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1 Gradient Copolymers Gradient Copolymers – Preparation, Properties and Practice Md. Mahbub Alam, a Kevin S. Jack, b David J.T. Hill,a,c Andrew K. Whittaker a,d and Hui Peng a,d * a Australian Institute for Bioengineering and Nanotechnology, The University of Queensland, Brisbane, Australia; b Centre for Microscopy and Microanalysis, The University of Queensland, Brisbane, Australia; c School of Chemistry and Molecular Biosciences, The University of Queensland, Brisbane, Australia; d ARC Centre of Excellence in Convergent Bio-Nano Science and Technology, Brisbane, Australia. Corresponding author: Hui Peng; Email: [email protected] Abstract: Gradient copolymers are members of a unique class of copolymer in which the monomer composition changes gradually from one end of the polymer chain to the other. In general, the properties of copolymers are strongly dependent on composition and sequence distribution, and hence gradient copolymers can exhibit unique properties compared to those of block and statistical copolymers. This review presents an overview of the methods of synthesis of gradient copolymers using the range of available polymerization techniques, and compares and contrasts the properties of gradient copolymers with properties of the analogous block and statistical copolymers. Keywords: Gradient copolymers, controlled radical polymerization, interfacial behaviour, critical micelle concentration, glass transition temperature. 1. Introduction to Gradient Copolymers Polymer chain microstructure plays an important role in determining the properties of polymeric materials [1]. Extensive theoretical and experimental investigations have suggested that beside the overall copolymer composition, the distribution of monomer units along the polymer chains can be an important microstructural parameter to control the physical and functional properties of polymeric materials by fine-tuning the nanomorphologies of the polymer chains [2-7]. 1 Figure 1. Illustration of the instantaneous copolymer composition and typical monomer distribution for (a) statistical copolymers, (b) diblock copolymers, and (c) gradient copolymers. The development of advanced radical polymerization techniques has provided the researcher significant freedom over the control of polymer properties, by controlling structural parameters such as molecular weight and its distribution, chain architecture and sequence distribution [8-11]. The class of copolymers named ‘gradient copolymers’, with unique chain microstructure, has attracted significant attention over the last couple of decades [6, 12-17]. As the name implies, gradient copolymers exhibit a gradual transition in composition from predominantly one monomer to the second monomer along the copolymer chains [18-21] as illustrated in Figure 1. Such a distribution of monomer units is markedly different from statistical and block copolymers. When formed by conventional polymerization methods, statistical copolymers will have a constant average composition along the polymer chain. Block copolymers on the other hand, exhibit an abrupt change in chemical composition at the point where the first-formed block was chain extended by reaction with a second monomer [22, 23]. Due to the continuously changing composition along the chains, gradient copolymers exhibit less intra- and inter-chain repulsion compared to statistical and block copolymers, and also show unique copolymer properties such as their behaviour at interfaces, thermal properties and properties in solution. [3, 24-31]. Gradient copolymers are predicted by theory and simulation to undergo microphase separation in a manner similar to block copolymers, however the morphologies formed may be different in reflection of the precise differences in chain structures [4, 24, 32, 33]. The continuous change in composition along the polymer chains in gradient copolymers leads to the formation of multiple separate microphase domains of different composition, and this is supported by theoretical simulations and experimental data [24, 34, 35]. The composition drift also results in the formation of unique broad glass transition temperatures (Tg) for gradient copolymers consisting of homopolymers with significantly different Tgs [4, 6, 29, 36, 37]. These distinct chain structures and properties, have applications in many different areas, such as compatibilizers of immiscible polymer blends [5, 14, 15, 38-40], stabilizers of emulsions or dispersions [41], damping materials [29, 36, 37], thermoplastic elastomers [42, 43] and multi-shape memory materials [44], etc. 2 Amphiphilic gradient copolymers are another unique class of responsive polymers, in which average properties of the monomers change from hydrophilic to hydrophobic gradually along the molecular chains, and consequently exhibit special properties [31, 45-56]. These polymers self- assemble in solutions and can be made to respond to environmental triggers such as changes in pH [30, 57-59], temperature [17, 53, 60], the nature of the solvent [61, 62]. Consequently such copolymers have a huge potential in the field of biomedical and pharmaceutical applications [5, 14, 15, 38, 39, 63-65]. 