Self-Assembly of Amphiphilic Block Copolymers during ... - Springer Link

ISSN 1811-2382, Polymer Science, Series C, 2018, Vol. 60, Suppl. 1, pp. S192–S218. © Pleiades Publishing, Ltd., 2018. Original Russian Text © E.V. Chernikova, E.A. Lysenko, N.S. Serkhacheva, N.I. Prokopov, 2018, published in Vysokomolekulyarnye Soedineniya, Seriya C, 2018, Vol. 60, No. 2, pp. 296–324.

Self-Assembly of Amphiphilic Block Copolymers during Reversible Addition-Fragmentation Chain Transfer Heterophase Polymerization: Problems, Achievements, and Outlook E. V. Chernikovaa,*, E. A. Lysenkoa, N. S. Serkhachevab, and N. I. Prokopovb aFaculty

bLomonosov

of Chemistry, Moscow State University, Moscow, 119991 Russia Institute of Fine Chemical Technologies, MIREA—Russian Technological University, Moscow, 119571 Russia *е-mail: [email protected] Received October 18, 2017

Abstract—The formation of dispersions of amphiphilic block copolymer particles with the controlled morphology via heterophase polymerization mediated by reversible addition-fragmentation agents is considered. Variants of dispersion, emulsion, and seeded polymerizations are analyzed, and the mechanism of this process is discussed. Special attention is focused on issues related to control over the morphology of the formed particles immediately during synthesis and methods of its transformation in the resulting dispersions. DOI: 10.1134/S1811238218020042

INTRODUCTION Block copolymers are microsegregated systems capable of self-assembly in bulk and in solution of a selective solvent to give rise to nanostructures; their size and morphology are determined by the length, chemical nature, and mutual arrangement of their constituent blocks [1]. The controlled synthesis of block copolymers based on vinyl monomers is traditionally conducted via ionic living polymerization [2]. However, in the case of amphiphilic block copolymers, this frequently necessitates the preliminary protection of functional groups of monomers and their removal after the synthesis [3]. It is more convenient to use reversible-deactivation radical polymerization, which is less sensitive to the nature of functional groups of monomers [4]. Among the known variants of reversible-deactivation radical polymerization, reversible addition-fragmentation chain transfer polymerization (RAFT) is the most attractive in terms of synthesis conditions and scope of applicable monomers and solvents [5]. The mechanism of this process was described in detail in many reviews [6–9]. For monofunctional RAFT agents of the general formula R–S–C(=S)–Z (Z and R are stabilizing and leaving groups), the synthesis of amphiphilic block copolymer AnBm requires two stages of polymerization. At the first stage, a polymer with the structure RAn–S– C(=S)–Z is obtained, while at the second stage the final product RAnBm–S–C(=S)–Z is formed. Thus, the monomer is inserted between the sulfur atom and the end unit of the polymeric substituent.

For bifunctional RAFT agents, for example, symmetric trithiocarbonates R–S–C(=S)–S–R containing one stabilizing moiety and two leaving groups R, triblock copolymer AnBmAn is produced during the two-step synthesis. Homopolymer RAn–S–C(=S)– S–AnR is obtained at the first stage, and at the second stage, the final product RAnBm/2–S–C(=S)–S– Bm/2AnR is synthesized. In this case, chains grow via both ends of the trithiocarbonate fragment. Another variant of bifunctional RAFT agent R'–S–C(=S)–S– R–R–S–C(=S)–S–R' with two stabilizing moieties also implies production of the triblock copolymer but of another structure BmAnBm. Here groups R are leaving, while R' groups are not leaving, and chains grow via two ends—from both sides of the central block. Naturally, further increase in functionality of the RAFT agent entails a rise in the number of blocks formed at the second stage. However, in practice, these systems are used very rarely. If blocks are composed of copolymers, then the structure of block copolymers (alternating, random, or gradient) depends on the composition of the monomer mixture and activity of monomers in copolymerization. For gradient copolymers, functionality of the RAFT agent determines the direction of a change in copolymer composition along the chain. For a monofunctional RAFT agent, the composition of a macromolecule alters from head to tail; for a bifunctional RAFT agent bearing one stabilizing moiety, from the end of the macromolecule to its center. All these nuances will define the physicochemical and physicomechanical properties of block copolymers.

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The level of control over the molecular-mass characteristics of amphiphilic block copolymers in RAFT polymerization depends on several factors. First, it depends on the order of addition of monomers [10], as directly follows from the mechanism of polymerization. In fact, the matter concerns the direction of fragmentation of intermediate АnS– C•(Z)(SBm) formed at the second stage of synthesis, which is guided by the nature of leaving groups—polymeric substituents Аn and Bm.

. B +S m

S AnR BmS

C

.C S

Z

AnR

Z S Bm

.

RAn + S C Z

.

.

RAnBm

RAn + mB

In general, these directions are inequivalent, and for the RAFT mechanism to be implemented, it is necessary that substituent RAn be a better leaving group than the attaching substituent Bm. Then the polymer generated at the first stage RAn–S–C(=S)–Z will be fairly quickly transformed into a narrowly dispersed block copolymer RAnBm–S–C(=S)–Z. Second, macromolecules of homopolymers and block copolymers free of thiocarbonyl groups appear during block copolymerization via the reaction of the square-law termination of propagating radicals. These undesirable processes cause widening of the MMD of reaction products and lead to formation of a compositionally heterogeneous product. These processes may be suppressed using at the most a 10-fold molar excess of the polymeric RAFT agent relative to the initiator [11].

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Third, block copolymerization is often conducted in solution; therefore, the thermodynamic quality of solvent/medium should be taken into account. In the synthesis of amphiphilic block copolymers, the uncontrolled aggregation of the polymeric RAFT agent (polymer formed at the first stage) sometimes proceeds upon addition of the second monomer, which is a precipitator for the polymeric RAFT agent. This phenomenon is frequently inherent in hydrophilic or amphiphilic polymers and hydrophobic monomers [12]. The aggregation of macromolecules of the polymeric RAFT agent apparently decreases the accessibility of its thiocarbonyl group to a macroradical. As a result, the apparent efficiency of the polymeric RAFT agent declines and control over molecular-mass characteristics of the polymer worsens. However, if the controlled self-assembly of the formed block copolymers takes place during synthesis, amphiphilic block copolymer nano- and microobjects of different morphology may be obtained [13–18]. Issues relevant to this synthesis and prospects of its application for designing functional materials are the subjects of this review. SELF-ASSEMBLY OF AMPHIPHILIC BLOCK COPOLYMERS IN SELECTIVE SOLVENT SOLUTIONS The self-assembling ability of amphiphilic block copolymers in a solvent selective for one of the blocks forms the basis for the heterophase synthesis of dispersions of particles with the controlled morphology. In general, amphiphilic diblock copolymers АВ consisting of thermodynamically immiscible polar block А and nonpolar/low-polarity block В during dispersion in a selective solvent self-assemble into micelles with the insoluble core consisting of lyophobic block В units and the soluble corona composed of lyophilic blocks А [1, 19, 20]: А corona

В

А

Solvent selective for A В core

As molecules of low-molecular-mass surfactants, the macromolecules of block copolymers self-assemble into micelles according to the close association model; that is, they demonstrate the existence of critical micellization concentration and can produce micellar structures of different morphology [1, 19]. POLYMER SCIENCE, SERIES C

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They are able to solubilize insoluble compounds in micelles in much the same way as surfactant micelles [19, 20]. However, in contrast, block copolymer micelles are characterized by an extremely low lability, that is, the kinetic retardation of all processes associated with change in the aggregation number and core

