDF-g-PHEA graft copolymer has a lower water contact angle(61.8°)and a higher final permeation flux(144.98 LMH)and normalized flux(0.51)。
Using PVDF as macromolecular initiator,NMP and DMSO as solvent,CuCl as catalyst,HMTETA as ligand,Kim et al realized the graft polymerization of sulfonated monomer sulfopropyl methacrylate(SPMA)or styrene sulfonic acid(SSA)with PVDF backbone by activating the secondary fluorine atom on PVDF backbone by ATRP,and prepared PVDF-g-PSPMA and PVDF-g-PSSA graft copolymers with the maximum grafting rate of 35%and 25%,respectively
[41]。 Subsequently,PVDF graft copolymer membranes were prepared by solution casting method using NMP and DMSO as solvents(PVDF dissolved in NMP,graft copolymer dissolved in DMSO).the introduction of PSPMA and PSSA functional chain segments can endow the grafted copolymer membrane with higher ion exchange capacity,water absorption and proton conductivity,so that the grafted polymer can be used as an electrolyte membrane material。
Cui et al used the halogen atom in PVDF as the initiation site,CuCl as the catalyst,DMDP as the ligand,and used ATRP method to graft poly(ethylene glycol)methyl methacrylate(POEM)monomer onto the secondary fluorine of PVDF main chain,and prepared PVDF-g-POEM amphiphilic copolymers with three different molecular weights(PVDF
275K,PVDF
320Kand PVDF
580K )
[42]。 Compared with the original PVDF membrane,the water flux of the PVDF-g-POEM copolymer membrane was increased by 39%,and the antifouling performance was significantly improved。
Park et al prepared PVDF-g-PtBA copolymer by ATRP with NMP as solvent,CuCl as catalyst and HMTETA as ligand
[43]。 Subsequently,at about 230℃,the ester was converted into the carboxylic acid(—COOH)group in the copolymer backbone by the driving force of in-situ pyrolysis,and the PVDF-g-PAA copolymer with polyacrylic acid(PAA)as the side chain was obtained.Due to its excellent mechanical properties and charge-discharge performance,the grafted polymer can be used as an electrode binder for various high-performance lithium-ion batteries。
2.2 ATRP grafted P(VDF-co-CTFE)
Using P(VDF-co-CTFE)(VDF/CTFE=97/3 mol%)as macroinitiator,NMP as solvent and CuCl/CuCl
2/Bpy as catalyst system,Zhu et al.Synthesized a series of P(VDF-co-CTFE)-g-PS graft copolymers with different lengths of PS side chains by ATRP
[44]。 Subsequently,as shown in fig.2,the C—Cl bond,which was not grafted in the graft polymerization,was converted into a C—H bond by tributyltin hydride(nBu
3SnH),that is,some CTFE units were directly dechlorinated into TrFE units to prepare a P(VDF-co-TrFE-co-CTFE)-g-PS graft copolymer.The obtained graft copolymer has lower dielectric loss(0.006)and higher energy storage density(10 J·cm
-3),and can be used as a dielectric material of a high energy storage capacitor 。
2.3 ATRP grafted polytetrafluoroethylene(PCTFE)
Zhang et al.used PCTFE oligomer and P(VDF-co-CTFE)copolymer with CTFE unit content of 12.0 wt%and 18.7 wt%respectively as macroinitiator,NMP as solvent,CuCl as catalyst,CuCl
2as deactivator,and Bpy or PMDETA as ligand to graft PS and PtBA onto the secondary chlorine of PCTFE and P(VDF-co-CTFE)respectively through ATRP to prepare PCTFE-g-PS and P(VDF-co-CTFE)-g-
[45]。 Since P(VDF-co-CTFE)and Cu(I)complex are insoluble in styrene and tert-butyl acrylate,the addition of polar solvents such as NMP is helpful to obtain a homogeneous polymerization system.Fig.3 is an ATRP of styrene initiated by PCTFE oligomer.Subsequently,P(VDF-co-CTFE)-g-PAA amphiphilic graft copolymer with hydrophilic PAA side chain was prepared by hydrolyzing the PtBA side chain of P(VDF-co-CTFE)-g-PtBA graft copolymer with hydrochloric acid in NMP solvent.Because the introduced PAA functional chain segment has good hydrophilicity,the obtained graft copolymer has potential application prospect in the aspect of filtration membrane materials。
