Preparation of Gradient Copolymers by Controlled Radical Copolymerization

AMPHIPHILIC GRADIENT COPOLYMERS: SYNTHESIS AND

3.2 SYNTHETIC STRATEGIES FOR THE PREPARATION

3.2.1 Preparation of Gradient Copolymers by Controlled Radical Copolymerization

Two experimental approaches have been reported to control the compositional gra- dient, namely the “spontaneous”—and the “forced”—gradient methods. The first is the use of batch copolymerizations in which a spontaneous gradient in instanta- neous composition is formed based on the differences in the reactivity ratios of the comonomers and the concentrations of comonomers in the monomer feed. The sec- ond method is the use of semi-batch copolymerizations to form “forced” controlled gradients in instantaneous composition.

3.2.1.1 Batch Process The simplest approach to prepare gradient copolymers, called spontaneous gradient polymerization, is a batch copolymerization in which the comonomer composition change is promoted by the difference in reactivity of the comonomers [68]. In the absence of either azeotropic points or ideal random copolymerizations the monomer composition will change gradually with the monomer conversion because of the different reactivities of the monomers.

In a copolymerization of two monomers, M1 andM2, with reactivity ratios,r1 andr2, the relative rate of consumption of each monomer (df1/df2) is given by the Mayo–Lewis equation [69]:

df1

df2 = (r1f12+f1f2) (r2f2

2 +f1f2) = F1

F2 (3.4)

in whichf1,f2are the mole fraction ofM1,M2in the feed solution andF1/F2represents the instantaneous copolymer composition. In most cases,F1/F2 is different tof1/f2, resulting in composition drift given by the integrated copolymerization composition equation (3.5) (Skeist’s equation [70]):

X=1− ( f1

f1,0 )𝛼(

f2 f2,0

)𝛽(

[f0−faz] [f1−faz]

)𝛾

(3.5)

k k where

𝛼= r2

1−r2, 𝛽= r1

1−r1, 𝛾 = 1−r1r2

(1−r1)(1−r2), faz= 1−r2 2−r1−r2, Xrepresents the conversion, andf1,0the initial mole fraction ofM1in the feed solution.

As determined by this Skeist’s equation [70] the composition along the polymer chain varies as a function of chain length.

To prepare a copolymer with a high gradient by batch polymerization, it is necessary to select monomer pairs for which the reactivity ratios differ, at least, by several times. The second condition for formation of gradient macromolecules under this regime is that high conversions close to ultimate values should be achieved, since only in this case will the head and tail of a molecule have appreciably different compositions.

In this subsection, we describe the synthesis of gradient copolymers using batch methods by the most relevant CRcoP processes used up to date, which are Atom-Transfer Radical Polymerization (ATRP), Nitroxide-Mediated (radical) Polymerization (NMP, currently named Aminoxyl-Mediated Radical Polymer- ization, AMRP [71]), and Reversible-Addition-Fragmentation chain-Transfer polymerization (RAFT).

3.2.1.1.1 ATRP Batch copolymerization using ATRP to prepare gradient copoly- mers has been shown to be successful for a number of monomer pairs [3, 4, 49, 72–96]. Matyjaszewskiet al. mainly contributed to the ATRP synthesis of gradient copolymers using the batch process [3, 4, 11, 49, 72, 80, 83, 84, 86, 97]. Batch copolymerization was performed and focused on styrene-acrylates and methyl methacrylate-acrylates gradient copolymers using catalytic systems based on copper complexes with various multifunctional nitrogen-containing ligands. Spontaneous gradient copolymers with a continuous tapering along the copolymer chain could be obtained for these systems. The gradient structure of these copolymers was confirmed by an increase in the instantaneous composition of the less active monomer (acrylates) with conversion and a decrease of the one of the more active monomers (styrene, methyl methacrylate) while maintaining a constant number of polymer chains.

