Amphiphilic block copolymer micro/nano-objects have attracted great attention due to their promising potential and practical applications in sensors, drug delivery, catalysis [1-3]. Traditionally, amphiphilic block copolymers (BCPs) micelles were prepared via self-assembly of BCPs in selective solvent for one of the blocks. However, this post-polymerization method involves multiple steps and leads to highly dilute dispersion (< 1 wt% solids) [4-6], which hinders large-scale preparation and impedes the practical applications of BCP self-assemblies. In the past decade, a new approach, named polymerization induced self-assembly (PISA) [7-10], has emerged as a powerful approach for the preparation of BCP micro/nano-objects. PISA combines the controlled radical polymerization process with in situ selfassembly of growing amphiphilic BCPs [11, 12], and represents an efficient and versatile strategy for the synthesis of BCPs micelles. The most active polymerization method in PISA is reversible addition-fragmentation chain transfer (RAFT) polymerization. In a RAFT PISA technique, a solvophilic macromolecular chain transfer agent (macro-CTA) is first synthesized by RAFT polymerization of A monomer, and then a dispersion polymerization of the second monomer B is mediated by polyA-CTA to get the AB diblock copolymer micelles [13-16]. Through designing different macro-CTA and core-forming monomers and varying the important factors affecting the self-assemblies, BCPs micelles with different morphologies, including sphere, worm, vesicle and other common forms obtained in traditional ways, have been fabricated by PISA methodology [17-21].

To date, most of the studied systems of PISA are based on diblock copolymers starting from mono-functional small molecular RAFT agent. Armes et al. [22] synthesized poly (2-(dimethylamino)-ethyl methacrylate) (PDMAEMA₃₁, the degree of polymerization (DP) of PDMAEMA was 31), macro-CTA, and employed it after purification to mediate the dispersion polymerization of benzyl methacrylate (BzMA) in ethanol. PDMAEMA-b-PBzMA with multiple morphologies like spheres, worms, vesicles were successfully prepared by changing the molar ratio of the solvophobic to solvophilic blocks. Yuan et al. [23] performed RAFT dispersion copolymerization of BzMA and 3-(triethoxysilyl)propyl methacrylate (TESPMA) in ethanol with purified PDMAEMA_51/70- CTA, and obtained wormlike and vesicular micelles. In contrast, spherical particles were obtained for the homopolymerization of BzMA under the same conditions. Zhang and co-workers synthesized poly(N, N-dimethyl acrylamide)-block-poly(styrene) (PDMAc-b-PSt) diblocks micelles in ethanol/water (85/15) utilizing PDMAc₆₇ macro-CTA. However, only spherical nano-objects were prepared with the increase of St conversion [24]. Generally, vesicles prepared by PISA have a relatively broad polydispersity index (PDI) [25] which is problematic for many potential applications. Recently, vesicles with narrow size distribution were achieved via some subtle techniques. Armes et al. [26] synthesized poly(methacrylic acid)-block-poly(benzyl methacrylate) (PMAAb-PBzMA) copolymer vesicles with narrow size distribution in ethanol by using a binary mixture of PMAA macro-CTAs with different DPs. An et al. [27] observed PDMAc₃₅-b-PDAAm (diacetone acrylamide, DAAm) copolymer vesicles with low PDI in aqueous dispersion polymerization by decreasing polymerization rate via reducing the amount of initiator or the temperature of polymerization. Zeng et al. [28] found that the position of the solvophilic macromolecular segment on the RAFT agent is crucial for RAFT PISA. The dispersion polymerization of St mediated with Z-type [poly-(ethylene glycol)monomethyl ether-3-(benzylthiocarbonothioylthio)propanoic acid (BTPA) (mPEG-BTPA), mPEG is on Z group] mPEG-CTA exhibited a poorer control character of polymerization, and the size of the copolymer spheres were extremely non-uniform. In contrast, the dispersion polymerization of St using R-type (mPEG-DDMAT (S-1-dodecyl-S'-(α, α'-dimethyl-α"-acetic acid) trithiocarbonate (DDMAT)) mPEG-CTA showed high controllability over the polymerization and the diameter of the uniform BCPs nano-spheres increased gradually with monomer conversion.