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and/or corona structure. The kinetic retardation becomes more pronounced with increasing length of both blocks and glass-transition temperature of the lyophobic block [1, 20]. The amphiphilic diblock copolymer becomes capable of micellization beginning from the length of lyophobic block В equal to 6–20 units; the value of CMC for the block copolymers is two to three orders of magnitude lower than that for a low-molecularmass surfactant and equals 10–5–10–8 mol/L [21]. At a degree of polymerization of block В of ~100 units or above, CMC value is predominantly determined by the chemical nature and length of the lyophobic block but depends only slightly on the length and chemical nature of the lyophilic block [21–25]. In general terms, the mechanism of micellization includes two stages [1, 26]. Initially macromolecules associate to small quasi-equilibrium micelles and the aggregation number of macromolecules in micelles is smaller than the equilibrium value. At the second stage, quasi-equilibrium micelles are transformed into equilibrium ones owing to the transfer of single macromolecules across solution or merging of quasi-equilibrium micelles, which is accompanied by the enlargement of micelles and reduction in their amount [26, 27]. Both processes are related to the overcoming of activation barriers associated with transfer of the insoluble block across the core–corona boundary, solvent, and corona composed of segments of the second block. The value of activation barrier grows with

increase in the thermodynamic incompatibility and the degree of polymerization of blocks [1]. Therefore, the synthesis of equilibrium micelles of amphiphilic block copolymers is a laborious task [19, 20]. As a rule, only block copolymers with fairly short (no more than 100 units) and soft (with a low glass-transition temperature) lyophobic blocks are dispersed spontaneously and yield equilibrium micelles [28]. The most important feature of block copolymer micelles important in practice is their ability to form particles of different morphology [20–22]. A number of morphological types of micelles from the same family of block copolymers with different ratios of lyophobic and lyophilic blocks were first synthesized by A. Eisenberg, as exemplified by the self-assembly of diblock copolymers of PS and poly(acrylic acid) (PAA) in the mixed solvent DMF–water selective for the PAA block [29–32]. Reduction in the length of the PAA block at a constant length of the PS block “stimulates” a change in the morphology of micelles in the following sequence: spherical and wormlike micelles, vesicles, and large compound micelles. The latter are spherical architectures consisting of a set of reverse micelles with the PAA core and PS corona coated by a thin layer of macromolecules with PAA blocks exposed to the outside [31]. The general scheme of morphological transitions of diblock copolymers АВ in a selective solvent with a change in the ratio of lengths of lyophobic В and lyophilic А blocks is presented below.

А corona

Lcorona Rcore В core Star-shaped micelles

#1

Short-haired micelles

Wormlike micelles

Lamellas (vesicles)

@1

>1

The morphology diversity is well known for lowmolecular-mass surfactants in aqueous media, for which the geometry of micelles is determined by the dimensionless packing parameter p expressed as p = Vs/(a0ls) (Vs and ls are the volume and length of the nonpolar fragment of a surfactant molecule, respectively; and a0 is the surface of a micelle per polar group). If p ≤ 1/3, spherical micelles are generated,

Large compound micelles PB/PA

and at 1/3 < p ≤ 1/2, wormlike micelles are formed. The interval 1/2 < p ≤ 1 corresponds to vesicles, and at p > 1, reverse micelles appear. In turn, the values of Vs, ls, and a0 can be easily estimated from the chemical structure of molecules [33]. As opposed to low-molecular-mass surfactants, because of a high looseness and conformational changeability of polymer coils, the geometric parame-

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ters of macromolecules in micelles are affected to a noticeable extent by the character of energy interaction of block copolymer segments between each other and with solvent molecules. In order to estimate this interaction, the free energy of a micelle recalculated per macromolecule (Fchain) can be written in the easyto-use form Fchain = FА + Fsur + FB [20, 22], where FА is the free energy of the corona-forming block related to the mutual repulsion of units in corona, Fsur is the surface energy at the core-corona boundary, and FB is the free energy of the insoluble block determined by the degree of deviation of linear sizes of block В in the core from the unperturbed state [22]. Regarding the geometry of micelles as a result of free energy minimization, the effect of various factors (the ratio of block lengths, the concentration of block copolymer, the presence of additives, the degree of solvent selectivity, solubilization in a core) on the equilibrium sizes and morphology of micelles may be interpreted and the thermodynamic conditions of morphological transitions may be ascertained [22, 34, 35]. For example, for block copolymers with РВ/РА ≤ 1 (Р is the degree of polymerization of the corresponding blocks), the main contribution to the free energy of micelles is provided by the repulsion of lyophilic blocks in the corona (FА). This corresponds to the equilibrium morphology of a star-shaped spherical micelle, for which corona sizes Lcorona are larger than core sizes Rcore; that is, the volume of corona per unit block А is maximal; this makes it possible to minimize the repulsion of blocks А [19, 21]. As the length of block А decreases, the contribution of FА diminishes and Fsurf becomes the dominant free energy component. Reduction in Fsurf may be achieved through a rise in core sizes; this process is therefore accompanied by a reduction in the core surface area per single macromolecule. Therefore, for block copolymers with РВ/РА > 1, the equilibrium morphology will be a short-haired spherical micelle with Rcore > Lcore [19, 21]. With increasing core sizes, the value of Fsur decreases, but simultaneously the value of FB increases because the growth of core sizes may be provided only by the unfolding of blocks В in the core; as a result, their conformational entropy decreases. Hence, for block copolymers with РВ/РА ≫ 1, the existence of spherical morphology becomes thermodynamically unfavorable and the spontaneous successive transition of spheres to wormlike micelles, vesicles, and reverse micelles occurs and makes it possible to “release” the conformation stress of block В [20, 22, 32]. In the region corresponding to the generation of nonspherical morphologies (РВ/РА ≫ 1), small changes in FА, Fsurf, or FB may induce a discontinuous transition of micelles from one morphology to another [20, 22]. For example, if block А is a weak polyelectrolyte, then even a small increase in the degree of its ionization (a rise in FА due to the electrostatic repulsion of POLYMER SCIENCE, SERIES C

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blocks) will stimulate transition from vesicles or wormlike micelles to spherical. At the same time, decrease in its degree of ionization or shielding of electrostatic interactions in the corona due to increase in ionic strength, on the contrary, will cause the transformation of spherical micelles into nonspherical ones [34–36]. An increase in the concentration of the block copolymer in solution or growth in solvent selectivity for the lyophilic block leads to a rise in the aggregation number of macromolecules in a micelle, causes an increase in core sizes and FB value, and facilitates the formation of nonspherical morphologies [37]. The solubilization of homopolymer В in the core enables enlargement of the core without extension of block В chains; this process is accompanied by the transition of cylindrical and lamellar structures to spherical micelles [32]. In other words, in the region РВ/РА ≫ 1, a fine adjustment of block copolymer morphology is possible not only owing to the chemical change in the ratio of block lengths but also owing to a slight variation in conditions of the medium (рН, ionic strength, solvent composition, concentration of block copolymer). Finally, taking into account the retardation of relaxation processes, including morphological transitions, typical of block copolymers, the kinetic factor, that is, the method of preparing micellar solutions, may also be morphogenic [20]. This may manifest itself as the simultaneous formation of micelles of different morphologies from the same block copolymer [37] and the obtainment of peculiar types of metastable micellar aggregates unknown for low-molecularmass surfactants, for example, large compound vesicles, tubular, and onion-type micelles [38]. On the whole, the micellization of amphiphilic triblock copolymers АВА, ВАВ, and АВС obeys the same thermodynamic and kinetic laws as the micellization of amphiphilic diblock copolymers АВ [19]. However, for block copolymers АВА, the tendency toward micellization is weaker (the CMC value is higher, and the aggregation number is lower) compared with diblock copolymers because of a better shielding of lyophobic block В by two lyophilic blocks А. Triblock copolymer ВАВ is characterized by formation of a large amount of loops from blocks А in the corona of micelles and gelation with increase in copolymer concentration [1]. Amphiphilic triblock copolymers АВС are prone to intermacromolecular aggregation even in nonselective solvents [39], while in selective solvents they generate micellar structures with the segregated core and/or corona. Attempts to classify these structures were made in a number of recent reviews [40–42]. The diversity of morphologies generated during the dispersion of amphiphilic block copolymers in a selective solvent makes it possible to expect that various types of architectures will also be generated in the heterophase synthesis of these block copolymers according to the RAFT mechanism.