Guet al. [87] synthesized gradient poly(methyl methacrylate-grad-n-butyl acry- late) (P(MMA-grad-nBA) copolymer chains from the surface of aluminum oxide particles by ATRP using the CuBr/PMDETA catalyst system. Here again, in ana- lyzing the kinetics of copolymerization, it was observed that the controlled gradient copolymers obtained are MMA-rich near the surface and become progressively richer innBA content on going further out. This observation has suggested more ways to synthesize novel ceramic/polymer architectures.

Indeed, the gradient copolymers of nBA/MMA were also synthesized by batch copolymerization in ATRP by Madrugaet al. both in bulk and in benzonitrile solu- tion [88, 98]. These studies were carried out on two copolymer series using CuCl/bpy as the catalyst systems with two different initiators at 100∘C. A Nuclear Magnetic Resonance (NMR) technique was used to determine the copolymer microstructure, particularly the stereochemical arrangement of monomers in poly(MMA-grad-nBA) copolymers. The relative intensity dependence of the monomer triad fractions as a

k 180 k

fB= 0.25 fB= 0.5 fB= 0.75

I

II IIIIV VI

VII V

179 178 177 176 ppm

175 174 173

Figure 3.3 Expanded13C NMR spectra showing the carbonyl region of P(MMA-grad-nBA) copolymers prepared by ATRP at different feed compositions. Reprinted from [88].

function of conversion was studied by analysis of the carbonyl region of these copoly- mers, and seven distinguishable signals in the carbon carbonyl region were observed in the 13C NMR spectra (Figure 3.3). Their relative intensities changed with the composition of the monomer in the copolymer and depended on both the diad tac- ticity (racemic (r) or meso (m)) and the proportion of the different diads and triads sequences. According to Aerdts’s method [99], four of these signals (I, II, IV, and VI; see Figure 3.3) were assigned to sequences and configurations of MMA-centered triads. The three others (III, V, and VII) corresponded tonBA-centered triads. Later, Braret al. [89, 90] reported the complete spectral assignments for the microstruc- ture of spontaneous poly(MMA-grad-nBA) copolymers produced by ATRP with a complete assignment of the peaks by 2D NMR spectroscopy.

Because of the differences in reactivity of acrylates and methacrylates, many methacrylates-acrylates-based spontaneous gradient copolymer systems were synthesized by ATRP using the batch process. The ATRP copolymerization of two (meth)acrylate-based PEO monomers—that is, a methacrylate PEO macromonomer with a methyl end-group (PEOMeMA, DPPEO = 23, r(PEOMeMA) = 1.51 +/− 0.02) and a PEO acrylate with a phenyl end-group (PEOPhA, DPPEO = 4,

k k

TABLE3.1ReactivityRatiosofComonomersReported M1M2ProcessSolventInitiatorT,∘C(controlagent)r1r2Ref. Methacrylate-acrylate AMAnBAATRPTolueneMBrP,100∘C(PMDETA/CuBr)2.580.51[93] HEMAtBABulkEBiB,50∘C(PMDETA/CuCl)4.490.1[94] PEOMeMAPEOPhAAnisoleEtBriBu,80∘C (Me6TREN/CuCl)1.510.67[92] MMAnBARAFTBulkAIBN,60∘C(CDB)1.70.2[124] MMAMAFRPBulkAIBN,50∘C20.40[129] AAEMFDARAFTTFTAIBN,65∘C(ECEDB)1.410.56[126] Methacrylate-methacrylate BMADMAEMAATRPWater/2-propanolMPEG,25∘C(Bpy/CuBr)1.241.07[23] MMABulkEBiB,80∘C(PCPI/CuBr)1.260.98[140] Methacrylate—other MMADMANMP1,4-dioxaneBB,100∘C (SG1)2.360.330[116] MMAVC2FRPBulkAIBN,50∘C20.36[129] TFEMAAARAFT1,4-dioxaneAIBN,70∘C(DSCSP)0.850.8[159]