To the best of our knowledge, triblock copolymers were rarely studied in PISA. Zhang et al. [29-31] synthesized several BAB triblock copolymers mediated by bi-functional macro-CTA agents, such as trithiocarbonate-terminated poly(ethylene glycol) (TTCPEG-TTC) [29], trithiocarbonate-terminated poly(N-isopropylacrylamide) (TTC-PNIPAM-TTC) [30], trithiocarbonate-terminated poly (4-vinylpyridine) (TTC-P4VP-TTC) [31]. Vesicular micelles was only achieved for PSt-b-P4VP-b-PSt triblock copolymer, which was prepared by "one-pot" method (macro-CTA was used directly after polymerization without purification). Spherical micelles (also called flower-like particles) or gel-like networks were mainly obtained in most BAB triblock copolymers, due to the loops or the bridge of middle solvophilic A formed on the surface of the micelles, which make the micelles unstable and difficult to evolve to higher order morphologies. This finding in PISA are consistent with the self-assembly behavior of BAB block copolymers by traditional self-assembly technique [32]. Biais et al. [33] successfully obtained vesicles of (PDAAm-b-PDMAc-b-PDAAm) (BAB type) via dispersion polymerization of DAAm in water. The author attributed the formation of stable vesicles to the stabilization of the particles via electrostatic repulsion originated from the ionization of the benzoic moieties in the middle of PDMAc loops at high pH value. Compared with BAB triblock copolymer, ABA copolymers show very different micellization behaviour in selective solvent [29] and micelles with different structures, such as vesicles [34] and ring shape [35], have been obtained for several ABA copolymers. However, the poly(acrylic acid)-block-poly(butyl acrylate)-block-poly(acrylic acid) (PAA-b-PBA-b-PAA) copolymer particles synthesized via photo-PISA in water by Li et al. [36], using bi-functional RAFT agent, S, S'-bis(α, α'-dimethyl-α"-acetic acid)- trithiocarbonate (BDMAT), remained spherical morphology throughout the whole RAFT polymerization process.

In this contribution, starting from bi-functional RAFT agent, BDMAT, ABA triblock copolymers micro/nano-objects with different morphologies and sizes were synthesized via dispersion polymerization of St mediated by PDMAc-TTC-PDMAc macro-CTA in ethanol/water mixed solvent. The effect of solvent composition and solid content on the morphology and size of BCPs were researched in detail. Spheres, worms, vesicles, vesicles of two different sizes and vesicles with hexagonally packed hollow hoops (HHHs) wall were obtained in different ratio of ethanol/water and at different solid content. In addition, the morphologies of typical diblock copolymers and triblock copolymers particles were compared under the same reaction conditions. Scheme 1 shows the synthetic routes of PDMAc-TTC and PDMAc-TTC-PDMAc macro-CTA, and dispersion RAFT polymerization of St in ethanol/water co-solvent.

As discussed previously [28], the location of solvophilic block on the macro-CTA plays an importance role on the morphology of the as-formed block copolymers particles by PISA process. In this paper, the RAFT dispersion polymerization of St mediated by mono-functional macro-CTA agent PDMAc₂₅-TTC and bi-functional macro-CTA agent PDMAc₁₂-TTC-PDMAc₁₃ in ethanol/water (9:1, w/w) were carried out under the same conditions. In both systems, the appearances of the reaction systems gradually changed from a clear homogeneous solution to a milky white suspension as the polymerization proceeded, presenting typical character of conventional dispersion polymerization.

The morphologies of the diblock copolymer particles mediated by PDMAc₂₅-TTC are shown in Figs. 1A–C. It is obvious that only spherical particles were observed during the whole polymerization process and the final particles had a mean diameter of about 42 nm. The morphologies of PDMAc₁₂-b-PSt-b-PDMAc₁₃ copolymer particles changed from spheres to worms/vesicles mixture, and further to vesicles with the extension of PSt block, as shown in Figs. 1D–F. In Fig. 1D, spherical micelles with a mean diameter of about 22 nm were observed at 10 h. At 13.5 h, a large number of worms and a little amount of vesicles were obtained (Fig. 1E). At this time, the viscosity of the system is very high, possibly due to the entangling of long worms. Vesicular particles of PDMAc₁₂-b-PSt₂₀₀-b-PDMAc₁₃ triblock copolymers were formed when St was completely consumed at 25 h. The average outer diameter and wall thickness of vesicles was 152 nm and 25 nm, respectively (Fig. 1F). The results indicate that it is more likely to obtain vesicle micelles for ABA triblock copolymers than AB diblock copolymers under identical reaction conditions. Owing to the distribution of solvophilic PDMAc block on both ends of macromolecular chain, the solvation of solvophobic block in triblock copolymer should be stronger [21, 37], accordingly, the fusion between nanoparticles would be easier, leading to the formation of vesicles. Table S1 (Supporting information) summarizes the size changes (measured by DLS), the molecular weights and molecular weight distributions (determined by GPC) of the copolymer aggregates. Figs. S1 and S2 (Supporting information) show the ¹H NMR spectra of PDMAcs and the corresponding block copolymers in CDCl₃.

As discussed previously [38], solvent composition has significant effects on the size and morphology of BCPs in PISA. To investigate the effects of solvent composition on the morphology of triblock copolymers, the dispersion polymerizations of St mediated by PDMAc₁₂-TTC-PDMAc₁₃ were performed in mixed solvent with different ethanol/water volume ratios (ratios of 8:2, 9:1 and 10:0). The morphologies of PDMAc₁₂-b-PSt-b-PDMAc₁₃ particles are shown in Fig. 2.

