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].

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.

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].

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 the polymerization, as illustrated in Figure 3. The addition of the second monomer leads to an increase in its content in the monomer feed, and thus results in a gradual increase in the composition of this monomer in the copolymer. The majority of gradient copolymers are made by this technique due to the flexibility and freedom it allows to design the copolymer composition and gradient. Theoretical and experimental studies suggested that through optimized feeding of the monomers, it is possible to design

4 and prepare copolymers with desired copolymer composition and sequence distribution in a semi-batch reactor [18, 81, 91-95].

All of the commonly-known living radical polymerization techniques have been extensively used to prepare gradient copolymers by the semi-batch techniques. For example, NMP has been used to synthesise gradient copolymers of St/AA [41, 85, 96], N,N-dimethylacrylamide (DMAA) /nBA [97, 98], St/nBA [99], octadecyl acrylate (ODA)/methyl acrylate (MA) [100], St/4-acytoxystyrene (AS) [15, 38, 101], St/MMA [102], St/4-methyl styrene (MSt) [72], St/tBA [6, 35, 103], St/nBA [35, 104], St/4- vinylpyridine (VP) [35, 105] and St/nBMA [35]. In all cases it was confirmed that a compositional gradient with good control over the copolymer molar mass and molar mass distribution could be achieved.

ATRP and RAFT are widely-used methods for gradient copolymer synthesis. ATRP has been reported to successfully produce a compositional gradient in the structure of St/nBA [28, 106], St/acrylonitrile (AN) [28], nBA/isobornyl acrylate (iBRA) [25], MMA/nBA [107, 108], MMA/tBA [75], 2-(dimethylamino)ethyl methacrylate (DMAEMA)/nBMA [109], MMA/2-(trimethylsiloxy)ethyl methacrylate (TMSEMA) [76, 110], tBA/2,2,3,3,4,4-heptafluorobutyl methacrylate (HFBMA) [77], and oligo(ethylene glycol) methyl ether methacrylate (OEGMA)/St [111] copolymers. Similarly St/MMA [112], St/nBA [67, 113], St/AN [82], fluorinated methyl acrylate (FMA)/BMA [66], AA/2,2,2-trifluoroethyl methacrylate (TFEMA) [62], AA/DMAA [114], OEGMA/ 3,3,4,4,5,5,6,6,7,7,8,8,8-tridecafluorooctyl acrylate (TFOA) [115] gradient copolymers have been reported to have been successfully synthesised by the RAFT method. Good control over the molar mass, molar mass dispersity and sequence distribution in gradient copolymers was confirmed for each technique.

Figure 3. A schematic diagram of the synthesis of amphiphilic gradient copolymers of OEGMA and TFOA using the RAFT semi-batch approach [115]. Reprinted with permission from reference 115. Copyright (2016) American Chemical Society.

ROMP is another well-established polymerization technique which has been reported to be used to synthesise gradient copolymers in several studies [3, 90, 116-121]. For example, Dettmer et al. [116] synthesised a gradient copolymer of exo-5-(benzyloxy)norbornene and exo-5-[(4-tert- butyl)benzyloxy]-norbornene, and Lefebvre et al. [3] synthesised a gradient copolymer of exo-5-

(benzyloxy)norbornene-d7 and exo-5-[(4-tert-butyl)benzyloxy]norbornene by ROMP semi-batch method. Gradient copolymers of endo,exo-norbornenyl dialkylesters and norbornene-functionalized PSt, polylactide, or polydimethylsiloxane have been reported to be synthesised by a ROMP macromonomer approach [120, 121].

In addition to feeding the second monomer at a single constant rate of addition, a related semi- batch strategy used to further control the microstructure of gradient copolymers involves feeding monomers with step-increasing additions. For example, Torkelson and co-workers have used this approach to synthesise St/MMA gradient copolymers [39, 122] where the St monomer was fed into the batch reactor initially and MMA was added to the reactor at three different constant rates in three different time segments. The addition of MMA started with 10 mL/h which ran for 3 hours (first segment) and then the addition rate was increased to 15 mL/h and 20 mL/h for second and final segments respectively. The same group have also used the same addition technique to prepare a series of St/nBA gradient copolymers [7, 35, 104, 105, 123, 124]. Guo et al. reported a slightly different approach which they referred to as a many-shot method to tailor the compositional gradient in St/nBA copolymer structure by RAFT emulsion polymerization [67]. They added the mixture of the monomers in many shots to the polymerization system, each shot having a different composition according to the desired copolymer composition to obtain St/nBA many-block copolymers at the end of polymerization, which was considered to be a gradient copolymer. Fu et al. [26] reported an interesting novel one-pot method which relies on enzymatic transformation of trifluoroethyl methacrylate (TFEMA) to produce gradient a copolymer of TFEMA and R-methacrylate by the RAFT method.

Although CRP in solution offers excellent control for many reactions, the dilution of reactants slows down the rate of reaction and this can be undesirable. Enhanced rates of polymerization can be achieved using emulsion or miniemulsion CRP. In these methods, the propagating radicals experience a segregation effect within the particles, and these methods can also be used to synthesise gradient copolymers [1]. Matyjaszewski and co-workers [125] successfully prepared a series of gradient copolymers of nBA/tBA, nBMA/MMA, and nBA/St monomer pairs by ATRP in a miniemulsion with activators generated by electron transfer (AGET). RAFT miniemulsion copolymerizations have also been reported to be used to prepare copolymers of BMA/dodecafluoroheptyl (DFMA) [126], MMA/St [87], St/butadiene (Bu) [127], and AA/TFEMA [31], with a constant rate, continuous monomer addition strategy. Charleux and co-workers [128] studied NMP miniemulsion polymerization of nBA/St in a batch process to create a gradient in the copolymer composition. 2-methyl-N-tosylaziridine and 2- decyl-N-tosyl-aziridine produces a statistical copolymer in homogeneous solution, but these same two monomers produce gradient copolymers in miniemulsion as a result of spatial separation due to their selective solubilities [129].

(a) (b)

Figure 4. (a) and (b) show the cumulative copolymer compositions and the change of molecular weight respectively for nBA/tBA copolymer in a batch ATRP process. The cumulative and instantaneous compositions of nBA and tBA in the forced gradient copolymer with a tBA feeding rate of 0.01 mL/min for 200 minutes is shown in (c) and (d) [125]. Reprinted with permission from reference 125. Copyright (2007) John Wiley and Sons.

An example is shown in Figure 4, where the composition profile for the copolymerization of nBA with tBA was studied in both batch and fed-batch modes [125]. The monomers have comparable reactivity ratios and therefore produce statistical polymers. However, in the forced copolymerization method using a continuous feeding of tBA, both the cumulative and instantaneous composition of tBA increased, resulting in nBA/tBA gradient copolymers. Moreover, the rate of addition of tBA could be optimized to achieve a smooth gradient in the copolymer composition.