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POLYMERIZATION-INDUCED SELF-ASSEMBLY General Ideas The key idea of the process known as polymerization-induced self-assembly relies on the abovedescribed self-assembling ability of amphiphilic block copolymers in a solvent selective for one of the blocks [43–45]. The polymerization of an alien monomer mediated by a living polymer (in the case of RAFT, a polymer with a thiocarbonyl group) is herein conducted in an appropriate solvent. The solvent (medium) is chosen so that it can be thermodynamically good for the living polymer and poor for the growing block. Given that the medium and order of monomer addition are chosen properly, a slowly growing block of the second monomer gradually loses its solubility and the generated amphiphilic block copolymer spontaneously assembles into nano- and micron-sized particles of a certain morphology in which further growth of the second block proceeds. Thus, during polymerization the resulting block copolymer serves two functions: as a polymeric RAFT agent and a stabilizer of the formed particles. The emergence and development of this direction are stimulated by many attempts to accomplish con-

Self-assembly

Chain propagation in water

Hydrophilic living polymer

trolled radical heterophase polymerization. Emulsion polymerization is actively employed in industry; it provides access to colloidally stable dispersions of hydrophobic polymers in aqueous or aqueous-organic media [46–50]. The first experiments on reversible deactivation emulsion polymerization were not up to the expectations of researchers: it was rarely possible to control the molecular-mass characteristics of polymers, the stability of the resulting dispersions was often low, and the size distribution of particles was usually broad [51–57]. This was primarily due to the impossibility of controlling the topochemistry of the process, specifically the location of controlling agent (e.g., RAFT agent) first in water and then in particles (see below). The replacement of a low-molecularmass RAFT agent with a hydrophilic polymeric one made it possible to overcome this challenge, because owing to its amphiphilic nature the resulting block copolymer represents a surfactant and participates in the stabilization of polymer-monomer particles. As a result, stabilizing moiety Z of the RAFT agent moves from the aqueous medium initially at the methanol/water/particle interface and then inside a particle with lengthening of the hydrophobic block [58, 59]:

Hydrophobic monomer

Further development of this direction revealed that, depending on the nature of the second monomer and reaction medium, polymerization-induced self-assembly processes can be implemented during dispersion and emulsion polymerizations [16, 43, 46, 60–62]. Dispersion Polymerization Dispersion polymerization is carried out in a medium in which a monomer is soluble, while a polymer is insoluble. As a consequence, dispersion polymerization begins under homogeneous conditions as solution polymerization and then (once the critical length of a macroradical is reached) the polymer segregates into a separate phase. In the conventional variant of dispersion polymerization, stable polymer dispersions are often created using stabilizers based on homopolymers, block and graft copolymers, and macromonomers. The rate of dispersion polymerization is

usually higher than the rate of solution polymerization at the same concentrations of components [63]. In the synthesis of amphiphilic block copolymers, the reaction medium consists of a monomer, initiator, solvent/precipitator, and polymeric RAFT agent. As was mentioned above, the amphiphilic block copolymer formed at the initial stage serves herein two functions—particle stabilizer and polymeric RAFT agent. Depending on the nature of monomer and polymeric RAFT agent, aqueous, aqueous-organic, and organic media are chosen for synthesis. Let us consider each of these variants in detail. Dispersion polymerization in aqueous media. Systems capable of dispersion polymerization in aqueous media are very few in number. This may be explained by the fact that many monomers soluble in water give rise to watersoluble polymers. An exception is thermosensitive polymers, which lose solubility at elevated temperatures (alkylated polyacrylamides, e.g., poly(N-isopropylacryl-


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amide) (PNIPAM) and poly(N,N-diethylacrylamide) (PDEAA), and water-insoluble poly(2-methoxyethyl acrylate) and poly(2-hydroxypropyl methacrylate) (PHPMA) [64–66]). Polymeric RAFT agents are usually monofunctional trithiocarbonates and less commonly dithiobenzoates based on PEO, poly(N,Ndimethylacrylamide) (PDMAA), poly(ethylene glycol polymethacrylate) (PPEGMA), poly(glycerol methacrylate) (PGMA), poly(N,N-dimethylaminoethyl methacrylate) (PDMAEMA), and PAA [62–80]. Polymerization is initiated by water-soluble initiators, such as potassium persulfate or 4,4'-azo-bis(4-cyanovaleric acid). The first studies in this field were aimed at the synthesis of thermosensitive linear and crosslinked (nanogels) block copolymers based on PEO and/or PDMAA and PNIPAM [64, 68, 69]. Later, diverse nanogels containing both thermosensitive and рНsensitive blocks were synthesized [73–75]. As a whole, the rate of dispersion RAFT polymerization in water is higher than that of solution polymerization [75, 76]. The kinetic features of this process were explored primarily in the case of 2-hydroxypropyl methacrylate (HPMA) [81, 82]. Regardless of the type of polymeric RAFT agent, three sections are seen on the time dependences of monomer conversion. The first section corresponds to the induction period, which is typical of RAFT solution polymerization: to a higher extent for dithiobenzoates and to a smaller extent for trithiocarbonates (its length depends on the nature of the stabilizing moiety). In the second section, the rate increases gradually; this corresponds to the onset of polymerization and formation of the soluble block copolymer. When moving to the third section, block copolymer nanoparticles also nucleate and grow; the rate of polymerization initially grows abruptly and then decays in the course of monomer conversion. It was supposed that this increase in rate is related to a change in the local concentration of the

monomer because of its solubilization to the block copolymer micelles being formed. The character of a change in MMD with monomer conversion depends on the efficiency of the polymeric RAFT agent. For example, the efficiency of the polymeric RAFT agent based on PPEGMA in the polymerization of 2-methoxyethyl acrylate is unexpectedly low, although the polymethacrylate substituent is a better leaving group than the polyacrylate one [75]. S

S

COOH

x

O CN

S

O

O

O 8−9

S 2-Methoxyethyl acrylate

S S

x

CN O

O O

S x

S

S

Ph

NC O

HO

O

HO HO

PGMA S

O x

HO NC

HPMA

O

OH

HO

O O HO

HO POLYMER SCIENCE, SERIES C

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Nevertheless, the polymeric RAFT agent is slowly consumed during monomer conversion, and even at limiting conversions, its traces are seen on the GPC curves [75]. In addition, shoulders are observed on the high-molecular-mass branch of GPC curves, indicating appearance of the dead polymer. According to [75], the coefficients of polydispersity of polymers are 1.1–1.3, in disagreement with their GPC curves. A different situation is typical of the dispersion polymerization of HPMA mediated by asymmetric trithiocarbonate based on PGMA [77].

RAFT agent

O

O

8−9

O

O

COOH

y

O

HO

HO Glycerol methacrylate

O

+

O PPEGMA

O O

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S

S

Ph

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(а)

(b)

M25H400-15

(c)

M25H400-16.2

1 µm

200 nm

(d)

M25H400-17

M25H400-25

1 µm

(e)

200 nm

Vesicles

PHPMA block length

400 Worms and vesicles Spheres and worms

300

Worms

200 Spheres

100 10 15 20 25 Content of the solid substance, wt %

Fig. 1. TEM images of particles with different morphology: (a) spherical and wormlike, (b) wormlike, (c) wormlike and vesicles, and (d) vesicles; (e) phase diagram of block copolymers PMEPC–block–PHPMA (denoted as MxHy, where x and y are the degrees of polymerization of blocks) at the fixed length of the PMEPC block and the variable length of the PHPMA block [83].