k k Acrylate—other VBPDEFDARAFTTFTAIBN,65∘C(ECEDB)1.410.24[126] nBAVAcCMRPTolueneV-70,30∘C(Co(acac)2,)150.02[167] MAVC2FRPwaterIPP,45∘C0.950.90[127] Styrene-acrylate SnBAFRPbulkAIBN,120∘C0.810.23[105] FRPbenzeneAIBN,50∘C0.6980.164[141] tBANMPbulkBenzoylperoxide,120∘C (TEMPO)0.90.3[6] MANMPbulkMONAMS,120∘C(SG1)0.890.22[107] AcGalEARAFTDMAcAIBN,90∘C(MCPDT)0.70.07[160] Styrene-methacrylate SMMAFRPbulkBenzoylperoxide,60∘C0.520.46[150] Styrene-acylicacid SAARAFTbulkAIBN,60∘C(BPATC)0.210.08[68] NMP1,4-dioxaneMONAMS,120∘C(SG1)0.720.27[77] BB,120∘C (SG1)0.930.25[171] PAA-SG1,120∘C(SG1)0.220.88[36] Styrene—other ASSFRPbulkAIBN,60∘C1.2180.887[150]

k k r(PEOPhA) = 0.67 +/− 0.02) (all reactivity ratios reviewed in this chapter are

summarized in Table 3.1)—was shown to lead to well-defined spontaneous gradient copolymers with narrow molecular weight distribution (1.2 < Mw/Mn < 1.4) [92]. Functional spontaneous gradient copolymers having a pendant, crosslinkable functional group of allyl methacrylate (AMA) and n-butyl acrylate (nBA) of P(AMA-grad-nBA), were also synthesized via ATRP polymerization [93]. The copolymer composition and microstructure, which demonstrated the gradient character, were analyzed. The experimental data agreed well with data calculated using the Mayo–Lewis terminal model and Bernoullian statistics, with monomer reactivity ratios of 2.58+/−0.37 and 0.51+/−0.05 for AMA andnBA, respectively (calculated by the Kelen–Tüdos method [100]). Functional spontaneous gradient copolymers of 2-hydroxyethylmethacrylate (HEMA) and tert-butyl acrylate (tBA) were also synthesized by ATRP in cyclohexanone at 50∘C [94]. The monomer reactivity ratiosr(HEMA)=4.50 andr(tBA)=0.21 were estimated by an extended Kelen–Tüdos method using copolymer composition data [101].

Acrylates derivatives often present similar reactivities, and therefore batch copoly- merization often results in a statistical distribution. However, blocky-gradient copoly- mers oftBA and 2-ethylhexyl acrylate (2EHA) were prepared by Vidtset al. [91] by polymerizing firsttBA alone until conversion occurred and then adding the 2EHA.

This procedure obtained copolymers that consist of a PtBA segment, followed by a random structure of both monomers with a high 2EHA content.

Different system based on styrene-(meth)acrylates copolymerization were also widely studied by ATRP batch copolymerization. For example, well-defined amphiphilic gradient copolymers of poly(ethylene glycol) methyl ether methacrylate (PEGMA) and styrene (S) were successfully synthesized by ATRP [95]. The authors reported that the initial feed ratio of the two monomers had a significant effect on the copolymer’s gradient composition. A gradient structure of the copolymer was seen when equimolar amounts of the comonomers were used, and the instantaneous composition changed continuously along the chain contour. The main reason for the formation of gradient copolymers might be considered to be the difference in relative reactivity between styrene and PEGMA due to their incompatibility.

The development of gradient copolymers is inhibited by the absence of a proper experimental technique to quantify the gradient quality, namely the gradient profile in the copolymer and the dispersity of this profile in the chains. However, the con- trol over chain length and end-group functionality should be related to the control over gradient quality, since a perfect gradient implies a uniform distribution of the comonomers. In other words, the gradient quality could be measured indirectly by a measurement of the PDI and/or end-group functionality. Recently, Van Stanbeerge et al. have verified this hypothesis by an evaluation of the gradient quality using computational simulation [96]. The linear gradient quality and the control over chain length and end-group functionality for the copolymerization of acrylates, methacry- lates, and styrenes via ATRP was evaluated by detailed kinetic Monte Carlo (kMC) simulations with explicit tracking of macromolecules. The kinetic Monte Carlo sim- ulations confirmed that batch ATRP conditions allow those copolymers with good linear gradient qualities to be synthesized, and this was quantified by the value of