Despite the success of these studies, the constant monomer feeding approach is not sufficiently versatile for designing and preparing polymers with specific comonomer composition profiles along the chain backbone [130]. Therefore, a kinetic model was developed based on an understanding of polymerization mechanism [76, 131-136] to allow preparation of copolymers with predesigned chemical composition distributions (CCDs); a so-called model-based monomer feeding strategy (MMFS). MMFS was first exploited by Zhu and co-workers [18, 58, 81, 93-95, 137], and has been described as an effective technology for precision production of polymer chains. Broadbelt [91, 92, 138, 139] predicted the copolymer sequence distribution (CSD) using Monte Carlo (MC) simulations in the NMP copolymerization of St/MMA, and the predicted CSD was controlled by employing MMFS in a semi-batch copolymerization. In addition, Schork and co-workers [140-142] used the method of moment analysis to develop a kinetic model and studied the CSD in a RAFT copolymerization under semi-batch operation.

1.0 (a) 1.0 (b) Predicted AAA Expt. AAA Predicted AAS/SAA Expt. AAS/SAA 0.8 0.8 Predicted SAS Expt. SAS

0.6 Predicted SSS Expt. SSS 0.6 Predictred ASS Expt. ASS/SSA Predicted ASA Expt. ASA 0.4 0.4 Triad fraction Triad fraction 0.2 0.2

0.0 0.0

0 20 40 60 80 0 20 40 60 80 100 Conversion (%) Conversion (%)

1.0 (c) 1.0 (d) Predicted SSS Expt. SSS 0.8 Predicted ASS/SSA Expt. SSA/ASS 0.8 Predicted ASA Expt. ASA

0.6 0.6 Predicted AAA Expt. AAA Predicted AAS/SAA Expt. AAS/SAA Predicted SAS Expt. SAS 0.4 0.4 Triad fraction Triad fraction 0.2 0.2

0.0 0.0

0 20 40 60 80 100 0 20 40 60 80 100 Conversion (%) Conversion (%)

1.0 (e) 1.0 (f)

Predicted SSS Expt. SSS 0.8 Predicted SSA/ASS Expt. SSA/ASS 0.8 Predicted ASA Expt. ASA Predicted AAA Expt. AAA 0.6 0.6 Predicted AAS/SAA Expt. AAS/SAA Predicted SAS Expt. SAS 0.4 0.4 Triad fraction Triad fraction 0.2 0.2

0.0 0.0

0 20 40 60 80 100 0 20 40 60 80 100 Conversion (%) Conversion (%) Figure 5. The experimental St centred and AN centred triad distributions for 20% [(a), (b)]; 60% [(c), (d)], and 80% [(e), (f)] St feed composition characterised by 13C NMR compared with predicted data [82]. Reprinted with permission from reference 82. Copyright (2017) John Wiley and Sons.

In most reported studies, the gradient in the copolymer structure has been characterised by the change in the average copolymer composition determined by 1H NMR spectroscopy [29, 66, 85]. In addition to the change in the average composition, the gradient structure can also be characterised by quantifying the triad distributions, as demonstrated for the synthesis of St/AN gradient copolymers in a RAFT batch process [82]. As shown in Figure 5, for an initial St feed composition of 60%, the triad fractions are almost constant throughout the whole chain, while for St feed compositions of 20% and 80%, the proportions of St-centred and AN-centred triads change with conversion, though the former shows a comparatively sharper change, indicating a stronger gradient in composition. Such a study provides a more detailed and direct understanding of the sequence distribution of this gradient copolymer system.

Table 1: A summary of polymerization techniques used to synthesise gradient copolymers of various monomer pairs.

Monomer pair Copolymer structure Polymerization Reaction Reaction Conv. Mol. FA:FB Ref. A/B technique media Temp. (%) weight (ºC) (g/mol) 2-Hydroxyethyl methacrylate/tert- ATRP Cyclohexa 50 36.2 92:08 [78] butyl acrylate Batch none 42.2 84:16 [78] O O 31.9 73:27 [78] O O 25.5 61:39 [78] 11.2 34:66 [78]

Styrene/poly(ethylene glycol) ATRP Anisole 130 68.5 5100 [111] methyl ether methacrylate Batch 81.5 19900 [111] O 36.9 7900 [111] O 43.2 12300 [111]

tert-Butyl acrylate/2,2,3,3,4,4,4- ATRP Toluene 80 73.0 10700 64:36 [77] heptafluorobutyl methacrylate Semi-batch O O O O F F

F CF3 F

Styrene/n-butyl acrylate ATRP Toluene 110 100 6000 55:45 [106] O Semi-batch O

n-Butyl acrylate/isobornyl acrylate ATRP Acetone/ 70 81.1 74800 69:31 [25] O O Semi-batch anisole O O

Methyl methacrylate/ tert-butyl ATRP DMF 25 53.3 13000 35:65 [75] acrylate Semi-batch DMF 65.5 15300 45:55 [75] O DMF 68.9 16000 47:53 [75] O O O DMF 56.8 13800 38:62 [75] DMF 69.5 16400 41:59 [75] DMF 53.2 13700 36:64 [75] DMSO 64.3 16600 34:66 [75] Toluene 87.4 20300 52:48 [75] Toluene 89.1 21100 48:52 [75] Methyl methacrylate/2- ATRP Toluene 90 56.0 38000 78:22 [76] (trimethylsilyl)-ethyl methacrylate Semi-batch 53.5 46000 48:52 [76]

O O O O

N 2-Methyl-2-oxazoline/2-n-propyl- N CROP CH3CN 75 100 11000 88:12 [89] 2-oxazoline Batch 9400 46:54 [89] O O 9700 28:72 [89] 9600 10:90 [89]

N 2-iso-Propyl-2-oxazoline /2-n- N CROP CH3CN 75 100 13100 85:15 [89] propyl-2-oxazoline Batch 12600 48:52 [89] O O 12800 39:70 [89] 11600 10:90 [89] Styrene/acrylic acid NMP 1,4- 120 15300 34:66 [30] Batch Dioxane 17000 36:64 [30]

HO O

n-Butyl acrylate/methyl NMP Bulk 115 70 48600 37:63 [143] methacrylate Batch 90 55800 73:27 [143]

O O O O

N,N-dimethylacrylamide/n-butyl NMP Toluene 112 82.0 33000 46:54 [97, 98] acrylate Semi-batch 55.0 24000 43:57 [97, 98] O 83.5 35000 40:60 [97, 98] O N O

Styrene/n-butyl acrylate NMP Bulk 120 77.9 51899 55:45 [99] O Semi-batch 73.7 40700 55:45 [99] O Bulk 120 96900 59:41 [35, 37] 95100 62:38 [35, 37] 100 40700 73:27 [35] Bulk 100 72000 60:40 [104] 95000 60:40 [104] 152000 61:40 [104]