This polymeric RAFT agent rapidly transforms into the block copolymer, and its ММ is easily controlled by the polymeric RAFT agent concentration and monomer conversion. As in the previous case, the GPC curves exhibit a shoulder in the high-molecularmass region; however, it is weaker than that for the system described above. The basic data array on the synthesis of amphiphilic block copolymers by RAFT dispersion polymerization in aqueous media concerns not kinetics of the process but ascertainment of relationship between the conditions of synthesis, the nature of the monomer and polymeric RAFT agent, and the morphology of the formed block copolymer nanoobjects. This circumstance may be apparently attributed to the fact that the control of morphology, being the predominant feature of RAFT dispersion polymerization, is of greater practical interest than the control of polymerization kinetics and molecular-mass characteristics of the amphiphilic block copolymer. The main parameters determining the type of particle morphology in dispersion polymerization in aqueous media include the ratios of ММs of hydrophobic and hydrophilic blocks, the chemical nature of the polymerizing monomer poly(ethylene glycol) and polymeric RAFT agent, the interactions of block copolymer segments with each other and with water, and the concentration of the hydrophobic monomer.

An analysis of the accumulated experimental results suggests that spherical and wormlike particles and vesicles are the most characteristic self-assemblies of dispersion polymerization products in aqueous media. As was demonstrated above, the type of morphology depends on the balance between the degree of extension of the core-forming hydrophobic block, the mutual repulsion of hydrophilic chains forming the corona, and the interfacial tension between the core and solvent. In fact, this means that different types of morphology may be obtained for the same system through variation in the initial concentrations of the monomer and polymeric RAFT agent and in monomer conversion, that is, by controlling the length of the core-forming block and setting the length of the corona block [67, 77, 81, 83]. The authors of several recent publications [81, 83] performed the painstaking work on constructing phase diagrams (dependences of the type of morphology on the degree of polymerization of the hydrophobic block PHPMA and the weight content of the organic phase—monomer and polymeric RAFT agent) [81, 83]. Polymeric RAFT agent was either poly(ethylene glycol) [81] or poly(2-(methacryloyloxy)ethylphosphorylcholin) (PMEPC) with the end dithiobenzoate group [83]. In order to construct phase diagrams, the authors synthesized in each case ~40 diblock copolymers, in which the length of the hydrophilic block remained constant


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and the length of the hydrophobic block was set by the conditions of synthesis. The resulting dispersions were investigated by dynamic light scattering, TEM, small-angle X-ray scattering, and AFM. A comparison of these results with kinetics of the process made it possible to ascertain conditions under which only one of the above-mentioned morphological types of particles is generated (Fig. 1). Transition from the spherical morphology to vesicles through a mixture of spherical and wormlike morpholigies, according to dynamic light scattering, is usually accompanied by a sharp rise in the average sizes of particles and their polydispersity. The interval of block lengths where only one morphological type of particles is formed is often very narrow; therefore, the construction of such diagrams is a very urgent called-for task.

One of the best studied monomers is benzyl methacrylate (BZMA). This is associated with its easy RAFT polymerization, a wide choice of solvents/precipitators, and a high rate of dispersion polymerization. Several leading research teams are involved in the study of its dispersion polymerization mediated by hydrophilic polymeric RAFT agents [62, 67]. The dispersion polymerization of BZMA initiated by 4,4'-azo-bis(4-cyanovaleric acid) at 80°С mediated by hydrophilic polymeric RAFT agents (e.g., monofunctional trithiocarbonate based on the copolymer of methacrylic acid (MAA) and poly(ethylene glycol methacrylate (PEGMA) of the equimolar composition) HOOC

In [79], the dispersion polymerization of N-isopropylacrylamide (NIPAM) mediated by PDMAA containing the end trithiocarbonate group and a small amount of acrylic acid units was conducted at a temperature above the LCST of PNIPAM. Conditions providing formation of particles of different morphology (spherical, wormlike, and vesicles) were unveiled and fixed via the crosslinking of acrylic acid units by diethylene amine. The morphology of particles may be controlled in another way. As was shown in [84], the dispersion polymerization of HPMA in water mediated by PGMA with the end dithiobenzoate group gives rise to wormlike particles that at 21°С form a soft hydrogel. Upon cooling to 4°С, the gel is destroyed and the particles switch their morphology to spherical. This transition is reversible. An analogous block copolymer synthesized by the dispersion polymerization of HPMA in water but mediated by asymmetric poly(glycerol methacrylate)based trithiocarbonate with the end carboxyl group [85] was capable of the same reversible changes in the morphology of particles from wormlike to spherical with variation in рН: in acidic media (рН < 4), the wormlike morphology was stable, while at a higher рН, the spherical morphology was stable. The replacement of the carboxyl group in the initial RAFT agent had no effect on the kinetics of polymerization and the control over the molecular mass of block copolymers; however, the capability for reversible changes in morphology was lost. Dispersion polymerization in polar media. Because the potential of dispersion aqueous polymerization is limited to a narrow range of monomers, widening the scope of research objects (styrene, acrylates) necessitated transition to other polar media, such as alcohols, mixed solvents (water-alcohol mixture), and acetonitrile [86–89]. POLYMER SCIENCE, SERIES C

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n

NC

m

COOH

S

S

O S O

O O

18

in ethanol or dioxane mixtures with water (solvent : water is no less than 80 : 20 vol %; at a higher content of water, the monomer segregates into a separate phase and the process follows the emulsion mechanism) allowed the synthesis of a number of nanoparticles of different morphology [61]. Along with the ratio of block lengths, the morphology is affected by the thermodynamic quality of the dispersion medium (organic solvent, water, and monomer function as dispersion media) with respect to the polymeric RAFT agent, the critical degree of polymerization of the hydrophobic block needed for self-assembly, the coefficient of monomer distribution between the continuous phase and particles, and the coefficient of organic solvent distribution between the continuous phase and particles. The organic solvent improves solubility of the stabilizing block in the continuous phase and leads to the formation of wormlike particles. In contrast, with an increase in the content of water, spherical particles are produced. The higher the thermodynamic quality of the solvent for the growing block, the greater the length of the hydrophobic block required for self-assembly. For example, 1,4-dioxane is a better solvent for poly(benzyl methacrylate) (PBZMA) than ethanol; therefore, the diversity of morphologies is easier to control in the ethanol–water mixture. Lengthening of the hydrophobic block provokes formation of anisotropic morphologies (spherical micelles → wormlike structures → large spherical aggregates → vesicles) (Fig. 2). The ability of

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(а)

200 nm (b)

500 nm

(c)

100 nm (d)

100 nm

(e)

rate corresponding to the period of particle formation cessation is observed ~2 h after the onset of polymerization. A good control over the molecular-mass characteristics of block copolymers is typical of this process: MMD is unimodal throughout polymerization, and the coefficients of polydispersity are 1.20–1.35. At the fixed length of the hydrophilic block, the authors changed the length of the hydrophobic block; for this purpose, synthesis was stopped at different monomer conversions. The dynamic light scattering and TEM studies of the obtained dispersions made it possible to construct the phase diagram demonstrating transition from one morphology to another and to seek conditions where only one variant of the structure (spherical, wormlike, or vesicle) rather than their mixtures exists. In some cases, self-assembled block copolymers based on BZMA were synthesized using hydrophobic polymethacrylates, for example, (poly(2hydroxyethyl methacrylate) and poly(2-hydroxypropyl methacrylate) as polymeric RAFT agents [92]. However, discussion of this issue is beyond the scope of our review. On the basis of analysis of the phase diagrams obtained for self-assembled diblock copolymer particles, the authors of [61, 87] managed to elaborate methods to prepare particles of the desired morphology which were stable upon displacement of dispersions synthesized in alcohol to the aqueous medium and vesicles with a narrow size distribution