2.4 ATRP graft-modified poly(vinylidene fluoride-co-trifluoroethylene-co-chlorotrifluoroethylene)(P(VDF-co-TrFE-co-CTFE))
Zhu et al.Prepared P(VDF-co-TrFE-co-CTFE)-g-PS graft copolymer by ATRP with NMP as solvent and CuCl/CuCl
2/Bpy as catalyst system
[46]。 Firstly,the P(VDF-co-TrFE-co-CTFE)(VDF/TrFE/CTFE=88.8/7.7/3.5 mol%)terpolymer containing TrFE unit was obtained by partial hydrogenation of activated C—Cl bond in CTFE unit.Then,a P(VDF-co-TrFE-co-CTFE)-g-PS(14%)graft copolymer with 14 wt%PS side chains was prepared by grafting PS onto the Cl site of P(VDF-co-TrFE-co-CTFE)via ATRP.Finally,the synthesized P(VDF-co-TrFE-co-CTFE)-g-PS was dechlorinated to avoid thermal crosslinking during subsequent melt processing(fig.4).the introduction of the PS side chain can reduce the compensation polarization generated by the interface between the crystalline region and the amorphous region of the fluoropolymer,and further accelerate the deorientation speed of the fluoropolymer crystal after the electric field is removed,so that the ferroelectric loss can be reduced,and the energy storage density of the graft copolymer can be improved。
Zhang et al synthesized a series of P(VDF-co-TrFE-co-CTFE)-g-SPS copolymers by ATRP using P(VDF-co-CTFE)(VDF/CTFE=94/6 mol%)copolymer as raw material,NMP as solvent and CuCl/Bpy as catalyst system
[47]。 First,a series of P(VDF-co-TrFE-co-CTFE)terpolymers with different TrFE/CTFE mol%were prepared by partial hydrogenation.Subsequently,the P(VDF-co-TrFE-co-CTFE)-g-PS copolymer was prepared by ATRP with the prepared terpolymer as macroinitiator.Finally,the P(VDF-co-TrFE-co-CTFE)-g-PS copolymers were sulfonated to prepare P(VDF-co-TrFE-co-CTFE)-g-SPS copolymers with different grafting density,grafting length and grafting amount.the sulfonated styrene side chain has good hydrophilicity and ion conductivity,which can endow the graft copolymer with higher water absorption,conductivity and ion exchange capacity,so that the graft copolymer can be used as a proton exchange membrane material。
A series of P(VDF-co-TrFE-co-CTFE)with different TrFE/CTFE mol%were prepared from P(VDF-co-CTFE)(VDF/CTFE=80/20 mol%)by the same method(hydrogenation followed by grafting)
[48⇓~50]。 A series of P(VDF-co-TrFE-co-CTFE)graft copolymers with different graft species were prepared by atom transfer radical polymerization(ATRP),including P(VDF-co-TrFE-co-CTFE)-g-PXMA(X=M,E,B)and VDF/TrFE/CTFE.By Graf a functional side chain on that side chain of the P(VDF-co-TrFE-co-CTFE),the content of an amorphous part in the fluorine-containing polymer can be increased,'cut'of large-size crystal grains of the fluorine-containing polymer is realized,the turnover speed of fluorine-containing polymeric crystals unde an electric field is improved,and the dielectric loss is reduced.Compared with the fluorine-containing polymer without grafting modification,the storage density can be increased from 4.2 J/cm
3to 14 J/cm
3 。
2.5 ATRP grafted P(VDF-co-HFP)
with DMSO as solvent,CuBr as catalyst and HMTETA as ligand,Xiao Shuqin et al.Directly initiated ATRP of 3-sulfopropyl methacrylate(SPMA)on The secondary fluorine atom of PVDF in P(VDF-co-HFP)to prepare P(VDF-co-HFP)-g-PSPMA graft copolymer With sulfonic acid group as side chain.the reaction formula is shown in Figure 5
[51]。 the successful synthesis of the P(VDF-co-HFP)-g-PSPMA graft copolymer was confirmed by 1H nuclear magnetic resonance(NMR)and Fourier transform infrared spectroscopy(FI-TR).the performance of the obtained graft copolymer is slightly lower than that of the commercialized Nafion 117 membrane,and the proton membrane with better performance is expected to be obtained by changing the membrane forming method and controlling the phase structure of the membrane to be applied to a fuel cell。
