AMPHIPHILIC GRADIENT COPOLYMERS: SYNTHESIS AND
3.2 SYNTHETIC STRATEGIES FOR THE PREPARATION
3.2.2 Preparation of Block-Gradient Copolymers Using Controlled Radical Polymerization
Block copolymers can be efficiently prepared if the first block is synthesized with high end-group functionality, followed by an efficient cross-propagation after the addition of the second monomer. Thus the ability to prepare block copolymers is one of the most useful attributes of controlled radical polymerization.
Block copolymers can be prepared by two general routes: (1) generation of reactive sites on the chain end of the polymer A, followed by polymerization of monomer B (C, D, etc.) using the activated polymer as (macro) initiator, or (2) by coupling of preformed polymers A and B (C, D, etc.) having at one chain end functional groups able to undergo covalent bond formation.
In the case of block-gradient copolymers, a spontaneous or forced gradient microstructure in the gradient block could be obtained depending on the reactivity ratios of the comonomers, as explained previously.
Amphiphilic block-gradient copolymers of 2-dimethylaminoethyl methacrylate (DMAEMA) with n-butyl methacrylate (nBMA) were synthesized by ATRP in aqueous media using a MPEG macro initiator previously synthesized by ATRP [23].
The gradient block formed by DMAEMA and BMA was obtained by semi-batch ATRP copolymerization, although these monomers have very similar reactivities (r(DMAEMA)=1.07,r(BMA)=1.24). Because of these similar values of reactivity ratios, the synthesis required slow feeding of one monomer to the ATRP of the second one. The molecular weight distributions of the resulting block copolymers were fairly narrow. Later, Phanet al. [116] prepared diblock and triblock copolymers made of poly(n-butyl acrylate) (PnBA) as the central block and PMMA/PDMA gra- dient copolymer as the external block using NMP method. Mono- and difunctional PnBA blocks were first synthesized using bulk polymerization of nBA at 120∘C
k k mediated by the alkoxyamines MAMA-SG1 or DIAMS (SG1-based dialkoxyamine),
respectively. A small excess of free SG1 was used to improve the control of the poly- merization, leading to polymers with narrow molecular weight distributions. After purification steps, the resulting SG1-PnBA and SG1-PnBA-SG1 were used as macro initiators for the copolymerization of MMA and DMA in 1,4-dioxane at 100∘C.
As was shown above, in this copolymerization system, the experimental obtained reactivity ratios (r(MMA)=2.36 andr(DMA)=0.33), which were in good agree- ment with the literature [163], led us to think that a spontaneous gradient copolymer could be formed, with densely MMA segments incorporated at the beginning of the copolymerization. The copolymerization kinetics of MMA/DMA system initiated by the macro initiators exhibited more similar data than that initiated by MAMA-SG1.
Recently, Steinhaueret al. [164] synthesized a triblock based on poly(hydroxyethyl acrylate) (P(HEA)) and poly(methoxyethylacrylate) (P(MEA)) blocks, and also P(HEA)-b-P(MEA)-b-P(HEA) and P(HEA)-b-P(HEA-grad-MEA)-b-P(HEA) via a RAFT process, using previously prepared HEA-based homopolymers with degrees of polymerization of DPn∼ 20−70 as the macro-chain transfer agents (MCTAs).
After adjusting the addition rate of MEA to the preset HEA/MCTA ratio during the polymerization reaction, good control was attained over the microstructure of the middle block of the P(HEA)-b-P(HEA-grad-MEA)-b-P(HEA)copolymers. Such copolymers are interesting due to their thermos-responsive properties in aqueous solutions, revealing adjustable cloud point (CP) temperatures between 0∘C and 80∘C for copolymers with a pure block microstructure and between 0∘C and 60∘C for copolymers with gradient microstructures.