500 nm

Fig. 2. TEM images of objects formed via the self-assembly of block copolymers P(MAA–со–PEGMA)–block– PBZMA formed in the dispersion polymerization of BZMA in the ethanol–water mixture (95 : 5 vol). Degrees of polymerization of the PBZMA block DPn: (a) 262, (b) 330, (c) 80, (d) 125, and (e) 983 [61].

organic solvent to penetrate into the core of particles also entails formation of morphologies different from spherical owing to the swelling of chains in the core and the enhancement of their mobility. In addition to the control of morphology, an increase in the content of water makes it possible to shorten the induction period, to raise the rate of polymerization, and to narrow the MMD of the reaction product. In the absence of water the dispersion polymerization of BZMA in alcohols (methanol, ethanol, isopropanol) mediated by polymeric RAFT agents, such as monofunctional trithiocarbonates based on PDMAEMA, PGMA, PMAA, and PMEPC, occurs to high conversions (above 95%) but at a lower rate for 24 h [87, 90–92]. The kinetics of polymerization is similar to that described above: the time dependence of conversion follows an S-shaped pattern and a rise in

In the former case, the dispersion polymerization of BZMA mediated by PMAA with the end trithiocarbonate group was conducted in the presence of a small amount (up to 10 mol %) of a crosslinking bifunctional monomer—ethylene glycol dimethylacrylate (EGDMA)—and the spherical morphology of particles was fixed. If after completion of the dispersion polymerization of BZMA, the dispersion postpolymerization of EGDMA mediated by the same PMAA is carried out, vesicles are generated and this morphology of particles no longer changes under any conditions [90]. In the latter case, narrowly dispersed vesicles were synthesized using two hydrophilic polymeric RAFT agents of the same chemical nature but of different molecular masses, for example, PMAA with the end trithiocarbonate group and degrees of polymerization differing by a factor of ~2.5 [91]. They were taken at different concentrations in order to grow the same PBZMA block. Self-assembly yielded vesicles in which a shorter PMAA block resided in the inner layer and the long block occurred in the outer layer:


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y

BZMA, ethanol, 70°С, 20 wt % x rods > spheres. A different strategy is used for the encapsulation of inorganic particles into polymer microspheres [46, 153–157]. Most hydrophilic polymeric RAFT agents (based on PAA, PDMAEMA) used in the heterophase synthesis of amphiphilic block copolymers can interact with inorganic nanoparticles. Adsorption of the hydrophilic polymeric RAFT agent on the surface of nanoparticles followed by the emulsion polymerization of the hydrophobic monomer (e.g., butyl acrylate or styrene) allows obtainment of hybrid nanocomposites in which nanoparticles are encapsulated into particles of amphiphilic block copolymers. In this case, the polymeric RAFT agent adsorbed on nanoparticles functions as a seed during the formation of polymermonomer particles. Particles of amphiphilic diblock copolymers based on PS and polyacrylamide were used for the vectorguided delivery of RNA [158]. Narrowly dispersed diblock copolymer particles containing end carboxyl groups (from the initial RAFT agent) in the corona were modified with 2-(2-pyridyldithio)ethylamine.

Owing to this procedure, microRNA (2–3 molecules per particle) was covalently attach via the disulfide bridge through the thiol-disulfide exchange. The microRNA was released under the action of L-glutathione at room temperature. The ability of dispersions of some amphiphilic block copolymer particles to reversibly switch morphology with a change in temperature is applied in the design of bactericidal thermosensitive gels [159] and in the sterilization of gels [84]. In the former case, wormlike particles composed of a mixture of diblock copolymers were synthesized by the dispersion polymerization of HPMA in water under the action of a mixture of nonionic PGMA and cationic poly((2-methacryloyloxy)ethyl)trimethylammonium chloride taken at different ratios. At a 12.5 wt % content of particles at 25°С, a gel is formed, which is destroyed at 7°С because the morphology of particles changes from wormlike to spherical. The cationic block imparts the bactericidal behavior to the gels, while the gels free of the cationic component do not exhibit antimicrobial activity. In the latter case, the dispersions of PHMA and PHPMA diblock copolymers with the constant length of the PGMA block and different lengths of the PHPMA block were described. The morphology of spherical particles (PGMA54–block–PHPMA90)1 and vesicles (PGMA54–block–PHPMA200) persists under heating/cooling of block copolymer dispersions. However, in the case of wormlike particles at a content of the block copolymer in water of 10 wt % and a temperature of 21°С, a gel is formed, which is reversibly destroyed at 4°С because the morphology of particles changes to spherical. This fact makes it possible to suggest a simple technique for the purification (sterilization) of similar hydrogels: to cause the transformation of morphology under cooling and to remove admixtures (e.g., bacteria) through filtration, because the size of spherical particles is smaller (by one to two orders of magnitude) than the size of bacteria. The same approach, namely, control over the morphology of dispersion particles, was successfully applied for the synthesis of nanoobjects of a more sophisticated architecture, so-called “tadpoles,” that is, particles with the desired geometry in which a head and a tail may be distinguished [160]. Similar nanoobjects show promise for numerous biomedical applications. The use of a set of NIPAM-based thermosensitive polymeric RAFT agents with different chemical compositions and, accordingly, LCST values with desired end groups (alkynyl or pyridyl disulfide) provided a way to the one-pot synthesis of “tadpole” structures. Upon dissolution of the polymeric RAFT agents in water and heating of the resulting solution above the LCST, their collapse took place. Therefore, 1 Hereinafter,

numerals designate the degree of polymerization of

blocks.


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the as-formed particles may be employed as a seed in the polymerization of styrene. Conditions were chosen so that the resulting diblock copolymers formed cylindrical micelles. Afterwards, toluene was added to the reaction mixture, and the mixture was cooled to a temperature lower than the highest LCST but higher than the lowest LCST. As a result, the desirable “tadpole” morphology was produced, in which the head was composed of PNIPAM collapsed globules and the tail included PNIPAM-based blocks soluble under these conditions. The ends of blocks contained groups of the initial RAFT agents. This makes it possible to accomplish further chemical modification without changing the morphology of particles. CONCLUSIONS Summing up the above data, the advantages of heterophase polymerization inducing self-assembly over conventional variants of heterophase polymerization may be formulated as follows: the formation of particles with the core–shell structure directly during the synthesis; easy creation of the specified amount of functional groups of different nature on the surface of particles owing to the use of an appropriate hydrophilic polymeric precursor; considerable opportunities to control the morphology of particles; control over the MMD of the formed amphiphilic block copolymers; and the absence of a specially added stabilizer that may be removed from dispersions, violating their stability. Despite understanding of the general principles of polymerization inducing self-assembly and considerable progress in the control of MMD of block copolymers and the morphology of particles formed on their basis, issues related to the topochemistry of this process—the distribution of the polymeric RAFT agent between particles, the mechanism governing the entry of radicals into particles, and the ratio of rates of chain propagation, termination, and transfer in PMPs— remain disputable. Thus, not only the synthesis of amphiphilic block copolymers, the preparation of particles of diverse morphologies on their basis, and the methods of their targeted transformation but also the fundamental study of the mechanism behind the elementary stages of heterophase polymerization is of interest. ACKNOWLEDGMENTS This work was supported by the Russian Science Foundation, project no. 15-13-30007. REFERENCES 1. Block Copolymers: Synthesis Strategies, Physical Properties, and Application, Ed. by N. Hadjichristidis, S. Pispas, and G. Floudas (Wiley, Hoboken, 2003). POLYMER SCIENCE, SERIES C