Chen et al.prepared P(VDF-co-HFP)-g-PPEGMA copolymer by grafting hydrophilic polyethylene glycol methyl ether methacrylate(PEGMA)onto the fluorine atom of the backbone of P(VDF-co-HFP)by ATRP with N,N-dimethylformamide(DMF)as solvent,CuBr as catalyst and HMTETA as ligand,as shown in Figure 6
[52]。 Then,the electrolyte microporous membrane with thickness of about 150~200μm was prepared by phase inversion method with acetone as good solvent and deionized water as non-solvent.The introduction of the hydrophilic PPEGMA functional segment can make the grafted polymer have higher saturated electrolyte absorptivity(323.5%),ionic conductivity(2.01×10
-3S·cm
-1)and good mechanical properties 。
the transition metal catalysts used in ATRP polymerization can effectively activate the carbon-fluorine(C—F)bond,which leads to the insertion of functional monomers and promotes the reaction.ATRP has the advantages of wide range of suitable monomers,adjustable grafting amount and high reaction rate.However,the unavoidable metal residues pose potential risks when applied in the fields of electrical,biological and environmental materials,and the high temperature required in the polymerization process and the participation of nitrogen-containing ligands may lead to side reactions such as elimination and hydrogenation。
3 Grafting modification of PVDF based fluoropolymer by single electron transfer living radical polymerization (SET-LRP)
In this paper, SET-LRP was used to modify PVDF based fluoropolymer, mainly using Cu (0) as catalyst, Bpy and Tris [2- (dimethylamino) ethyl] amine (Me6-TREN) as ligands.
3.1 Modification of PVDF by SET-LRP
Using DMSO as solvent,Cu(0)as catalyst and Bpy as ligand,Zhang et al.activated the C—F bond in the main chain of PVDF by Cu(0)-mediated controlled/living radical polymerization,and successfully introduced MMA chain segment into PVDF to prepare PVDF-g-PMMA graft copolymer
[53]。 During polymerization,the initiation(activation)step(k
act0)proceeds via an outer shell electron transfer(OSET)mechanism from the electron donor Cu(0)to the electron acceptor alkyl halide(R—F),as shown in fig.7.During the reaction,Cu(0)is the main catalyst of C-F bond,and Cu(Ⅱ)produced by the oxidation or disproportionation of Cu(Ⅰ)acts as a passivator,which can passivate the(k
act0)radical into a dormant species of alkyl halide.The graft copolymers were characterized by FI-TR,
1H NMR,gel permeation chromatography(GPC)and diffusion-ordered spectroscopy(DOSY),and the results showed that the graft copolymers were successfully synthesized 。
3.2 SET-LRP grafted P(VDF-co-CTFE)
Zhang et al.synthesized a series of P(VDF-co-CTFE)-g-PAN graft copolymers with different PAN contents by SET-LRP at room temperature using P(VDF-co-CTFE)as macroinitiator,DMSO as solvent,trace Cu(0)as catalyst and Me
6-TREN as ligand
[21]。 In the SET-LRP process,Cu(0)is the only catalyst source,and a balance between active and dormant species is established through a rapid activation/deactivation process.Among them,Cu(0)transfers electrons to the macroinitiator through the outer-shell electron transfer(OSET)mechanism,which activates the C—Cl bond to form anionic radicals.Compared with traditional ATRP,SET-LRP requires less energy to activate the C—Cl bond through the inner-shell electron transfer(ISET)mechanism,so the grafting reaction is more efficient and the reaction conditions are milder.the grafting content can be controlled by changing the reaction conditions。
Although Me
6-TREN ligands have extremely high reactivity,they are expensive.Therefore,our research group designed and synthesized a series of pyridine diimine compounds with different electronic and steric effects as ligands,and used them to catalyze the graft polymerization of MMA initiated by P(VDF-co-CTFE)with Cu(0)to synthesize a series of P(VDF-co-CTFE)-g-PMMA graft copolymers with different graft lengths