Styrene/ tert-butyl acrylate NMP Bulk 115 135100 53:47 [35] Semi-batch 85200 67:33 [35] O O

Styrene/methyl methacrylate NMP Bulk 93 55200 51:49 [102] Semi-batch Bulk 90/110 94000 76:24 [144] 90/110 17 64000 71:29 [144] O O

Styrene/4-methyl styrene NMP Cyclohexa 90 84600 42:58 [72] Semi-batch ne

Octadecyl acrylate/methyl acrylate NMP Toluene 112 44.5 21300 60:40 [100] Semi-batch 42.0 28500 60:40 [100]

O H3C(H2C)17O O O

12 n-Butyl acrylate/methyl NMP Bulk 115 91 61900 56:44 [143] methacrylate Semi-batch 88 55100 46:54 [143]

O O O O

Styrene/4-acetoxystyrene NMP Bulk 115 93800 56:44 [29] Semi-batch 75600 58:42 [29] 115 51000 64:36 [123] 94000 56:44 [123] Bulk 100 51300 64:36 [35] 58800 84:16 [35] O O Cyclohexa 90 84100 25:75 [101] ne 57700 60:40 [101] 81000 37:63 [101] 144700 23:77 [101] 48000 76:24 [101] 59200 55:45 [101] 63900 35:65 [101] 50200 49:51 [101] 67000 25:75 [101] Bulk 90 60100 59:41 [38] 65600 25:75 [38] Styrene/4-hydroxystyrene NMP Bulk 115 127100 39:61 [29] Semi-batch 80100 71:29 [29]

Styrene/4-vinyl pyridine NMP DMF 90 23:73 [35] Semi-batch 33:63 [35]

Styrene/n-butyl methacrylate NMP Bulk 115 83000 71:29 [35] Semi-batch 57000 49:51 [35]

O O

Styrene/acrylic acid NMP Bulk 115 135100 53:47 [6] Semi-batch 85200 67:33 [6] 1,4- 120 14000 27:73 [30] HO O Dioxane 21000 32:68 [30]

Styrene/acrylic acid RAFT Bulk 60 76.3 2800 13:87 [57] Batch 86.1 9400 12:88 [57] 85.6 5300 22:78 [57] HO O 88.0 4200 32:68 [57] 87.5 12100 35:65 [57]

Styrene/acrylonitrile RAFT Bulk 80 74.2 16700 35:65 [82] Batch 64.5 14200 77:23 [82]

Acrylic acid/N,N- RAFT Water 85 78.0 12900 [145] dimethylacrylamide Semi-batch O HO O N

Methacrylic acid/N,N- RAFT Water 85 83.0 18300 [145] dimethylacrylamide Semi-batch

O HO O N

N,N-dimethylacrylamide/vinyl RAFT EtOAc 100 68.5 18600 [145] acetate Semi-batch O O

N O

Butyl acrylate/ 3-chloro-2- RAFT CH3CN 85 36.0 4600 [145] hydroxypropyl methacrylate Semi-batch O O

O O

Styrene/n-butyl acrylate RAFT Emulsion 70 95-97 89000 62:38 [67] O Semi-batch 95-97 90000 60:40 [67] O

Styrene/methyl methacrylate RAFT Emulsion 70 100 80800 55:45 [112] Semi-batch

O O

Poly(ethylene glycol) methyl ether RAFT Butyl 80 11500 85:15 [115] methacrylate/3,3,4,4,5,5,6,6,7,7,8,8 Semi-batch acetate 11100 [115] ,8-tridecafluorooctyl acrylate

O O O

CH2CH2(CF2)5CF3 O

Hexafluorobutyl methacrylate/ RAFT Miniemul 70 87±2 25800 [66] n-butyl methacrylate Semi-batch sion 91±3 29100 29:71 [66] 93±2 31500 64:36 [66] O O O O

F F3C

Acrylic acid/2,2,2-trifluoroethyl RAFT Emulsion 70 92.5 8000 57:43 [31] methacrylate Semi-batch 98.5 9800 53:47 [31]

O 93.5 8700 53:47 [31] HO O 96.7 8100 53:47 [31] O 97.5 7100 51:49 [31] 1,4- 70 81.2 7300 74:26 [62] F3C Dioxane

2-Methacryloyloxyethyl RAFT Ethanol/ 70 53000 39:61 [146] phosphorylcholine/2,2,2- Semi-batch 1,4- 40000 52:48 [146] trifluoroethyl methacrylate O O Dioxane

O O

CF3 O - O O P N+ O

Styrene/2,2,3,4,4,4-hexafluorobutyl RAFT Emulsion 70 89.0 119100 59:41 [80] acrylate O Semi-batch 88.5 32900 67:33 [80]

O 80.2 15900 62:38 [80] F

F F F F F endo-Tricycle[4.2.2.0]deca-3,9- ROMP THF 50 100 613000 [90] diene/norbornene O Batch N N O

O O

O G4 exo-1,4,4a,9,9a,10-Hexahydro- ROMP CH2Cl2 Room 100 109000 88:12 [119] 9,10(10,20)-benzeno-l,4- Batch temp. 105000 78:22 [119] methanoanthracene/tricyclo[4.3.0.1 114000 71:29 [119] 2,5]deca-3-ene 111000 60:40 [119] 106000 45:55 [119]

17 exo-1,4,4a,9,9a,10-Hexahydro- ROMP CH2Cl2 Room 100 98000 95:5 [119] 9,10(10,20)-benzeno-l,4- Batch temp. 89000 88:12 [119] methanoanthracene/ 5-n- 100000 81:19 [119] hexylnorbornene 95000 64:36 [119]

exo-1,4,4a,9,9a,10-Hexahydro- ROMP CH2Cl2 Room 55.6 11200 87:13 [118] 9,10(1’,2’)-benzeno-l,4- Batch temp. 71.9 12100 69:31 [118] methanoanthracene/cis-cyclooctene 33.3 8000 75:25 [118] 63.5 6800 43:57 [118] 75.4 9000 40:60 [118] 72.0 8500 90:10 [118] 73.9 15300 91:09 [118] 77.4 12300 85:15 [118] exo-5-(Benzyloxy)norbornene/exo- ROMP THF Room 171000 54:46 [116] 5-[(4-tert-butyl)benzyloxy]- Semi-batch temp. 164000 68:32 [116] norbornene O O

C(CH3)3 exo-5-[(4-tert- ROMP THF Room 141000 48:52 [3] Butyl)benzyloxy]norbornene/exo- Semi-batch temp. O O 5-(benzyloxy)norbornene-d7 D D D D

D D

D C(CH3)3

3. The Properties of Gradient Copolymers Gradient copolymers are of particular interest because of the unique properties they exhibit compared to their statistical, alternating and block copolymer counterparts, which originates from the difference in their chain microstructures [147]. In addition to the overall composition, the properties of a copolymer are also strongly dependent on the distribution of monomer units along the polymer chains [18, 67]. The properties of polymers can be controlled by fine-tuning the polymer chain structure to achieve a specific microstructural morphology, as suggested by theoretical and experimental studies [3, 7, 18, 29, 37]. The properties of the gradient copolymers are discussed in detail and compared with the properties of conventional block and gradient copolymers in the following sections.