k k a parameter⟨GD⟩that ranges between 0 and 1. This parameter denotes the average

deviation for each polymer chain from the corresponding theoretically perfect linear gradient of the same chain length (e.g., for a uniform distribution of comonomers, the⟨GD⟩value is close to 0) and hence was reported to be a new copolymer property besides the average chain length, composition, andÐ. Moreover, for sufficiently high conversions and well-chosen comonomers, it was demonstrated that a strong correla- tion exists between⟨GD⟩andÐ. Thus the measuredÐwas used as a first assessment of the linear gradient quality. The physical meaning of⟨GD⟩was illustrated by dis- cussing copolymer compositions with varying linear gradient qualities. Under batch ATRP conditions, copolymerization ofnBA and MMA could lead to a copolymer with good linear gradient quality if high conversion is reached and an appropriate catalytic system is selected, and thus lead to a lowÐ. Further it was shown that this correlation between⟨GD⟩andÐdepends on the monomer reactivity ratios.

3.2.1.1.2 NMP Nitroxide-mediated polymerization (NMP) is a rather widespread method for preparing gradient copolymers but mostly using the semi-batch pro- cess [6], as will be discussed later in this chapter. Charleux et al. have mostly contributed to the NMP synthesis using the batch process [33, 76, 77, 102–104].

These authors reported on the batch copolymerization of styrene with n-butyl acrylate [76] and acrylic acid [33, 77, 102] using an alkoxyamine initiator based on the N-tertbutyl-N-(1-diethylphosphono-2,2-dimethylpropyl)nitroxide, SG1, in mini-emulsion and in 1,4-dioxane, respectively. As in these cases the reactivity differences of the comonomers were not so strong (r(S) = 0.81 and r(nBA)) = 0.23 at 120∘C in bulk [105];r(S)=0.72 for styrene andr(AA)=0.27 for acrylic acid at 120∘C in 1,4-dioxane [77]) (Figure 3.4), the chains exhibited no or small composition drift depending on the initial comonomer composition, and hence a slightly pronounced gradient structure was obtained. The gradient microstructure was confirmed experimentally by liquid adsorption chromatography (LAC) analyses, performed at different conversions showing a net composition drift of the copolymer chains toward higherFAAvalues with the increase in comonomer conversion, on the one hand, and a narrow composition distribution, on the other hand. A comprehensive characterization of the controlled feature of NMP copolymerization of styrene and n-butyl acrylate comonomers in bulk and in butyl acetate solution, using SG1 as the control agent and methyl propionate-SG1 (MONAMS) as the initiator, which led to a gradient microstructure, was obtained by automatic continuous online monitoring of polymerization reactions (ACOMP) [106].

NMP copolymerization of styrene and methylacrylate in bulk, with SG1 used as the control agent and MONAMS as the initiator, was also found to lead to a gradient distribution along the polymer chain [107]. Here too, a slightly pronounced gradient structure was obtained due to the value of the reactivity ratios of the comonomers (r(S)=0.89 andr(MA)=0.22) and was confirmed by a1H NMR study that showed the triad MA-MA-MA to increase with the time of polymerization.

The NMP of methacrylates does not exhibit any features of a controlled system, typically reaching a low conversion plateau in a short time period. This is due to an extensive irreversible termination of the propagating radicals caused by

k k

0.0 0.0

0.2 0.2

0.4

Molar fraction of acrylic acid in the monomer mixture r(acrylic acid)= 0.27

r(styrene)= 0.72

Molar fraction of acrylic acid in the copolymer 0.4

0.6 0.6

0.8 1.0

0.8 1.0

Figure 3.4 Reactivity ratios for acrylic acid and styrene determined in 1,4-dioxane at 120∘C using MONAMS/SG1 system. Reprinted from [77].