There remain two major drawbacks to the current block copolymer preparation techniques via controlled radical polymerization. The first is that two separate poly- merization reactions are required to produce a diblock copolymer. These reactions, with their preparation and workup stages, are time-consuming, and it typically takes several days to produce a single block copolymer. Second, the nature of controlled radical polymerization, with low but unavoidable levels of termination, requires that polymerizations be stopped before reaching full conversion in order to limit the for- mation of dead polymer chains. This leads to lower reproducibility in their synthesis, as it is difficult to stop a polymerization at a predetermined target conversion. As a result, besides being time-consuming, the preparation of block copolymers by con- trolled radical polymerization can be expensive and poorly reproducible. Thus the development of a general, one-step strategy for the preparation of block-gradient copolymers presents a significant advance. Indeed, block-gradient copolymers can be prepared by a one-step strategy in two stages: (1) the polymerization of one monomer until a certain conversion and then (2) the addition of the second monomer in the reaction mixture (either straight or at a continuous rate) to obtain the spontaneous or forced gradient, respectively. The advantage of this method is that, depending on the reactivity ratio and on the order of the two stages (either pure block first and then gradient block or first gradient and then pure block), two different architectures can be obtained (Figure 3.10).
Amphiphilic block-gradient copolymers of poly(maleic anhydride-grad-styrene)- b-polystyrene were obtained for the first time using a one-step strategy by an NMP
k k
(a)
(b)
Figure 3.10 Schematic representation of the architectures of block-gradient copolymers in which the open circles denote monomer A and the closed circles monomer B.
copolymerization of a 9:1 mixture of styrene and maleic anhydride [165]. Thanks to the dramatically different reactivity ratios of the comonomers, a preferential incor- poration of maleic anhydride occurred, and upon depletion of the maleic anhydride, the monomer feed consisted of essentially pure styrene. The authors reported that, in contrast to the normal free radical case in which pure homopolystyrene chains are produced at this stage, chain growth of a polystyrene block occurred in the con- trolled free radical case yielding poly(maleic anhydride-grad-styrene)-b-polystyrene block-gradient copolymers.
Later, Luo et al. [166] reported a RAFT copolymerization of MMA and S in mini-emulsion. Well-defined P(S-co-MMA) gradient copolymers and P(S-co-MMA)-b-PS block gradient copolymers were proved to be very effi- ciently synthesized by a RAFT mini-emulsion polymerization using 1-phenylethyl phenyldithioacetate (PEPDTA) as CTA. Because the reactivity ratios of MMA/S arer(MMA)=0.46 andr(S)=0.52 [150], an azeotropic point atf1=0.471 could be predicted, and the copolymerization occurred without a drift in the monomer composition. When the molar fraction of MMA in the feed monomers was lowered below 0.471, MMA was consumed faster during the copolymerization, resulting in well-controlled P(MMA-grad-S) gradient copolymers. P(MMA-grad-S)-b-PS block-gradient copolymers were then obtained by the addition of pure styrene using a syringe pump at 8 mL/h. The comparison of the SEC chromatograme before and after the S addition demonstrated that the molecular weight rose after the addition, indicating the formation of a well-defined block-gradient copolymers.
Poly(n-butyl acrylate-grad-vinyl acetate)-block-P(vinyl acetate) (P(nBA-grad- VAc)-b-P(VAc) were also obtained using a one-step synthetic strategy involv- ing Cobalt-Mediated Radical Polymerization (CMRP) [167]. Here again, the reactivity ratios of the comonomers were dramatically different (rnBA
= 15 and rVAc = 0.02), and the copolymerization of nBA and VAc con- ducted at VAc 77% in the initial feed resulted in a block-gradient copolymer
k k composed of a poly(nBA-grad-VAc) segment and a pure poly(VAc) segment.
By using another comonomer with a higher reactivity ratio than VAc—that is, Poly(ethyleneglycol) acrylate (PEGA)—P(PEGA-grad-VAc) gradient copolymers and P(PEGA-grad-VAc)-b-PVAc block-gradient copolymers were also obtained by the same strategy [168]. The kinetic reveals that that both radicals add PEGA around 30 times faster than VAc. After PEGA was completely consumed, the CMR polymerization of the remaining VAc led to a diblock graft copolymer P(PEGA-grad-VAc)-b-PVAc in one step. The amphiphilic nature of these copoly- mers and their (quasi-)diblock structure led to the formation of well-defined micelles in water.