3.1. Interfacial Behaviour and Blending Properties

The phase behaviour of the gradient copolymers and their interfacial activity have been intensively studied by a number of different research groups [3-5, 7, 14, 15, 32, 33, 103, 105, 148-153]. Shull and co-workers used self-consistent field (SCF) theory to study the interfacial behaviour of gradient copolymers, and introduced a gradient parameter, λ, which describes the length of the composition gradient relative to the entire length of the copolymer [4, 32]. For AB copolymers, when λ =0, the copolymer is a conventional block copolymer consisting of separate blocks of A and B units. When λ = 1, the composition varies smoothly along the chain from pure A to pure B. The scattering function was analytically calculated using a random phase approximation (RPA) to locate the critical order- disorder transition for the gradient copolymer as a function of the gradient parameter.

Figure 6. Demonstration of the gradient parameter λ [32]. Reprinted with permission from reference 32. Copyright (2002) American Chemical Society.

By combining the SCF and RPA methods, the phase segregation behaviour of four symmetric A-B copolymers, namely a block copolymer, two linear gradient copolymers, and a tanh gradient copolymer was examined [4]. It was found that, for a fixed value of χN (χ = Flory Huggins interaction parameter, N = degree of polymerization), increasing the width of the composition gradient along the chain decreases the lamellar repeat length, which makes phase separation more difficult to achieve for

19 gradient copolymers than for block copolymers. The order-disorder transition for block copolymers, known to occur at (χN) = 10.5, was reported to be raised to 29.3 for a melt of a fully tapered gradient copolymer (λ = 1) [4]. This team also experimentally examined the interfacial segregation of a diblock, gradient, and statistical copolymers at the interface of immiscible polymer blends using forward recoil spectroscopy (FRES) and SCF theory [3]. The norbornene-based monomers with reactivity ratios close to 1 gave the freedom to tailor the degree of the gradient in the structure and therefore was an ideal selection for studying the interfacial properties of the copolymers. In contrast to the formation of a monolayer and a wetting layer observed for block copolymers and statistical copolymers respectively, the study found a monolayer with a larger interfacial excess and width was observed for gradient copolymers.

Marić and co-workers studied the self-assembly of block and gradient copolymers of St/MMA with identical properties and compared the morphologies of the copolymers [144]. They reported that the gradient copolymers exhibited poor self-assembly compared with the block copolymers due to the long transition length and statistical copolymer-like structure in the transition from the MMA-rich to St-rich domains. In another study, a drop-shape analysis was used to determine the interfacial tension of St/AA gradient and block copolymers at the liquid/liquid interface [103]. Figure 7 illustrates the nature of interfaces formed by diblock and gradient copolymers inferred from this study. The block copolymer exhibits a single junction at the interface of the oil and water, while gradient copolymers adopt a parallel structure as a result of multiple junction points along the chains. However, the overall interfacial properties of the gradient copolymers were reported to be mainly dependent on the geometrical distribution of the copolymer chains, while the geometrical distribution is in turn dependent on the sequence distribution along the copolymer backbone [103].

Figure 7. Schematic diagram of the chloroform/water interface due to the presence of a diblock copolymer (left) and a gradient copolymer (right) [103]. Reprinted with permission from reference 103. Copyright (2010) American Chemical Society.

Wang and co-workers [5] also studied the effect of monomer unit distribution on the phase behaviour of ternary homopolymer/gradient copolymer blends using RPA and SCF theory and found that the composition profile of the gradient copolymers plays a strong role in the phase behaviour of the

20 polymer blends. The sequence distribution of the gradient copolymers also has a significant effect on the microphase structure of the blends which includes the width and breadth of the interfacial region as well as the distribution of the homopolymer and copolymer layers. Therefore, by designing the copolymer composition profile, the interfacial properties of immiscible homopolymer blends could be fine-tuned depending on specific applications.

Figure 8. Schematic presentation of microstructures formed by copolymer blends of (a) steep (b) smooth and (c) intermediate gradient distributions. The chain section composed of A and B are denoted by the solid and dashed lines respectively, and the middle section of the gradient copolymer chains are denoted by circles. The darkness of the background represents the presence of A monomer fraction in the copolymer layer [5]. Reprinted with permission from reference 5. Copyright (2009) American Chemical Society.

Torkelson and co-workers [14, 15, 38, 39] produced a series of gradient copolymers and showed that macrophase separated polymer/polymer blends can be rendered thermally stable by including a gradient copolymer as an additive. The gradient copolymer localizing at the interface lowers the interfacial tension, and as a result, suppresses the phase separation process. Wang et al. [80] also found similar improvements in the compatibility of St/fluorobutyl acrylate (FBA) blends after adding a gradient St/F6BA copolymer. The copolymer molecular weight, strength of the gradient and the proportion of added gradient copolymer are a few of the factors on which the extent of compatibilization is mostly dependent.

Gradient copolymers were also found to modulate and suppress the phase separation in polymer/polymer and polymer/fullerene blends, and have been shown to improve the long-term thermal stability of photovoltaic devices when used as additives, though at the expense of reducing the filling factor [154]. Physical blends of polymers of 3-hexylthiophene (3HT) and 3-(6-bromohexyl) thiophene (3BrHT) showed extensive micron-scale phase separation as found by AFM and transmission electron microscopy (TEM) studies [155]. Addition of a gradient copolymer to the blend resulted in a substantial reduction in the domain size, and with increasing amount of gradient copolymer, the domain size decreases. In comparison, statistical and block copolymers were less effective in promoting mixing in these systems, which suggests that gradient copolymers are effective compatibilizers for incompatible homopolymer blends (Figure 9) [155]. This result was further supported by photoluminescence (PL) data. A blend containing P3HT/phenyl-C61-butyric acid methyl ester (PCBM) and 10 wt% gradient

21 copolymer additive exhibited marked PL quenching even after prolonged thermal annealing, which suggests that gradient copolymers are effective compatibilizers and are likely to improve the thermal stability of the corresponding bulk heterojunction-based solar cells [154].

Figure 9. The microphase distribution for 1 : 1 (v/v) P3HT-P3BrHT blend (A) without copolymer additive, (B) with 20 wt% gradient copolymer, (C) with 20 wt% statistical copolymer, (D) with 20 wt% block copolymer are shown by STEM images. The domain size distribution histogram is shown in (E) [155]. Reprinted with permission from reference 155. Copyright (2013) Royal Society of Chemistry.