2. The Synthesis of Gradient Copolymers The synthesis of gradient copolymers requires concurrent initiation and uniform growth of all propagating chains involved in the polymerization process to create a continuous change in the copolymer composition from one end of the chain to the other [66]. Controlled radical polymerization (CRP) techniques are therefore widely used to prepare gradient copolymers [18, 31, 52, 62, 67, 68]. Well-defined gradient copolymers for a range of monomers have been reported to be successfully synthesised using CRP techniques such as nitroxide mediated polymerization (NMP) [22, 29, 69-73], atom transfer radical polymerization (ATRP) [28, 74-78] and reversible addition-fragmentation chain transfer (RAFT) polymerization [31, 52, 57, 79-81]. A list monomer pairs and their polymerization techniques is provided in Table 1. Gradient copolymers are prepared either by exploiting a natural or spontaneous gradient in composition formed during copolymerization, or by producing a forced gradient. Spontaneous gradient copolymers are prepared by the batch technique relying on the differences in the reactivity ratios of the two monomeric species [57, 82-84]. Examples of monomer pairs forming spontaneous gradients in controlled free radical polymerization include styrene (St)/acrylic acid (AA) [85], tert-butyl acrylate (tBA)/octadecyl methacrylate (ODMA) [86], St/methyl methacrylate (MMA) [87], n-butyl acrylate (nBA)/n-butyl methacrylate (nBMA) [84], St/tBA [88], etc. In such polymerizations, one monomer is consumed more rapidly than the other, which results segments of the copolymer rich in that monomer and preferential depletion of the monomer in the reaction mixture. As the polymerization proceeds, the second, more slowly reacting monomer is incorporated to a greater extent as a consequence of depletion of the first monomer at early stage of polymerization. The resulting polymer chains will therefore have varying chemical composition distributions (CCDs) depending on the monomer reactivity ratios and initial feed composition [1]. Figure 2 shows the calculated instantaneous and cumulative compositions of St in a St/nBA gradient copolymer (rSt = 0.8, rnBA = 0.2) at different initial monomer feed ratios. As appears in the Figure 2, the compositional gradient is more apparent in the plots of instantaneous composition, and the strength of the gradient is significantly dependent on the initial monomer feed ratio [28]. 3 Figure 2. Finst (left) and Fcum (right) of M1 for a simulated living batch copolymerization of St and nBA with reactivity ratios rSt = 0.8 and rnBA = 0.2 with different monomer feed ratios assuming that the monomer rate constants for initiation are equal. [M]0 = 10 M; [I]0 = 0.1 M [28]. Reprinted with permission from reference 28. Copyright (2000) John Wiley and Sons. Further examples of batch copolymerization, by various methods of initiation, where the compositional gradient in the copolymer structures were achieved by exploiting the difference in the monomer reactivity ratios include: Oleszko-Torbus et al. who synthesised a series of thermoresponsive gradient copolymers of 2-n-propyl-2-oxazoline (nPrOx) with 2-methyl-2-oxazoline (MOx) or 2- isopropyl-2-oxazoline (iPrOx) by living cationic ring opening polymerization (CROP) [89]; and Kim and Choi [90] who used ring-opening metathesis polymerization (ROMP) to synthesise dendronized gradient copolymers of endo-tricycle[4.2.2.0]deca-3,9-diene (TD) monomers via a macromolecular approach. In this later work the authors demonstrated gradient profiles within single chains using atomic force microscopy (AFM). Whilst the simplicity of the batch approach to prepare gradient copolymers makes this method very attractive, there are a very limited number of monomer combinations with sufficiently different reactivity ratios to produce copolymer chains with the desired gradient architectures [90]. This limitation narrows down the scope for preparing bespoke gradient copolymers and, therefore, requires alternative approaches. The second approach used to create a gradient in the copolymer structures is known as the ‘forced gradient’ method, and is a semi-batch technique. In forced-gradient copolymerization, a second monomer is continuously added to the polymerization mixture during the reaction using an external device such as a syringe pump, so as to change the instantaneous monomer composition in
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