[22]。 the pyridine diimine ligands with adjustable catalytic activity can be obtained by adjusting the electronic effect and the steric effect of the substituent.the functional grafting modification of the fluorine-containing polymer by SET-LRP can be carried out under mild reaction conditions without a nitrogen-containing solvent,so that side reactions such as elimination,hydrogenation and the like caused by high temperature and a nitrogen-containing solvent during the ATRP functional modification of the P(VDF-co-CTFE)can be avoided。
Due to the influence of geometric shape and size,the traditional kettle reactor usually leads to the problems of unstable batch of free radical polymerization products,uneven thermal diffusion and attenuation of light intensity with penetration distance,which limits its application.Microfluidic technology provides a good solution to this problem.Microreactors have a large specific surface area,and the microscale effect can greatly improve the efficiency of mass and heat transfer and chemical selectivity
[54⇓⇓~57]。 the continuous flow characteristics of microfluidic technology can realize the precise control of reaction conditions,inhibit the occurrence of side reactions and stably realize the amplification reaction.In recent years,microfluidic technology has been applied to various polymerization reactions,including ionic polymerization,radical polymerization,ATRP,O-ATRP,SET-LRP,RAFT,nitrogen-oxygen mediated radical polymerization(NMP),Iniferter polymerization and ring-opening polymerization,and continues to flourish
[58][59⇓~61][62][23,63][64,65][23,24,66][12⇓⇓~15,67⇓~69][70][71][72⇓⇓⇓⇓⇓⇓⇓⇓~81]。 the application of microfluidic technology in The preparation of functional materials has broad research prospects。
Using P(VDF-co-CTFE)(VDF/CTFE=94/6 mol%)as macroinitiator,DMSO as solvent,Cu(0)as catalyst and Me
6-TREN as ligand,Guo et al.Realized the graft polymerization of MMA and AN in a microchannel reactor with the functions of both reactor and catalyst by SET-LRP,and prepared a series of P(VDF-co-CTFE)-g-PMMA and P(VDF-co-CTFE)-g-PAN copolymers with different graft lengths
[25]。 Fig.8 is a schematic diagram of a SET-LRP and copper tube reactor for MMA and AN initiated by P(VDF-co-CTFE).Compared with the macro-scale reaction,the micro-scale living radical polymerization shows incomparable advantages,including:(1)the induction period is significantly shortened(from 30 min to 10 min);(2)effectively inhibiting the"hot spot"effect(the self-polymerization of the monomer caused by the"hot spot"effect);(3)improving the polymerization reaction rate(the time for reaching the same grafting amount is shortened by at least 50%);(4)reducing the reaction temperature by at least 10 deg C;and(5)reducing the amount of expensive ligand by at least 50%.the fluorine-containing polymer is subjected to functional grafting modification by the method,so that a copolymer with a more precise grafting structure can be obtained,and meanwhile,the metal content in a grafting product is effectively reduced.the obtained product is expected to be applied to dielectric energy storage materials.This technology provides a new idea for the large-scale and stable preparation of functional grafted polymer materials。
The SET-LRP has the advantages of high polymerization rate,low metal catalyst consumption,easy removal and the like,and simultaneously,the reaction condition is mild,so that side reactions such as elimination,hydrogenation and the like caused by fluorine-containing polymers under the conditions of high temperature and nitrogen-containing reagents can be effectively avoided.In addition,the graft content can be easily controlled by changing the reaction conditions.However,the ligand Me6-TREN needed in the polymerization process of SET-LRP is expensive,so the development of ligands with high activity and low cost is an important direction of SET-LRP development 。