Scanning electron microscopy (SEM) [156, 157], AFM [158, 159] and TEM [160-162] have been extensively used in determining the boundaries between different morphologies in block copolymers [156-162]. However, there are only few reports on imaging of gradient copolymers, due to the low contrast offered by the continuous variation in the composition. Mok et al. [105] studied the surface patterns of St/nBA gradient and block copolymers formed in thin films by using three different types of microscopic analysis (Figure 10). Each of the three microscopic techniques clearly showed the well-defined distinct phase-segregated domain patterns for block copolymers. However, such distinct domains do not appear for gradient copolymers due to in-effective phase segregation which results from the gradual transition of monomer units in the copolymer chains. This example also highlights that the weak phase segregation within gradient copolymers is extremely difficult to observe by direct microscopic imaging techniques due to low mean compositional contrast [105].

St-block-nBA St-grad-nBA

Figure 10. AFM, SEM and TEM images of St-block-nBA and St-grad-nBA phase segregation [105]. Reprinted with permission from reference 105. Copyright (2012) John Wiley and Sons.

Guo and co-workers used AFM to study the morphology of St/nBA copolymers, and reported that the block copolymers showed clear boundaries between the blocks [67]. However, no evident boundaries could be seen for the gradient copolymers, due to much weaker phase separation, and instead indistinct boundaries were observed. Figure 11 shows the high resolution AFM images of films of gradient and block dendronized copolymers prepared from these same two monomers [90]. Due to the considerably large size of the dendronized polymers, the difference in the microstructures between the dendrons are clearly differentiable by AFM imaging. The block copolymer showed an abrupt change in height and thickness with a sharp domain boundary. On the other hand, a smooth and gradual change in both height and thickness was found from the images of the gradient copolymer films, and no appearance of any interfacial boundary was observed.

The findings of the above studies are quite similar to and agree with the theoretical predictions that the interfacial behaviour of gradient copolymers are significantly different to those of analogous statistical and block copolymers. The abrupt switch in the composition between two blocks, results in clear phase boundaries. In contrast, gradient copolymers produce broad interfaces with no distinct boundaries, consistent with the compositional gradient in their chain structures.

Figure 11. AFM images of a single chain of dendronized block (left) and gradient (right) copolymers showing the structural differences [90]. Reprinted with permission from reference 90. Copyright (2013) American Chemical Society.

3.2. Thermal Properties

Both differential scanning calorimetry (DSC) [6, 18, 19, 25, 28, 29, 35, 36, 97, 163-165] and dynamic mechanical analysis (DMA) [25, 28, 37, 97, 99] have been extensively used to study the thermal properties of gradient copolymers. Multiple studies have suggested that, the Tgs of gradient copolymers are markedly different from those of the analogous statistical and block copolymers, despite having similar molecular weights and copolymer compositions [6, 29, 35, 37].

Torkelson and co-workers studied the thermal behaviours of a range of gradient copolymers, including St/MSt, St/AS, St/HST, St/AA, St/nBA, St/tBA, St/BMA, and compared their properties with those of the corresponding block and statistical copolymers [6, 29, 35, 72, 101, 123, 166]. Conventional thermal analysis did not reveal significant differences in the Tg of statistical and gradient copolymers when the glass temperatures of the homopolymers of the constituent monomers are close to each other, for example the St/MSt system where the values of Tg are between 100 °C to 110 °C [72]. However, for copolymers comprised of monomers whose homopolymers have significantly different values of Tg (e.g. St/AS or St/HST), the DSC heating curves were found to be very different for statistical, block and gradient copolymers (Figure 12) [29]. DSC produced a single, narrow Tg for the statistical copolymers indicative of a single phase, while two narrow transitions were found for the block copolymers, one transition corresponding to each of the homopolymer blocks. In contrast, the gradient copolymers showed unusually broad glass transitions. Recent work by Mok et al. [35] suggested that the Tg and breadth of the Tg transition can be tuned through a choice of the monomer pairs and copolymer compositions. Chain and sequence lengths are other factors which can influence the glass transition [5, 25, 29].

(a) (b) i i

ii ii

iii iii

iv iv

Figure 12. The DSC heating curves (a) and first derivatives of the heating curves (b) for [i] a block, [ii] a statistical copolymer and two different gradient copolymers ([iii], [iv]) of St and AS [29]. Reprinted with permission from reference 29. Copyright (2006) American Chemical Society.

Matyjaszewski and co-workers [25] also synthesised well-defined statistical, block and gradient copolymers of isobornyl acrylate (iBRA) and nBA, and investigated their thermomechanical behaviours using DSC, DMA and small-angle X-ray scattering (SAXS). The study showed that, while statistical

copolymers showed a single Tg, block copolymers showed two distinct transitions and the DSC thermograms for the gradient copolymer indicated a single, but distinctly broad, glass transition. Zhang et al. [66] and Guo et al. [67, 113] also reported similar thermal behaviour for FMA/BMA and St/nBA gradient copolymers respectively. The transition for the gradient copolymers were distinctly different to those of the block and statistical copolymers. Figure 13 shows how closely the breadth of the Tg transitions are dependent on the strength of the gradient in the copolymer chains. By characterising the sample aliquots collected from the reaction system, it has been found that, the breadth of the transition increased as a result of the gradual incorporation of HST units into the copolymer chains. (a) (b)

i ii ii

iii iii

iv iv

Figure 13. (a) DSC heating curves and (b) the first derivatives of DSC heating curves for St-HST

gradient copolymers. The arrows in (b) indicate the breadth of Tg and the broken arrow indicates the

increase in breadth Tg as polymerization proceeds [29]. Reprinted with permission from reference 29. Copyright (2006) American Chemical Society.

3.3. Solution Properties and Micellization

Extensive studies of the association behaviour of gradient copolymers in selective solvents have been reported over the past few years [49, 50, 55, 109, 150, 167-174]. A significant finding was that gradient copolymers have a higher solubility than the corresponding diblock copolymers, leading to significantly higher values of the CMC [122, 170, 175]. Monte Carlo (MC) simulations have been employed to study the self-assembly of gradient copolymers with varying degrees of gradient in the chain structures [30, 173, 176]. The ordering of the monomer units along the copolymer chains plays a significant role in the association behaviour in addition to the nett copolymer composition. In selective solvents, block copolymers form micelles with well-defined spherical shapes. In contrast, gradient copolymers have a tendency to associate in such a way as to form aggregates of less-well-defined shapes and wide size distributions [30, 173].

Figure 14. Fluorescence intensity data for St-b-MMA (represented by squares, dashed lines) and St-g- MMA (represented by circles, solid lines) copolymers. CMC values are estimated from the intersections of the fits to low and high concentration data [177]. Reprinted with permission from reference 177. Copyright (2007) American Chemical Society.