Vol. 60

Suppl. 1

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S215

2. Controlled and Living Polymerization: From Mechanisms to Applications, Ed. by A. H. E. Muller and K. Matyjaszewski (Wiley-Intersci., New York, 2009). 3. A. Hirao, R. Goseki, and T. Ishisone, Macromolecules 47, 1883 (2014). 4. Handbook of Radical Polymerization, Ed. by K. Matyjaszewski and T. P. Davis (Wiley, New York, 2003). 5. Handbook of RAFT Polymerization, Ed. by C. BarnerKowollik (Wiley-VCH, Weinheim, 2008). 6. G. Moad, J. Chiefari, Y. K. Chong, J. Krstina, R. T. A. Mayadunne, A. Postma, E. Rizzardo, and S. H. Thang, Polym. Int. 49, 993 (2000). 7. G. Moad, R. T. A. Mayadunne, E. Rizzardo, M. Skidmore, and S. H. Thang, Macromol. Symp. 192, 1 (2003). 8. G. Moad, E. Rizzardo, and S. H. Thang, Aust. J. Chem. 58 (6), 379 (2005). 9. G. Moad, E. Rizzardo, and S. H. Thang, Polymer 49, 1079 (2008). 10. G. Moad and S. H. Thang, Aust. J. Chem. 62 (11), 1402 (2009). 11. G. Moad, E. Rizzardo, and S. H. Thang, Aust. J. Chem. 65 (8), 985 (2012). 12. E. V. Chernikova and E. V. Sivtsov, Polym. Sci., Ser. B 59 (2), 117 (2017). 13. E. Rizzardo, J. Chiefari, Y. K. Chong, F. Ercole, J. Krstina, J. Jeffery, T. P. T. Le, R. T. A. Mayadunne, G. F. Meijs, G. Moad, C. L. Moad, and S. H. Thang, Macromol. Symp. 143, 291 (1999). 14. N. S. Serkhacheva, O. I. Smirnov, A. V. Tolkachev, N. I. Prokopov, A. V. Plutalova, E. V. Chernikova, E. Yu. Kozhunova, and A. R. Khokhlov, RSC Adv. 7, 24522 (2017). 15. Amphiphilic Block Copolymers Self-Assembly and Applications, Ed. by P. Alexandridis and B. Lindman (Elsevier, Amsterdam, 2000). 16. S. L. Canning, G. N. Smith, and S. P. Armes, Macromolecules 49, 1985 (2016). 17. L. A. Fielding, M. J. Derry, V. Ladmiral, J. Rosselgong, A. M. Rodrigues, L. P. D. Ratcliffe, S. Sugihara, and S. P. Armes, Chem. Sci. 4, 2081 (2013). 18. A. A. Cockram, T. J. Neal, M. J. Derry, O. O. Mykhaylyk, N. S. J. Williams, M. W. Murray, S. N. Emmett, and S. P. Armes, Macromolecules 50, 796 (2017). 19. G. Reiss, Prog. Polym. Sci. 28 (7), 1107 (2003). 20. Y. Mai and A. Eisenberg, Chem. Soc. Rev. 41 (18), 5969 (2012). 21. M. Moffitt, K. Khougaz, and A. Eisenberg, Acc. Chem. Res. 29 (2), 95 (1996). 22. E. B. Zhulina and O. V. Borisov, Macromolecules 45 (11), 4429 (2012). 23. M. Wilhelm, C. L. Zhao, Y. C. Wang, R. L. Xu, M. A. Winnik, J. L. Mura, G. Reiss, and M. D. Croucher, Macromolecules 24 (5), 1033 (1991). 24. I. Astafieva, X. F. Zhong, and A. Eisenberg, Macromolecules 26 (26), 7339 (1993). 25. I. Astafieva, K. Khougaz, and A. Eisenberg, Macromolecules 28 (21), 7127 (1995). 26. C. Honda, Y. Hasegawa, R. Hirunuma, and T. Nose, Macromolecules 27 (26), 7660 (1994).

S216

CHERNIKOVA et al.

27. E. E. Dormidontova, Macromolecules 32 (22), 7630 (1999). 28. O. Colombani, M. Ruppel, M. Burkhardt, M. Drechsler, M. Schumacher, M. Gradzielski, R. Schweins, and A. H. E. Muller, Macromolecules 40 (12), 4351 (2007). 29. K. Khougaz, I. Astafieva, and A. Eisenberg, Macromolecules 28 (21), 7135 (1995). 30. D. Kiserow, K. Prochazka, C. Ramireddy, Z. Tuzar, P. Munk, and S. E. Webber, Macromolecules 25 (1), 461 (1992). 31. L. F. Zhang and A. Eisenberg, Science 268 (5218), 1728 (1995). 32. L. Zhang and A. Eisenberg, J. Am. Chem. Soc. 118 (13), 3168 (1996). 33. Intermolecular and Surface Forces, Ed. by J. N. Israelachvili (Elsevier, Amsterdam, 2011). 34. O. V. Borisov and E. B. Zhulina, Macromolecules 36 (26), 10029 (2003). 35. E. B. Zhulina and O. V. Borisov, Macromolecules 38 (15), 6726 (2005). 36. L. Zhang and A. Eisenberg, Macromolecules 29 (27), 8805 (1996). 37. L. Zhang and A. Eisenberg, Macromolecules 32 (7), 2239 (1999). 38. N. S. Cameron, M. K. Corbierre, and A. Eisenberg, Can. J. Chem. 77 (8), 1311 (1999). 39. D. V. Vishnevetski, E. A. Lysenko, A. V. Plutalova, and E. V. Chernikova, Polym. Sci., Ser. A 58 (1), 1 (2016). 40. T. Smart, H. Lomas, M. Massignani, M. V. FloresMerino, L. R. Perez, and G. Battaglia, Nanotoday 3 (3–4), 38 (2008). 41. I. W. Wyman and G. Liu, Polymer 54 (8), 1950 (2013). 42. A. H. Gröschel and A. H. E. Müller, Nanoscale 7 (28), 11841 (2015). 43. B. Charleux, G. Delaittre, J. Rieger, and F. D’Agosto, Macromolecules 45 (17), 6753 (2012). 44. J. Rieger, Macromol. Rapid Commun. 36 (16), 1458 (2015). 45. Macromolecular Self-Assembly, Ed. by L. Billon and O. Borisov (Wiley, Hoboken, 2016). 46. P. B. Zetterlund, S. C. Thickett, S. Perrier, E. Bourgeat-Lami, and M. Lansalot, Chem. Rev. 115 (18), 9745 (2015). 47. P. B. Zetterlund, Y. Kagawa, and M. Okubo, Chem. Rev. 108, 3747 (2008). 48. J. Qiu, B. Charleux, and K. Matyjaszewski, Prog. Polym. Sci. 26, 2083 (2001). 49. M. F. Cunningham, Prog. Polym. Sci. 27, 1039 (2002). 50. J. B. McLeary and B. Klumperman, Soft Matter 2, 45 (2006). 51. W. Zhang, F. D’Agosto, O. Boyron, J. Rieger, and B. Charleux, Macromolecules 45, 4075 (2012). 52. W. Zhang, F. D’Agosto, O. Boyron, J. Rieger, and B. Charleux, Macromolecules 44, 7584 (2011). 53. X. Zhang, F. Boisson, O. Colombani, C. Chassenieux, and B. Charleux, Macromolecules 47 (1), 51 (2014). 54. G. Delaittre, J. Nicolas, C. Lefay, M. Save, and B. Charleux, Chem. Commun. 2005, 615 (2005).