4 Photoinduced Cu (Ⅱ) -mediated RDRP Graft Modification of PVDF-Based Fluoropolymer
4.1 Photoinduced Cu(Ⅱ)-mediated RDRP Graft Modification of PVDF
Li et al.,using CuCl
2as catalyst,Me
6-TREN as ligand and NMP as solvent,for the first time used photoinduced Cu(II)mediated RDRP to catalyze the graft polymerization of tetrahydrofurfuryl methacrylate(THFMA)initiated by PVDF to prepare a new amphiphilic copolymer PVDF-g-THFMA.The grafting reaction is shown in Figure 9
[82]。 PVDF homopolymer membranes and PVDF-g-THFMA copolymer membranes were prepared by non-solvent induced phase separation method with NMP as solvent and deionized water as non-solvent coagulation bath.During the synthesis of PVDF amphiphilic copolymer membranes,a large number of hydrophilic segments were transferred to the membrane surface and self-assembled into regular pores,thus showing excellent hydrophilicity,good filtration and antifouling properties。
4.2 Photoinduced Cu(Ⅱ)-mediated RDRP Graft Modification of P(VDF-co-CTFE)
Using P(VDF-co-CTFE)(VDF/CTFE=94/6 mol%)as macroinitiator,DMSO as solvent,CuCl
2as catalyst and Me
6-TREN as ligand,Guo et al.prepared P(VDF-co-CTFE)-g-PAN and P(VDF-co-CTFE)-g-PMMA copolymers with different grafting amounts by RDRP,as shown in figure 10
[83,84]。 Through the photoinduced reversibly deactivated radical polymerization,the amount of Cu(II)catalyst added can be reduced to 0.1 mol%of the initiator concentration,that is,the supported concentration of the copper catalyst(CuCl
2)can be as low as 1/64 equivalent of the chlorine atom content in P(VDF-co-CTFE)in the presence of a Me
6-TREN under UV irradiation.At the same time,the reaction also has good space-time control,and the reaction rate is fast,and the functional monomer shows high grafting efficiency 。
Photoinduced Cu(Ⅱ)-mediated RDRP has the advantages of good space-time control,fast reaction rate,mild reaction conditions and high grafting efficiency,which can effectively avoid the side reactions of fluorine-containing polymers and highly reactive monomers at high temperatures.Compared with the traditional ATRP and SET-LRP,the concentration of the metal catalyst can be greatly reduced while the grafting amount is maintained,but expensive Me6-TREN are still needed as ligands 。
5 Organocatalytic atom transfer radical polymerization (O-ATRP) grafting modification of PVDF based fluoropolymer
Fluorine-containing polymer based on PVDF was grafted by O-ATRP, and N-phenylphenothiazine, perylene, eosin, fluorescein and p-anisaldehyde were used as organic photo-redox catalysts. Compared with other organic catalysts, p-anisaldehyde has the advantages of simple structure, low cost and good catalytic activity when used for atom transfer radical polymerization.
Guo et al successfully prepared a series of P (VDF-co-CTFE) -g-PMMA copolymers with different graft lengths by O-ATRP of MMA initiated by P (VDF-co-CTFE) macroinitiator under the action of ultraviolet light, using DMSO as solvent and N-phenylphenothiazine as organic catalyst
[85]. The MMA side chain of the obtained P (VDF-co-CTFE) -g-PMMA copolymer still contains a C-Cl bond chain end and can be used as a macroinitiator to be activated again to initiate methacrylate (MA) to carry out O-ATRP under the action of ultraviolet light and N-phenylphenothiazine,The obtained P (VDF-co-CTFE) -g- (PMMA-b-PMA) copolymer (as shown in Figure 11) contains 6.94% of PMMA and 25.71% of PMA respectively. Since the reaction completely avoids the use of a metal catalyst, the material is expected to be applied to the field of electrical materials.