Using SCF theory, Shull [32] predicted significantly higher CMC values for gradient copolymers than those of the block copolymers, having similar compositions and molecular weights. This prediction was later experimentally confirmed by Torkelson and co-workers [122] for St/MMA copolymers. Figure 14 shows the plots of the ratio of fluorescence intensity at 330nm to that at 280nm

(I330/I280) as a function of logarithmic copolymer concentration in PMMA and the estimation of the

CMCs of copolymers from the plots. The ratio I330/I280 increases sharply as the concentration of block copolymer exceeds 0.2 wt%, whereas the gradient copolymer shows a slight concentration dependence below 2 wt%. The CMC values were estimated from the intersection of the linear lines describing the

low and high concentration data. The I330/I280 values increase sharply above 2 wt% for the gradient copolymers, leading to an estimated CMC of 2 wt%, which is an order of magnitude higher than that of the CMC for the block copolymer. Due to the higher CMC values, the gradient copolymers behave differently when used as interfacial agents in immiscible blends. The probability to be entrapped in

26 micelles is less for the gradient copolymers, and therefore they can have a greater effect on the interfacial energy of the blends compared with the analogous block copolymers [102].

Pandav and co-workers [150] studied the phase separation behaviour of amphiphilic gradient copolymers using kinetic MC simulations, and found that the copolymer chains collapse and form micelle-like aggregates in poor solvents. They also reported that the degree of gradient in the copolymer chain structure is an important parameter which affects the critical temperature for the transition, and they reported a linear relationship (of negative slope) between the critical temperature and the strength of the copolymer gradient; where the gradient strength is defined as the largest difference in the instantaneous composition along the theoretical chain. This is in good agreement with the results reported by Gallow et al. [171]. An interesting observation is that the core region of spherical micelles formed by gradient copolymers can expand or shrink under the influence of environmental stimuli, such as temperature [45, 61, 96], pH [46, 58, 59, 77] and changes in solvents [45, 54, 169]. Such stimuli- responsive behaviour was examined by Seno et al. [169] for gradient, block and statistical copolymers of 2-ethoxyethyl vinyl ether (EOVE) and 2-hydroxyethyl vinyl ether (HOVE). It was found that, the micellar corona was reduced and the micellar core was expanded in EOVE-HOVE gradient copolymer micelles as a result of temperature stimulus and solvent stimulus. But no such changes were observed for the block and statistical copolymers.

The solution behaviour of EOVE and 2-methoxyethyl vinyl ether (MOVE) gradient copolymers was studied and compared with the corresponding block copolymers in multiple studies [17, 53, 60, 169]. Due to the differences in the hydrophilicity of the core and the shell, the dimension of the block copolymer micelles were found to be insensitive to the changes in temperature. However, an increase in temperature resulted in an increase in the size of the micellar core and a contraction of the micellar shell for the EOVE-MOVE gradient copolymer micelles. These phenomena were explained as being due to a “reeling-in” process, shown schematically in Figure 15 [169]. The hydrophobic interactions between the core segments are weaker for the gradient copolymers than for the block copolymers due to the diffuse structure of the gradient copolymer micelles. As the temperature increases, the gradient copolymers form smaller micelles having larger cores due to the dominance of hydrophobic interactions over hydrophobic hydration of the MOVE segment, similar to reeling in of a fishing wire. This reel-in process continues until the whole polymers collapse to result in precipitation at a lower critical solution temperature (LCST) of homopolymers of MOVE.

Figure 15. Schematic illustration of the micellization behaviour in solutions of stimuli-responsive gradient, block and statistical copolymers [169]. Reprinted with permission from reference 169. Copyright (2008) John Wiley and Sons.

A number of other studies of the micellization of gradient copolymers also reported similar behaviours [46, 49, 77, 178, 179]. Borisova and co-workers [30, 46, 178] used small-angle neutron scattering (SANS), dynamic light scattering (DLS), isothermal titration calorimetry (ITC) and TEM to examine the responsiveness of the micelles of St/AA block and gradient copolymers with the variation in pH and ionic strength of solutions. Based on the experimental results, they suggested that due to irreversible association the block copolymer micelles are unable to re-arrange with the change in pH and ionic strength, and they therefore referred to these micelles as being ‘kinetically frozen’. PSt domains form a glassy-state core and therefore the aggregation number of the block copolymer micelles remain unchanged. In contrast, St/AA gradient copolymers are capable of forming nano-sized aggregate-like micelles by reversible association due to the presence of the hydrophobic St units and the pH-sensitive AA comonomer units in the terminal gradient blocks. At pH values higher than 7.5, the strong electrostatic inter-molecular repulsion of the ionized AA units causes the gradient copolymers to be in a dissolved molecular state. A decrease in pH results in protonation of AA units and reduces the inter-molecular repulsion. This therefore leads to the association of the gradient copolymer into micelle-like aggregates below a threshold pH value. Similarly, the addition of salt, which changes the ionic strength of the solution can only influence the conformation of AA units in the coronae of the block copolymer micelles and has no effect on the aggregation number. However, addition of salt ions had a strong influence on the aggregation number as well as on the hydrodynamic radius of the gradient copolymer micelles [30]. A reduction in the size of methacrylic acid (MAA)/MMA gradient copolymer micelles with increasing pH was also demonstrated by Zhao et al. [58]. In this study it was shown that the amount of reduction observed for a linear gradient copolymer was intermediate to that observed in a statistical copolymer (completely dissolves at higher pH) and a diblock copolymer. Through a potentiometric titration study they found that the degree of dissociation

(α) and the dissociation constant (Ka) of the –COOH group of MAA/MMA copolymers was clearly

28 influenced by the electrostatic interaction of neighbouring –COO- anions and H+ cations, which in turn depends on the copolymer composition profile. Diblock copolymers exhibit maximum change in the free energy of electrostatic interaction (ΔGel) due to a high MAA unit density, while ΔGel was a minimum for statistical copolymers in which the MAA units are separated by MMA units in the

copolymer chains. ΔGel for a gradient copolymer on the other hand, was intermediate to those of the diblock and statistical copolymers owing to their chain composition profiles.

Zheng et al. [179] studied the micellization behaviour of St/MMA gradient copolymers in binary acetone-water mixtures. As a result of a change in the water content in the acetone-water mixture, three different types of transitions were observed for St/MMA gradient copolymers which were assigned to different relaxation processes. The first type of transition is unimer-to-micelle transition due to association and dissociation of unimers. Shrinkage and stretching results in the second type of transition which is a ‘star-like’ micelle to ‘crew-cut’ micelle transition, and the third type of transition takes place from spherical micelles to cylindrical micelles and to vesicles, a morphological transition resulting from fission/fusion of micelles (Figure 16). Chen et al. [77] and Pispas and co-workers [49] also reported similar structural changes for gradient copolymer micelles.