55. G. Delaittre, J. Nicolas, C. Lefay, M. Save, and B. Charleux, Soft Matter 2, 223 (2006). 56. G. Delaittre, M. Save, and B. Charleux, Macromol. Rapid Commun. 28, 1528 (2007). 57. G. Delaittre and B. Charleux, Macromolecules 41, 2361 (2008). 58. C. J. Ferguson, R. J. Hughes, B. T. T. Pham, B. S. Hawkett, R. G. Gilbert, A. K. Serelis, and C. H. Such, Macromolecules 35 (25), 9243 (2002). 59. C. J. Ferguson, R. J. Hughes, D. Nguyen, B. T. T. Pham, R. G. Gilbert, A. K. Serelis, C. H. Such, and B. S. Hawkett, Macromolecules 38, 2191 (2005). 60. M. Chenal, J. Rieger, C. Véchambre, J. M. Chenal, L. Chazeau, C. Creton, L. Bouteiller, Macromol. Rapid Commun. 34, 1524 (2013). 61. X. Zhang, J. Rieger, and B. Charleux, Polym. Chem. 3, 1502 (2012). 62. Z. An, Q. Shi, W. Tang, C. K. Tsung, C. J. Hawker, and G. D. Stucky, J. Am. Chem. Soc. 129, 14493 (2007). 63. C. Gao, S. Li, Q. Li, P. Shi, S. A. Shah, and W. Zhang, Polym. Chem 5, 6957 (2014). 64. J. M. Weissman, H. B. Sunkara, A. S. Tse, and S. A. Asher, Science 274, 959 (1996). 65. M. Das, S. Mardyani, W. C. W. Chan, and E. Kumacheva, Adv. Mater. 18, 80 (2006). 66. S. Nayak, H. Lee, J. Chmielewski, and L. A. Lyon, J. Am. Chem. Soc. 126, 10258 (2004). 67. S. Chen, P. Shi, and W. Zhang, Chin. J. Polym. Sci 35 (4), 455 (2017). 68. C. Grazon, J. Rieger, N. Sanson, and B. Charleux, Soft Matter 7 (7), 3482 (2011). 69. J. Rieger, C. Grazon, B. Charleux, D. Alaimo, and C. Jérôme, J. Polym. Sci., Part A: Polym. Chem. 47 (9), 2373 (2009). 70. Q. Qu, G. Liu, X. Lv, B. Zhang, and Z. An, ACS Macro Lett. 5 (3), 316 (2016). 71. W. Zhou, Q. Qu, Y. Xu, and Z. An, ACS Macro Lett. 4 (5), 495 (2015). 72. Y. Jiang, N. Xu, J. Han, Q. Yu, L. Guo, P. Gao, X. Lu, and Y. Cai, Polym. Chem. 6 (27), 4955 (2015). 73. Q. Qiu, G. Liu, and Z. An, Chem. Commun. 47, 12685 (2011). 74. G. Liu, Q. Qiu, and Z. An, Polym. Chem. 3 (2), 504 (2012). 75. G. Liu, Q. Qiu, W. Shen, and Z. An, Macromolecules 44 (13), 5237 (2011). 76. A. Blanazs, J. Madsen, G. Battaglia, A. J. Ryan, and S. P. Armes, J. Am. Chem. Soc. 133 (41), 16581 (2011). 77. A. Blanazs, A. J. Ryan, and S. P. Armes, Macromolecules 45 (12), 5099 (2012). 78. W. Shen, Y. Chang, G. Liu, H. Wang, A. Cao, and Z. An, Macromolecules 44 (8), 2524 (2011). 79. C. A. Figg, A. Simula, K. A. Gebre, B. S. Tucker, D. M. Haddleton, and B. S. Sumerlin, Chem. Sci. 6, 1230 (2015). 80. N. J. Warren and S. P. Armes, J. Am. Chem. Soc. 136, 10174 (2014).


Vol. 60

Suppl. 1

2018

SELF-ASSEMBLY OF AMPHIPHILIC BLOCK COPOLYMERS 81. N. J. Warren, O. O. Mykhaylyk, D. Mahmood, A. J. Ryan, and S. P. Armes, J. Am. Chem. Soc. 136 (3), 1023 (2014). 82. N. J. Warren, O. O. Mykhaylyk, A. J. Ryan, M. Williams, T. Doussineau, P. Dugourd, R. Antoine, G. Portale, and S. P. Armes, J. Am. Chem. Soc. 137 (5), 1929 (2015). 83. S. Sugihara, A. Blanazs, S. P. Armes, A. J. Ryan, and A. L. Lewis, J. Am. Chem. Soc. 133, 15707 (2011). 84. A. Blanazs, R. Verber, O. O. Mykhaylyk, A. J. Ryan, J. Z. Heath, C. W. I. Douglas, and S. P. Armes, J. Am. Chem. Soc. 134, 9741 (2012). 85. J. R. Lovett, N. J. Warren, L. P. D. Ratcliffe, M. K. Kocik, and S. P. Armes, Angew. Chem., Int. Ed. Engl. 54, 1279 (2015). 86. W. Cai, W. Wan, C. Hong, C. Huang, and C. Pan, Soft. Matter 6 (21), 5554 (2010). 87. M. Semsarilar, E. R. Jones, A. Blanazs, and S. P. Armes, Adv. Mater. 24 (25), 3378 (2012). 88. M. Semsarilar, V. Ladmiral, A. Blanazs, and S. P. Armes, Langmuir 28 (1), 914 (2012). 89. Y. Li and S. P. Armes, Angew. Chem., Int. Ed. Engl. 49 (24), 4042 (2010). 90. M. Semsarilar, V. Ladmiral, A. Blanazs, and S. P. Armes, Polym. Chem. 5, 3466 (2014). 91. C. Gonzato, M. Semsarilar, E. R. Jones, F. Li, G. J. P. Krooshof, P. Wyman, O. O. Mykhaylyk, R. Tuinier, and S. P. Armes, J. Am. Chem. Soc. 136, 11100 (2014). 92. D. Zehm, L. P. D. Ratcliffe, and S. P. Armes, Macromolecules 46, 128 (2013). 93. E. R. Jones, M. Semsarilar, A. Blanazs, and S. P. Armes, Macromolecules 45, 5091 (2012). 94. J. Yeow, J. Xu, and C. Boyer, ACS Macro Lett. 4, 984 (2015). 95. H. Liu, C. Gao, Z. Ding, and W. Zhang, Macromol. Chem. Phys. 217, 467 (2016). 96. W.-D. He, X.-L. Sun, W.-M. Wan, and C.-Y. Pan, Macromolecules 44, 3358 (2011). 97. C. Q. Huang and C. Y. Pan, Polymer 51, 5115 (2010). 98. F. Huo, X. Wang, Y. Zhang, X. Zhang, J. Xu, and W. Zhang, Macromol. Chem. Phys. 214, 902 (2013). 99. J. R. McKee, V. Ladmiral, J. Niskanen, H. Tenhu, and S. P. Armes, Macromolecules 44, 7692 (2011). 100. X. Wang, J. Xu, Y. Zhang, and W. Zhang, J. Polym. Sci., Part A: Polym. Chem. 50 (12), 2452 (2012). 101. P. Shi, Q. Li, X. He, S. Li, P. Sun, and W. Zhang, Macromolecules 47 (21), 7442 (2014). 102. L. Houillot, C. Bui, C. Farcet, C. Moire, J. A. Raust, H. Pasch, M. Save, and B. Charleux, ACS Appl. Mater. Interfaces 2 (2), 434 (2010). 103. J. A. Raust, L. Houillot, M. Save, B. Charleux, C. Moire, C. Farcet, and H. Pasch, Macromolecules 43 (21), 8755 (2010). 104. L. Houillot, C. Bui, M. Save, B. Charleux, C. Farcet, C. Moire, J. A. Raust, and I. Rodriguez, Macromolecules 40 (18), 6500 (2007). 105. M. J. Derry, L. A. Fielding, and S. P. Armes, Polym. Chem. 6, 3054 (2015). POLYMER SCIENCE, SERIES C