Zhang et al. Prepared a series of P (VDF-co-CTFE) -g-PMMA copolymers with different grafting amounts by using N-phenylphenothiazine, perylene, eosin and fluorescein as organic catalysts to catalyze the O-ATRP of MMA initiated by P (VDF-co-CTFE) under ultraviolet light
[86]. The number of side chains of the synthesized graft copolymer is small, and the chain length changes obviously. Subsequently, the P (VDF-co-CTFE) -g-PMMA graft copolymer film was prepared by solution casting method with DMF as solvent, and the dielectric properties of the material were improved by grafting modification, which could be used as high pulse capacitor materials.
Lu et al. developed a new method to functionalize and modify PVDF-based fluoropolymers by O-ATRP using commercially available p-anisaldehyde as an organic catalyst, and the polymerization process showed good space-time control
[87]. According to the method, P (VDF-co-CTFE) is used as a macroinitiator, p-anisaldehyde which is simple in structure and cheap and easy to obtain is used as an organic catalyst, DMSO is used as a solvent, and MMA and glycidyl methacrylate (GMA) monomers are respectively grafted onto a main chain of the P (VDF-co-CTFE) through O-ATRP under the condition of ultraviolet light, so that a series of metal-free P (VDF-co-CTFE) -g-PMMA and novel P (VDF-co-. The resulting graft copolymer can be used as a macroinitiator to reinitiate MMA for O-ATRP, demonstrating good fidelity of the chain ends. The novel copolymer is expected to be applied to the field of dielectric energy storage materials.
Compared with the Cu (Ⅱ) -mediated photoinduced RDRP system, the grafting efficiency of O-ATRP on fluorine-containing polymers is reduced, but the metal residues can be completely avoided, and more pure PVDF-based grafted products can be obtained. In addition, the O-ATRP process has good space-time control and high end group fidelity, and the functionalized graft polymer with adjustable grafting amount and grafting structure can be prepared.
6 Graft modification of fluoropolymer by free radical polymerization
Free radical polymerization can be divided into traditional free radical polymerization and radiation-induced grafting polymerization for grafting modification of PVDF based fluoropolymer. Among them, the traditional free radical polymerization mainly uses azobisisobutyronitrile (AIBN) with high chemical resistance and high temperature resistance as initiator, which has the characteristics of slow initiation, fast growth and fast termination. Radiation polymerization is a process in which polymers are irradiated by high-energy rays such as electron beams, gamma rays, ultraviolet rays and ozone to form free radicals, thus forming active centers and eventually grafting monomers to the surface of polymers. Since radiation can penetrate the entire material, radiation-induced graft polymerization can introduce graft chains not only on the surface of the solid material but also inside the solid material.
6.1 Graft Modification of PVDF by Initiator and Double Bond
Kulshrestha et al.synthesized the PVDF-g-P(Sty-co-VBC)graft copolymer by grafting polystyrene-co-4-vinylbenzyl chloride(Sty-co-VBC)onto the unsaturated fluorinated backbone by free radical polymerization with AIBN as initiator and DMSO as solvent
[88]。 PVDF was dehydrogenated in alkaline solution to form An unsaturated structure,and then St and VBC were copolymerized with AIBN to form a PVDF-g-P(Sty-co-VBC)graft copolymer.Finally,the copolymer was quaternized in situ with trimethylamine.an anion exchange membrane with good thermal stability,chemical stability,mechanical stability and methanol permeability is prepare by a simple solution casting method and apply to that field of fuel cells。
Using NMP as solvent and AIBN as initiator,Liu et al.Grafted N-(3-dimethylaminopropyl)methacrylamide(DMAPMA)functional monomer with high reactivity,good water solubility and alcohol solubility onto the main chain of PVDF by free radical polymerization,and synthesized a new type of amphiphilic copolymer PVDF-g-PDMAPMA
[89]。 The amphiphilic copolymer PVDF-g-PDMAPMA was introduced into PVDF matrix as a hydrophilic modifier,and PVDF-g-PDMAPMA/PVDF blend membranes were prepared by non-solvent induced phase separation method with NMP as solvent and deionized water as non-solvent coagulation bath.Finally,the PVDF-g-PDMAPMA/PVDF membrane was quaternized by alkyl bromide(tertiary amine group on DMAPMA(—NR