More recently, the synthesis of amphiphilic block-gradient copolymers con- taining 2-(2′, 3′, 4′, 6′-tetra-O-acetyl-𝛽-D-galactosyloxy)ethylacrylate (AcGalEA) glycomonomer and styrene (S) by RAFT was reported for the first time [160].
The copolymerizations were performed in dimethylacetamide (DMAc) using S-methoxycarbonylphenylmethyldodecyltrithiocarbonate (MCPDT) as the control agent. The reactivity ratios of both monomers were estimated by in situ NMR experiments using Skeist’s equation [70] along with the Fineman–Ross [169] and Kelen–Tudos [100] methods:r(AcGalEA)=0.07+/−0.01 andr(S)=0.7+/−0.1.
Three different kinds of amphiphilic copolymers—namely statistical P(S-co-GalEA), block PS-b-PGalEA, and block-gradient PS-b-P(S-grad-GalEA)—were prepared by deacetylation of the AcGalEA moiety in order to study their ability to form honeycomb porous films by the breath figure process. The block copolymer was synthesized by a two-stage experiment, whereas the statistical and gradient copoly- mers were obtained in one-pot synthesis using batch and semi-batch processes, respectively. For an initial monomer fraction of 80% styrene, required for the formation of honeycomb film, the batch process led to statistical copolymers based on the azeotropic curve. Gradient copolymers were obtained using the semi-batch process. By this method, S was polymerized first and then the AcGalEA monomer was slowly added. The molar fraction of AcGalEA in the copolymer for each sample versus the normalized chain length evolved in an “S-shaped” curve, and this can be associated with the slight increase in the instantaneous AcGalEA molar fraction in the copolymer up to 12%.
Block copolymers of styrene (S) and acrylic acid (AA) present the most typ- ical examples of amphiphilic ionic/hydrophobic block copolymers capable of self-assembly in aqueous solutions. The synthesis of di- and triblock gradient copolymers of PAA-b-P(AA-grad-S) and P(AA-grad-S)-b-PAA-b-P(AA-grad-S) was developed by Borisovaet al. using NMP in a one-step strategy [35, 36]. The reaction was performed in dioxane at 120∘C in the presence of alkoxyamine, 2-methylaminoxypropionic-SG1 (BlocBuilder® or MAMA) as the initiator and a slight excess of N-tert-butyl-(1-diethyl-phosphono-2,2-dimethylpropyl) nitroxide SG1 as the counter radical in order to avoid side reactions and to ensure a good control of the chains’ growth. The procedure consisted of two stages: in the first stage the AA was polymerized for 4 hours, to obtain a mono- or bifunctional macro initiator to use for the second reaction stage. At the second stage S was added stepwise to the reaction mixture in the amount equal to the residual amount of
k k AA, and the reaction was continued. The gradient sequence of the P(AA-grad-S)
block(s) with increasing instantaneous fraction of styrene monomer units toward the end(s) was confirmed by 1H NMR. This gradient type was not expected from the reactivity ratios determined earlier for this system (rS=0.93 andrAA=0.25;
BlocBuilder/SG1 at 120∘C in 1,4-dioxane) [170]. This gradient sequence arose because of the increase in the apparent reactivity ratio of AA in the polymerization process due to the PAA macro-initiator (rS=0.22,rAA=0.88). The increase in the apparent activity of AA monomers was explained by the “boot-strap” effect [37, 171–177], that is, by enrichment of the vicinity of the reaction center, the AA monomers were preferentially adsorbed by the PAA chain. This effect was reported to be of practical importance for the synthesis of block-gradient copolymers. For example, if the controlled radical copolymerization of styrene with AA is performed in 1,4-dioxane in the presence of a low molecular mass initiator, copolymerization will produce a copolymer in which the head is enriched with styrene units and the tail is enriched with AA units. However, if the functionalized PAA is taken as an initiator, the reaction gives rise to the block-gradient copolymer, in which the content of styrene gradually increases to the chain’s end. The self-assembly of the obtained amphiphilic ionic gradient copolymers in water was further characterized, as will be discussed later in this chapter.