Vol. 60

Suppl. 1

S217

106. L. A. Fielding, J. A. Lane, M. J. Derry, O. O. Mykhaylyk, and S. P. Armes, J. Am. Chem. Soc. 136, 5790 (2014). 107. G. Zheng and C. Pan, Macromolecules 39 (1), 95 (2006). 108. M. Dan, F. Huo, X. Zhang, X. Wang, and W. Zhang, J. Polym. Sci., Part A: Polym. Chem. 51 (7), 1573 (2013). 109. Y. Kang, A. Pitto-Barry, H. Willcock, W.-D. Quan, N. Kirby, A. M. Sanchez, and R. K. O’Reilly, Polym. Chem 6, 106 (2015). 110. Y. Kang, A. Pitto-Barry, A. Maitland, and R. K. O’Reilly, Polym. Chem. 6 (27), 4984 (2015). 111. W. Ji, J. Yan, E. Chen, Z. Li, and D. Liang, Macromolecules 41 (13), 4914 (2008). 112. L. P. D. Ratcliffe, B. E. McKenzie, G. M. D. le Bouëdec, C. N. Williams, S. L. Brown, and S. P. Armes, Macromolecules 48, 8594 (2015). 113. J. Jennings, G. He, S. M. Howdle, and P. B. Zetterlund, Chem. Soc. Rev. 45 (18), 5055 (2016). 114. A. B. Lowe, Polymer 106 (5), 161 (2016). 115. Emulsion Polymerization, Ed. by I. Piirma (Acad. Press, New York, 1982). 116. J. Ugelstad, P. C. Mork, and J. O. Aasen, J. Polym. Sci., Part A-1: Polym. Chem. 5, 2281 (1967). 117. B. T. T. Pham, M. J. Monteiro, and R. G. Gilbert, Macromol. Symp. 150, 155. 118. S. C. Thickett and R. G. Gilbert, Polymer 48, 6965 (2007). 119. S. C. Thickett and R. G. Gilbert, Macromolecules 39, 2081 (2006). 120. S. C. Thickett and R. G. Gilbert, Macromolecules 39, 6495 (2006). 121. B. T. T. Pham, H. Zondanos, C. H. Such, G. G. Warr, and B. S. Hawkett, Macromolecules 43, 7950 (2010). 122. M. S. El-Aasser, in Emulsion Polymerization and Emulsion Polymers, Ed. by P. A. Lovell and M. S. ElAasser (Wiley, New York, 1997), p. 37. 123. R. M. Fitch, Br. Polym. J. 5, 467 (1973). 124. F. K. Hansen and J. Ugelstadt, J. Polym. Sci., Polym. Chem. Ed. 16, 1953 (1978). 125. F. K. Hansen and J. Ugelstad, J. Polym. Sci. A-1: Polym. Chem. 17 (10), 3069 (1979). 126. A. M. Van Herk, Adv. Polym. Sci. 233, 1 (2010). 127. J. W. Ugelstad, M. S. El-Asser, and J. W. Vanderhoff, J. Polym. Sci., Part C: Polym. Lett. 11, 503 (1973). 128. F. J. LuoY. Schork, W. Smulders, J. P. Russum, A. Butte, and K. Fontenot, Adv. Polym. Sci. 175, 129 (2005). 129. T. Boursier, I. Chaduc, J. Rieger, F. D’Agosto, M. Lansalot, and B. Charleux, Polym. Chem. 2 (2), 355 (2011). 130. J. Rieger, F. Stoffelbach, C. Bui, D. Alaimo, C. Jerome, and B. Charleux, Macromolecules 4065 (2008). 131. M. Manguian, M. Save, and B. Charleux, Macromol. Rapid Commun. 27 (6), 399 (2006). 132. E. V. Chernikova, N. S. Serkhacheva, O. I. Smirnov, N. I. Prokopov, A. V. Plutalova, E. A. Lysenko, and E. Y. Kozhunova, Polym. Sci., Ser. B 58 (6), 629 (2016).

2018

S218

CHERNIKOVA et al.

133. E. V. Chernikova, A. V. Plutalova, K. O. Mineeva, I. R. Nasimova, E. Y. Kozhunova, A. V. Bol’shakova, A. V. Tolkachev, N. S. Serkhacheva, S. D. Zaitsev, N. I. Prokopov, and A. B. Zezin, Polym. Sci., Ser. B 57 (6), 547 (2015). 134. M. Chenal, L. Bouteiller, and J. Rieger, Polym. Chem. 4, 752 (2013). 135. D. E. Ganeva, E. Sprong, H. de Bruyn, G. G. Warr, C. H. Such, and B. S. Hawkett, Macromolecules 40, 6181 (2007). 136. X. Wang, Y. Luo, B. Li, and S. Zhu, Macromolecules 42, 6414 (2009). 137. Y. Luo, X. Wang, B.-G. Li, and S. Zhu, Macromolecules 44, 221 (2011). 138. I. Chaduc, M. Girod, R. Antoine, B. Charleux, F. D’Agosto, and M. Lansalot, Macromolecules 45, 5881 (2012). 139. M. Li, M. Schellekens, J. de Bont, R. Peters, A. Overbeek, F. A. M. Leermakers, and R. Tuinier, Macromolecules 48 (4), 1194 (2015). 140. I. Chaduc, A. Crepet, O. Boyron, B. Charleux, F. D’Agosto, and M. Lansalot, Macromolecules 46 (15), 6013 (2013). 141. N. P. Truong, J. F. Quinn, A. Anastasaki, D. M. Haddleton, M. R. Whittaker, and T. P. Davis, Chem. Commun. 52, 4497 (2016). 142. J. Xu, X. Xiao, Y. Zhang, W. Zhang, and P. Sun, J. Polym. Sci., Part A: Polym. Chem. 51, 1147 (2013). 143. H. Zhou, C. Liu, Y. Qu, C. Gao, K. Shi, and W. Zhang, Macromolecules 49 (21), 8167 (2016). 144. P. Chambon, A. Blanazs, G. Battaglia, and S. P. Armes, Macromolecules 45, 5081 (2012). 145. M. Dan, F. Huo, X. Xiao, Y. Su, and W. Zhang, Macromolecules 47, 1360 (2014). 146. M. J. Derry, L. A. Fielding, and S. P. Armes, Prog. Polym. Sci. 52, 1 (2016).

147. R. Wei, Y. Luo, and Z. Li, Polymer 51, 3879 (2010). 148. W. Zhou, W. Yu, and Z. An, Polym. Chem. 4, 1921 (2013). 149. F. Huo, S. Li, X. He, S. A. Shah, Q. Li, and W. Zhang, Macromolecules 47 (23), 8262 (2014). 150. X. Xiao, S. He, M. Dan, F. Huo, and W. Zhang, Chem. Commun. 50, 3969 (2014). 151. F. Huo, C. Gao, M. Dan, X. Xiao, Y. Su, and W. Zhang, Polym. Chem. 5, 2736 (2014). 152. L. Qiu, C.-R. Xu, F. Zhong, C.-Y. Hong, and C.-Y. Pan, ACS Appl. Mater. Interfaces 8, 18347 (2016). 153. D. Nguyen, H. S. Zondanos, J. M. Farrugia, A. K. Serelis, C. H. Such, and B. S. Hawkett, Langmuir 24, 2140 (2008). 154. S. C. Thickett and P. B. Zetterlund, ACS Macro Lett. 2, 630 (2013). 155. S. H. Che Man, D. Ly, M. R. Whittaker, S. C. Thickett, and P. B. Zetterlund, Polymer 55, 3490 (2014). 156. SC. Thickett, N. Wood, Y. H. Ng, and P. B. Zetterlund, Nanoscale 6, 8590 (2014). 157. J. Ma, M. Lu, C. Cao, and H. Zhang, Polym. Compos. 34, 626 (2013). 158. C. K. Poon, O. Tang, X.-M. Chen, B. T. T. Pham, G. Gody, C. A. Pollock, B. S. Hawkett, and S. Perrier, Biomacromolecules 17 (3), 965 (2016). 159. M. Williams, N. J. W. Penfold, J. R. Lovett, N. J. Warren, C. W. I. Douglas, N. Doroshenko, P. Verstraete, J. Smets, and S. P. Armes, Polym. Chem. 7, 3864 (2016). 160. V. A. Bobrin and M. J. Monteiro, J. Am. Chem. Soc. 137, 15652 (2015).


Translated by T. Soboleva

Vol. 60

Suppl. 1

2018