3—))to obtain quaternary ammonium salt with antibacterial properties.The synthesis and quaternization process of the amphiphilic copolymer is shown in Figure 12.The prepared membrane has good hydrophilicity,antifouling property,permeability and antibacterial property,and can be used as an ultrafiltration membrane 。
the traditional free radical polymerization has the advantages of mild polymerization conditions,water resistance,suitability for various polymerization methods and wide range of suitable monomers,and the prepared graft copolymer also has the characteristics of chemical resistance and high temperature resistance.However,the microstructure,degree of polymerization,and polydispersity of the polymer cannot be controlled。
6.2 Radiation grafting modified PVDF
Chen et al.Prepared PVDF-g-St(91.8%)membrane byγ-ray pre-irradiation grafting method
[90]。 the grafting process is mainly divided into two steps.Firstly,the chemical bonds such as C—H or C—F in the main chain of PVDF were broken byγ-ray irradiation,and then the free radicals were generated.Subsequently,the irradiated PVDF was immersed in a styrene/water/methanol solution at 60°C to initiate radical graft polymerization,resulting in the formation of C—C chemical bonds between the graft chains and the PVDF backbone,thus chemically bonding the PS graft chains to the PVDF backbone to prepare PVDF-g-St with one or two graft chains pendant from each PVDF backbone.in order to determine the molecular weight of the grafted chain in the grafted membrane,the PS grafted chain of the PVDF-g-St membrane synthesized by pre-irradiation grafting method was separated by pyrolysis with boiling xylene extraction method.Fig.13 shows the preparation and extraction process of PVDF-g-St membrane.the extracted material and the residual membrane were analyzed by FT-IR,NMR,GPC and other methods,which confirmed that the PS chain was dissolved in xylene,while the PVDF skeleton was retained in the membrane during the extraction process。
Cohen et al.Grafted The hydrophilic sulfonic acid group onto PVDF by pre-irradiation grafting method to prepare PVDF-g-PSSA copolymer.the specific reaction process is shown in Figure 14
[91]。 Firstly,irradiating a PVDF raw material by gamma-rays to generate free radicals on a PVDF main chain;Ub,the irradiated PVDF raw material is soaked in a styrene/toluene solution for graft reaction to obtain a styrene graft modified PVDF-g-PS copolymer;Finally,the PVDF-g-PS copolymer was sulfonated(sulfonation of the benzene ring)by soaking in 0.5 M chlorosulfonic acid solution to prepare the PVDF-g-PSSA copolymer,which can be used as a novel air-drying membrane。
Lee et al.Grafted hydrophilic monomer 2-hydroxyethyl methacrylate(HEMA)with high dielectric constant onto pre-irradiated PVDF powder by irradiation(electron beam)method,so that hydroxyl groups were combined into PVDF polymer to prepare PVDF-g-PHEMA graft copolymer
[92]。 the PVDF polymer is first exposed to ionizing radiation,and the chemical bonds in the PVDF polymer are broken by an electron beam,and free radicals are generated according to the radiation dose,thereby generating reaction sites;the pre-irradiated PVDF powder is then immersed in a HEMA/water solution at elevated temperature within hours or days after irradiation to contact the vinyl monomer and initiate grafting from the reaction sites,the grafting first forming on the powder surface and then the monomer diffusing through the grafted portion and reacting with more free radicals.They also used electron beam radiation to induce free radicals to graft copolymerization in solution
[93]。 In vacuum,the C—H bond was easily broken by electron beam irradiation,and then PVDF was dehydrogenated to produce a large number of free radicals,and then hydrophobic styrene monomer was grafted onto PVDF matrix to prepare PVDF-g-PS graft copolymer。
Golcuk et al.Prepared PVDF-g-PS graft copolymer by UV-induced graft polymerization of PVDF and styrene,and the grafting rate could reach 100%
[94]。 Firstly,the PVDF membrane was prepared,then the PVDF membrane coated with photoinitiator was mixed with styrene/deionized water/acetone solution,and the mixture was placed in a photoreactor(λ=254 nm)for induced grafting.Free radicals were first formed on the PVDF,and then grafted with styrene under ultraviolet light to obtain PVDF-g-PS graft copolymer.Finally,the PVDF-g-PS proton conducting membrane was prepared by sulfonation of PVDF-g-PS graft copolymer with chlorosulfonic acid as sulfonating agent and dichloromethane as solvent.Chen et al.Also used UV light to modify the surface of PVDF film and prepared PVDF-g-PMMA polymer brushes