the large activation–deactivation equilibrium constant and 𝛽-hydrogen transfer from the propagating radical to the nitroxide no matter which nitroxide is used (TEMPO [108, 109] or SG1 [110–114]). However, a strongly improved, controlled nitroxide-mediated polymerization of methyl methacrylate was attempted using a small amount of styrene [103, 115]. The controlled feature of this copolymerization was demonstrated both experimentally [103, 115] and theoretically [104] and was explained by the fact that due to the low value of the activation–deactivation equilibrium constant K of styrene (KS = 4 × 10−10 mol.L−1 for SG1-mediated polymerization at 90∘C), the average activation–deactivation equilibrium constant

<K> of the copolymerization of S with a large K value such as MMA (KMMA

= 1 × 10−7 mol.L−1 for SG1-mediated polymerization at 90∘C) remains close to the lowest K value, and thus the presence of styrene promotes the reversible deactivation of the propagating radicals by SG1 and the impact of the side reactions is decreased. The controlled feature of the NMP of MMA was also observed by adding a small proportion of dimethyl acrylamide (DMA−10 mol%), which resulted in well-defined P(MMA-grad-DMA) gradient copolymers [116].

More recently, Kalugin et al. studied the optimum condition for the NMP copolymerization of styrene andtBA mediated by TEMPO [117]. It is known that, in contrast to styrene,tBA is little involved in the controlled radical polymerization mediated by TEMPO [118]. This result may be due to the high bond strength in the PtBA–TEMPO polymer adduct, that is, its inability to reinitiate polymerization at temperatures usual for this process (110–130∘C). However, the copolymerization of

k k tBA with styrene can proceed in a controlled manner to high conversions because

the probability of occurrence of the styrene unit at the end of the propagating radical is much higher than that of the tert-butyl acrylate unit (r(S) = 0.9 and r(tBA) = 0.3) [6]. Therefore, at a sufficient content of styrene in the monomer mixture, the adducts with styrene–TEMPO end-groups would be the main products.

An initial content of styrene of 30 and 15 mol% was found to lead to a controlled feature of the copolymerization as well as to a gradient structure. Indeed, at initial contents of styrene of 30 and 15 mol%, the changes in instantaneous compositions Finstof the copolymer were 45 and 30%, respectively. These values correspond to a change in cumulative compositionFcumof the copolymer equal to 15% in both cases.

However, the use of an additional source of initiation, obtained via the addition of cumenehydroperoxide (CHP), was necessary to increase the rate of polymerization in order to maintain the control feature of the copolymerization.

3.2.1.1.3 RAFT Another way of preparing gradient copolymers is by controlled radical polymerization with a Reversible Addition–Fragmentation chain Trans- fer (RAFT). Copolymerization conducted under RAFT conditions can generate copolymers with targeted compositions and controlled architectures [119–122].

Since RAFT is classed as a controlled radical polymerization [123], it is possible to obtain gradient copolymers, depending on the reactivity ratios. For instance, Rizzardoet al. studied the copolymerization of an equimolar mixture of MMA and nBA (r(MMA)=1.7,r(nBA)=0.2) in the presence of a dithio-compound as chain transfer agent (CTA) [124]. The values of the reactivity ratios clearly indicated that this copolymerization provides copolymer chains that should be rich in MMA at one end and rich innBA at the other. The authors reported that when an initial mixture of MMA andnBA in the molar ratio of 1:0.91 was polymerized with AIBN in the presence of CTA at 60∘C, the ratio of MMA tonBA in the polymer was 1:0.45 at 22% conversion and reached 1:0.80 at 93% conversion.

Harrison et al. [68] attempted RAFT copolymerizations using styrene (S) and acrylic acid (AA) to prepare block-like copolymers. The reactivity ratios of the S–AA system are unusually sensitive to temperature and solvent (due to the formation of hydrogen-bonded AA dimers). So under NMP conditions (120∘C, 1,4-dioxane solu- tion, r(S) = 0.72,r(AA) = 0.27) [77], reported above, relatively weak gradients were formed. The use of RAFT allowed Harrisonet al. to conduct the polymer- ization at lower temperatures and in bulk, and the disparate reactivity that resulted in the comonomers (r(S)=0.21 andr(AA)= 0.082) created block-like structures comprising a S-rich segment, a relatively short transitional segment, and a segment of AA homopolymer. The copolymers displayed properties characteristic of block copolymers, such as microphase separation (observed by AFM) and self-assembly in a selective solvent (measured by DLS).