Figure 16. Schematic representation of three different types of transitions of the gradient copolymer micellar system for variation of water content in acetone–water mixtures [179]. Reprinted with permission from reference 179. Copyright (2013) John Wiley and Sons.

The group of Zheng has also studied the thermal responsiveness of St/MMA block and gradient copolymers for varying water contents in acetone-water mixtures [61]. Due to the gradual transition of monomer units in the polymer chains, the temperature dependence of the solubility of the gradient copolymers would also vary from one end to the other of the copolymer chain. Hence, the micelles formed by the gradient copolymers are expected to be more responsive to temperature than those of the block copolymer micelles. It was found that the shrinkage/stretching of a gradient copolymer of St/MMA induced by a change in temperature was reversible nature even after multiple heating-cooling

29 cycles within a certain temperature range [61]. This thermodynamically controlled behaviour of the gradient copolymer micelles has been proposed to be an intrinsic and universal property originating from the gradient structure. Based on the block copolymer microfluidic self-assembly, Zhu et al. [180] analysed the self-assembly of fluorinated gradient copolymers by combining a COMSOL Multiphysics software simulation and experimental data. They used a 3D co-flow focusing microfluidic device to investigate the self-assembly of AA-TFEMA gradient copolymers and found that their self-assembly can be controlled to achieve copolymer aggregates with a variety of morphological structures.

Potemkin and co-workers [181] studied the micelles formed by block and gradient copolymers of similar compositions using molecular dynamics simulations to examine the differences in internal structures of micelles in a selective solvent. Block and gradient copolymers having the same number of solvophobic and solvophilic monomer units have very different chain conformations in the micelles, which results in different aggregation numbers. In contrast to the strongly segregated core-corona interface of the block copolymer micelles, the interfaces in the gradient copolymer micelles are broad due to wide distribution of the solvophobic monomer units along the chains. Therefore, the segregation of different groups in the gradient copolymer micelles are not as pronounced as those of the block copolymers, even in strongly-selective solvents.

Figure 17. Chain conformation of diblock (left) and gradient (right) copolymer micelles. The loop formed by the soluble block near the core-shell interface is indicated by the green circle [181]. Reprinted with permission from reference 181. Copyright (2016) American Chemical Society.

The internal structure of the micelles of amphiphilic block and gradient copolymers of 2- oxazolines based on the hydrophilic poly(2-methyl-2-oxazoline) (PMeOx) and the hydrophobic poly(2- phenyl-2-oxazoline) (PPhOx) was studied by SAXS, SANS, static and dynamic light scattering (SLS/DLS), and 1H NMR spectroscopy in water and water-ethanol mixtures to demonstrate the differences in the micellar structures [182]. In contrast to the uniform core density of the micelles formed by block copolymers, the gradient copolymer micelles were found to have a higher density at the periphery compared with the centre of the core. The hydrophilic-hydrophobic interactions cause back-folding of chains which in turn produces cores with a high-density outside and a liquid-like inside. Such a structure of gradient copolymer micelles were referred to as micelles with a ‘bitterball-core’ structure [182]. They found that in addition to the difference in aggregation numbers, the micelles

30 resulting from the gradient copolymers also have a different core size, eccentricity and solvent volume fraction compared with those resulting from the block copolymers. Ribaut and co-workers [175] studied

the solubility of gradient copolymers in supercritical CO2, and compared the results with those for the analogous block copolymers. Cloud point measurements showed that the gradient copolymers were more soluble under milder conditions of pressure and temperature than the block copolymers. Similar to the observations in aqueous solutions, the block copolymers form ‘frozen’ aggregates in scCO2, whereas the gradient copolymers form ‘dynamic’ aggregates which are environmentally sensitive [170].

3.4. Mechanical and other Properties

Gradient copolymers also possess unique mechanical properties compared with block and statistical copolymers. Zaremski et al. [183] reported that gradient copolymers of styrene-methyl acrylate possessed a lower elastic modulus, higher elongation at break and higher tensile strength compared to statistical copolymers of the same composition at room temperature. This is in agreement with the results reported by Michler and co-workers [184], in which a St/butadiene tapered copolymer showed superior tensile properties compared to the corresponding neat block copolymer. These observations were ascribed to a reduction in the stress concentration resulting from a gradual change in the local modulus within different nanodomains in the gradient copolymers [113]. As shown in Figure 18, the diblock copolymer behaved like a brittle plastic by failing at only 6% elongation, whereas the linear gradient copolymer showed an elongation at break of over 300% and exhibited elastomeric properties [113].

Linear gradient V-shape gradient Triblock copolymer Statistical copolymer Diblock copolymer

Figure 18. Tensile tests on statistical, linear and V-shape gradient, diblock and triblock St/nBA copolymers at 25 °C. The strain portion is enlarged in the inset (Figure adapted from [113]). Adapted with permission from reference 113. Copyright (2015) John Wiley and Sons.

The conductivity of gel polymer electrolytes from different copolymers with a range of contents of electrolytes has been measured by Zheng et al. [112] They found that the gradient copolymers showed

31 significantly higher ion conductivity than those for the block and statistical copolymers at the same liquid electrolyte uptake, and the differences in behaviour clearly originated from the different chain structures. It was proposed that in a film, the conducting polar domains connect to each other to build conducting pathways which are circuitous. However, distinctive domain boundaries resulted from the stronger phase separation in block copolymers which disrupt the conductive pathways by isolating the polar domains, and therefore creating numerous dead ends [185, 186]. In comparison, the gradient copolymers are more weakly phase separated with no sharp domain boundaries which eliminates or minimizes the interruptions of the conducting domains to produce highly connected conducting pathways, which facilitates fast and efficient lithium ion transport (Figure 19).

Figure 19. Proposed continuous conducting pathways for gradient copolymers, and pathways with dead ends for block copolymers [112]. Reprinted with permission from reference 112. Copyright (2016) American Chemical Society.

3.5. A Summary of Applications of Gradient Copolymers

The uniqueness in the structure of the gradient copolymers furnishes this class of polymers with a number of unique properties, and as a result gradient copolymers have immense potential in a number of applications. A very attractive aspect of gradient copolymers is that their properties can be optimized by fine-tuning the copolymer structure, e.g. by tuning the degree of the gradient. One of the most commonly reported applications is as compatibilizers for immiscible polymer blends [5, 14, 15, 38-40]. Multiple reports have experimentally shown the impact of incorporation of gradient copolymers on mixing in otherwise immiscible homopolymer blends. Other applications reported for gradient copolymers include as stabilizers of emulsions or dispersions [41], as thermoplastic elastomers [42, 43], and multi-shape memory materials [44]. Gradient copolymers with broad glass transition behaviour can be designed to yield materials with excellent damping behaviour [29, 36, 37, 166].