[95]。
Kang et al.Grafted propargyl methacrylate(PMA)onto the PVDF skeleton preactivated by ozone through thermally induced graft copolymerization to prepare PVDF-g-PPMA copolymers with propargyl pendant groups in the side chain of PPMA
[37]。 First,a solution of NMP containing PVDF was bubbled through a continuous stream of O
3/O
2mixture with NMP as the solvent to produce a peroxide capable of initiating monomer graft polymerization.Then,PVDF-g-PPMA copolymer was synthesized by inducing the decomposition of the peroxide groups on the PVDF chain to initiate the graft copolymerization of PMA onto PVDF preactivated by ozone.Finally,PVDF and PVDF-g-PPMA membranes were prepared by phase inversion of NMP solution in deionized water.Fig.15 shows the ozone preactivation of PVDF and the thermally induced graft copolymerization of PMA.Peng et al.Grafted poly(n-vinyl-2-pyrrolidone)(PNVP)onto the PVDF backbone preactivated by ozone via RAFT polymerization to prepare PVDF-g-PNVP copolymers
[96]。
radiation polymerization induced by high-energy rays usually has the advantages of strong penetration and high activation efficiency,so it can be used for solid phase polymerization.However,the controllability of radiation polymerization is poor,and the structure of fluorine-containing polymer matrix is easily damaged by high-energy radiation。
7 Conclusion and prospect
With the continuous development of the technology of functionalized graft modification of fluorine-containing polymers,Various functional monomers have been successfully introduced into fluorine-containing polymers by various living radical polymerizations (atom transfer radical polymerization (ATRP), single electron transfer living radical polymerization (SET-LRP), photoinduced Cu (Ⅱ) -mediated RDRP, organocatalyzed atom transfer radical polymerization (O-ATRP)) and high-energy ray irradiation. Compared with the radiation grafting modification induced by high-energy rays, the introduction of functional side chains into fluorine-containing polymers through various living radical polymerization shows more significant advantages.Comprises mild reaction conditions, controllable graft polymerization process and graft structure, more accurate distribution of the introduced functional chain segment, no damage to the original structure of the fluorine-containing polymer and the like. It has become the mainstream of functional modification of fluorine-containing polymers. By grafting functional monomers on fluorine-containing polymers, their corresponding properties (hydrophilicity, ionic conductivity, etc.) Can be improved and their application fields (such as sewage separation membranes, proton exchange membranes, high-performance capacitor dielectric materials, antibacterial materials, etc.) Can be expanded. A comparison of the various methods of functionalizing the graft modified fluorocopolymer is summarized in Table 1.
Among the various methods for functionalizing fluoropolymers, photoinduced RDRP has shown great potential for grafting fluoropolymers. The use of organic compounds instead of transition metal catalysts (such as O-ATRP) can completely avoid the use of metal catalysts in the polymerization process and obtain more pure polymerization products, which is expected to have better performance in high electric field and biomedical environment. Functionalization of fluorine-containing polymers by photoinduced RDRP will be the future development trend. However, most of the existing systems focus on inducing the activation of the corresponding sites of fluoropolymers in the UV band. Ultraviolet light has high energy and limited penetration, so living radical polymerization in the visible-infrared band has been developed to modify fluoropolymers.In particular, the development of efficient activation of PVDF-based macroinitiators/new organic catalysts for regulation of macromolecular free radicals is the main direction of future development of graft modification of fluorine-containing polymers, which has broad research prospects.
the National Natural Science Foundation of China(21604037)
the Priority Academic Program Development of the Jiangsu Higher Education Institutions(PAPD)
the Qing Lan Project, and the Six Talent Peaks project in Jiangsu Province(XCL-029)
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