The group of Lacroix-Desmazes widely used batch RAFT polymerization for the preparation of new functional copolymers [125, 126]. Rixenset al. studied the prepa- ration of terpolymers containing vinylidenechloride (VC2), methyl acrylate (MA), and phosponated groups by RAFT polymerization using 1-(ethoxycarbonyl)-ethyl dithiobenzoateas CTA [125]. Such copolymer combining barrier properties (VC2),

k k solubility in organic solvents (MA), and good adhesion and anticorrosion properties

(phosphonated moiety) could be useful as additives in many applications, including paints and surface treatments. Two routes were tried: a direct RAFT copolymerization of a phosphonated methacrylate with VC2 and MA and the preparation of functional polymers by RAFT copolymerization of hydroxyethylacrylate (HEA) with VC2 and MA, followed by a chemical modification of the hydroxyl pendant groups of the HEA units with a phosphonatedepoxide. Two architectures were obtained that were predicted from the reactivity ratios of the monomers involved in RAFT polymeriza- tion. The gradient terpolymer was prepared in one step by RAFT terpolymerization of VC2 (1), MA (2), and a phosphonated methacrylate monomer (3) (reactivity ratios arer1−2=0.90 andr2−1 =0.95 [127],r1−3=0.36 andr3−1=2 [128, 129],r2−3= 0.40 andr3−2=2 [129, 130]), whereas the terpolymer poly(VC2-co-MA-co-HEA) was expected to be almost ideal statistical because the reactivity ratios of VC2/MA and MA/HEA and VC2/HEA are very similar. This was confirmed experimentally by a composition drift in the case of the gradient terpolymers, whereas no composition drift was observed for the statistical case.

Fluorinated gradient copolymers with complexing groups, Poly(1,1,2,2-tetrahydro- perfluorodecyl acrylate-co-acetoacetoxyethyl methacrylate) (poly(FDA-co-AAEM)) and poly(1,1,2,2-tetrahydroperfluorodecyl acrylate-co-vinylbenzylphosphonic acid diethylester) (poly(FDA-co-VBPDE)), were prepared by Ribaut et al. using batch RAFT polymerization [126]. The RAFT polymerization was performed in α,α,α-trifluorotoluene using 1-(ethoxycarbonyl)-ethyl dithiobenzoate as CTA.

Because the reactivity ratio of an acrylate monomer (r1) is less than the reactivity ratio of the methacrylate and styrenic monomers (r2), the AAEM (M2) thus reacted faster than FDA (M1) and the obtained copolymer was a gradient copolymer (calculated reactivity ratios were r2 = 1.61, r1 = 0.56). The same held for the copolymerization of FDA with the styrenic monomer VBPDE (M2) (calculated reactivity ratios: r2 =1.41, r1 = 0.24). The faster consumption of AAEM and VBPDE in comparison with FDA was confirmed experimentally by performing a kinetic analysis of the copolymerization. Such gradient copolymers were soluble in supercritical carbon dioxide (scCO2) and thus could be advantageously synthesized directly in CO2 medium, which allowed the use of organic solvents to be avoided, promoting a green chemistry. Moreover, the authors reported that these gradient copolymers exhibit remarkable CO2 solubility that may allow them to be used as complexing surfactants in many applications, particularly for extraction of metal derivatives in scCO2in the context of the nuclear decontamination.

Other functional gradient copolymers have been prepared by batch RAFT poly- merization. Godyet al. reported the first synthesis of glycopolymer architectures by the RAFT process [131]. In this work, biotin end-functionalized hydrophilic gly- copolymers were synthesized using copolymerization of an acrylamide galactose derivative (GalAm) withN-acryloylmorpholine (NAM) in the presence of a biotin CTA. The individual monomer conversions indicated that the conversion of NAM is slightly higher than that of the acrylamide galactose derivative, so that a slight gradient microstructure can be obtained. Subsequently, this glycopolymer has been successfully used to prepare gold nanoparticles [132].