Amphiphilic fluorinated gradient copolymers with optimized hydrophilicity and low surface energy segments exhibit excellent antifouling performance [115]. Besides the well-known application of such amphiphilic structures as detergents and stabilizers, the micelles formed by self-assembly of amphiphilic gradient copolymers in an aqueous medium can be used as micellar catalysis and in drug delivery [187, 188]. Amphiphilic gradient copolymers can be used to fabricate nano-carriers in

32 biocompatible media [45]. Stable spherical nanostructures formed by gradient copolymers of 2-methyl- 2-oxazoline and 2-phenyl-2-oxazoline (MPOx) are capable of delivering DNA chains [64], and the stable MPOx/DNA complexes demonstrate potential in a broad range of biomedical applications. Gradient copolymer of EOx and 2-(4-dodecyloxyphenyl)-2-oxazoline (DPOx) have been found to yield polymeric nanoparticles by assembling in water[189]. These nanoparticles are capable of encapsulating, e.g., curcumin, and in vitro experiments, suggesting that the nanoparticles are highly cytocompatible and able to deliver bioactive compounds in cellular environments.

Zheng et al. [112] evaluated a series of statistical, block and gradient St/MA copolymers as polymer electrolytes. They found that, the gradient copolymers yielded the highest lithium ion conductivity (10- 3 Scm-1), by providing continuous ion conductive pathways. Therefore, gradient copolymers have potential as replacements for commonly used statistical copolymer gel electrolytes. Gradient copolymers have also been found to successfully extract up to 40% palladium from commercial

Pd/Al2O3 catalysts through a non-destructive process carried out under scCO2 at 40 ºC and 25 MPa [190]. This greener approach could be a potential replacement for the destructive pyrometallurgical or wet-chemical processes which involve a complex post-treatment process and provide only 10% extraction.

4. Summary The field of free radical polymerization has experienced enormous development over the past two to three decades. In particular, the invention of controlled radical polymerization techniques such as ATRP, NMP, RAFT provide the freedom to design and prepare homo- and copolymers with precise control over molecular weight, molar mass dispersity and the distribution of monomer units in the copolymer chains, for a wide range of monomer pairs. These advances in the polymerization techniques have allowed the introduction of ‘gradient copolymers’, a novel class of copolymers with unique structure compared with conventional statistical and block copolymers. Theory tells us that the properties of copolymers are strongly dependent on, not only their overall compositions, but also on the distribution of monomer units along the copolymer chains. This has been confirmed by comparison of interfacial properties, thermal properties and micellization properties of gradient copolymers with those of statistical and block copolymers. Microscopic analyses suggest no evident phase boundaries are formed in gradient copolymers due to weak phase segregation, in contrast with the equivalent block copolymers. The uniqueness of the properties of the gradient copolymers owing to their unique structures is also evident in their thermal properties. Gradient copolymers show a broad glass temperature due to the continuous variation in composition along the chain, while block copolymers

generally possess two transitions, and statistical copolymers generally show a single sharp Tg. Gradient copolymers also demonstrate unique self-assembly behaviour in solution. Micelles of gradient copolymers can be designed to be sensitive to the changes in temperature, pH and solvent quality. This unique class of copolymers have already been successfully applied in many different areas, for example

33 as compatibilizers for immiscible polymer blends, as stabilizers of emulsions or dispersions, as multi- shape memory materials, and also in the biomedical field. With their unique structures and properties, gradient copolymers possess immense potential to be applied in many other different polymer-based applications.

5. Future Directions The discovery of the living radical polymerization techniques has already made it possible to synthesise gradient copolymers with excellent control over the compositions and sequence distributions. Especially, the use of mathematical modelling to predict and design the composition and sequence distribution enables gradients of required strength and length to be achieved. However, further studies are required on the structure-properties relationships of the copolymers in order to exploit the full benefit of this control in this notable class of copolymers. A complete understanding of the structure- property relationships would provide opportunities to tune and achieve a particular gradient structure for specific applications.

List of Abbreviations AA - Acrylic acid AFM - Atomic force microscopy AGET - Activators generated by electron transfer AN - Acrylonitrile AS - 4-Acetoxystyrene ATRP - Atom transfer radical polymerization BMA - Butyl methacrylate 3BrHT - 3-(6-Bromohexyl) thiophene CCD - Chemical composition distribution CMC - Critical micelle concentration CROP - Cationic ring opening polymerization CRP - Controlled radical polymerization CSD - Copolymer sequence distribution DLS - Dynamic light scattering DMA - Dynamic mechanical analyses DMAA - N,N-Dimethylacrylamide DMAEMA - 2-(Dimethylamino)ethyl methacrylate

DMF - N,N-dimethylformamide DSC - Differential scanning calorimetry EOx - 2-Ethyl-2-oxazoline EOVE - 2-Ethoxyethyl vinyl ether FBA - 2,2,3,4,4,4-Hexafluorobutyl acrylate FMA - 2,2,3,4,4,4-Hexafluorobutyl methacrylate FRES - Forward recoil spectroscopy HEMA - 2-Hydroxyethyl methacrylate HOVE - 2-Hydroxyethyl vinyl ether 3HT - 3-Hexylthiophene HFBMA - 2,2,3,3,4,4-Heptafluorobutyl methacrylate HST - Hydroxystyrene iBRA - Isobornyl acrylate LG - Linear gradient MA - Methyl acrylate MAA - Methacrylic acid MC - Monte Carlo MiPrOx - 2-Methyl- or 2-isopropyl-2-oxazoline MMA - Methyl methacrylate MMFS - Model-based monomer feeding strategy MOVE - 2-Methoxyethyl vinyl ether MPC - 2-Methacryloyloxyethyl phosphorylcholine MSt - 4-Methyl styrene nBA - n-Butyl acrylate nBMA - n-Butyl methacrylate NMP - Nitroxide mediated polymerization nPrOx - 2-n-Propyl-2-oxazoline ODA - Octadecyl acrylate ODMA - Octadecyl methacrylate OEGMA - Oligoethylene glycol methyl ether methacrylate PCBM - Phenyl-C61-butyric acid methyl ester RAFT - Reversible addition-fragmentation chain transfer

ROMP - Ring-opening metathesis polymerization SANS - Small-angle neutron scattering SAXS - Small-angle X-ray scattering SCF - Self-consistent field Styrene -St tBA - tert-Butyl acrylate

TCP- Cloud point temperature TEM - Transmission electron microscopy TFEMA - 2,2,2-Trifluoroethyl methacrylate TFOA - 3,3,4,4,5,5,6,6,7,7,8,8,8-Tridecafluorooctyl acrylate

Tg - Glass transition temperature TMSEMA - 2-(Trimethylsiloxy)ethyl methacrylate TRI - Triblock copolymer VG - V-shape gradient

4VP - 4-Vinylpyridine

Acknowledgements The authors would like to acknowledge financial support from the Australian Research Council

(DP180101221, CE140100036, DP130103774) and the Asian Office of Aerospace Research and

Development (AOARD).

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