For the polymerization in ethanol/water (8:2, v/v) reaction medium, the morphology transformation of PDMAc₁₂-b-PSt-b-PDMAc₁₃ micelles with the increase of St conversion is shown in Figs. 2A–D. The final sample (at 16 h) has a large number of spherical particles and a small amount of vesicle micelles (Fig. 2D). Figs. 2E–H show the series of PDMAc₁₂-b-PSt-b-PDMAc₁₃ triblock copolymers synthesized in ethanol/water (9:1, v/v) medium. Spheres were obtained with a mean diameter of approximately 18 nm at 6 h (Fig. 2E). Worms and small amount of vesicles were observed at 9 h (Fig. 2F). Pure vesicles of about 144 nm in size and 23 nm in wall thickness were formed at 14 h (Fig. 2G). Pure vesicles with average diameter of 180 nm and ultrathick wall thickness up to 40 nm were formed when the conversion of St reached 95.1% at 23 h (Fig. 2H). It can be found that lower water content is beneficial to form vesicles, which is consistent with the findings in previous report [39]. In pure ethanol, a mixed phase of spheres (size of 20 nm) and short sticks (diameter of 17 nm) was formed at 11.5 h (Fig. 2I). Worms along with small amount of vesicle micelles (Fig. 2J, 15.5 h), about 869 nm lamellas along with worm with diameter of 22 nm (small amount) (Fig. 2K, 19 h) were formed with the increase of monomer conversion. When the monomer conversion was 100% at 36 h, 1030 nm large vesicles were observed by TEM (Fig. 2L). Interestingly, ordered tubular structure can be observed clearly on the wall of the vesicle. The diameter of the tube and the intermediating void were 25 nm and 21 nm, respectively, as measured from the TEM picture. Pan et al. [40] have ever obtained such ordered microscopic micelles in their PISA work, which is named by hexagonally packed hollow hoops (HHHs). From these results, we can conclude that as the water content in the co-solvent gradually decreased, higher order morphologies become more common, such as vesicles and hexagonally packed hollow hoops structure observed in ethanol/water (9:1, v/v) and pure ethanol, respectively. When the water content is excess (ethanol/water = 8:2), we speculate that the ionization of the carboxylic acid terminal on PDMAc stabilizer chains [41] enhance the inter-particle repulsion, thus preventing the sphere-sphere fusion events that are required for the evolution in morphology of the copolymer particles. Table S2 (Supporting information) summarizes the size changes (measured by DLS), the molecular weights and molecular weight distributions (determined by GPC) of the copolymer aggregates.

As reported previously, solid content also have significant effect on the morphology and size of the polymer particles in PISA and traditional self-assembly method [1, 42-44]. A series of experiments in ethanol/water (9:1, v/v) mediated by PDMAc₁₂- TTC-PDMAc₁₃ macro-CTA at solid contents ranging from 20 wt% to 40 wt% were performed to evaluate the effect of total solid content on the morphology of the assembly. The results are summarized in Table S3 (Supporting information) and the final morphologies of the samples are shown in Fig. 3. The morphology transformation of PDMAc₁₂-b-PSt-b-PDMAc₁₃ assemblies at 20 wt%, 30 wt% and 40 wt% solid content are shown in Figs. S3, S5 and S6 (Supporting information), respectively. Pure vesicles with 180 nm average diameter and 40 nm wall thickness were obtained at 20 wt% solid content (Fig. 3A). Vesicles with two different sizes were obtained in 30 wt% solid content system, in which the average diameter of the large-sized vesicles was about 600 nm with 52 nm wall thickness, while the average diameter of the small-sized vesicles was ca. 264 nm with 47 nm wall thickness (Fig. 3B). The bimodal distribution of the particles can be observed clearly by DLS test (Fig. 3D). Again, large vesicles with tubular wall features were obtained at 40 wt% solid content. Especially, the vesicles are not regular spheres, which might be the result of kinetically "frozen". It is clear that high monomer concentrations tend to form large-size vesicles and particles with special structures. At high monomer concentrations, the chance for the inter-sphere fusion is more frequent, and the excess monomers also act as co-solvent to solvate the solvophobic PSt block, which could promote the adhesion of particles and further induce phase separation of the immiscible blocks, leading to the formation of higher order morphologies [42, 45].

RAFT dispersion polymerization of St mediated by PDMAc-TTC and PDMAc-TTC-PDMAc macro-CTA is performed in ethanol/water mixed solvent. Under identical reaction conditions, pure vesicles can be obtained for PDMAc-b-PSt-b-PDMAc triblock copolymers, while spherical particles were obtained dominantly for diblock copolymers. In the polymerization system mediated by bi-functional PDMAc-TTC-PDMAc macro-CTA, vesicle with HHHs wall can be prepared in pure ethanol or high solid content (40 wt%). Vesicles with two sizes were achieved in 30% solid content, which has never been reported in the literature. The present work provides an efficient and simple method to prepare BCPs micelles with high order morphologies by PISA mediated with bi-functional RAFT agent. The synthesis of ABA copolymers by combination of different solvophobic and solvophilic blocks (Fig. S7 in Supporting information presented the results of a kind of ABA copolymer samples) and bi-functional RAFT agents is under way in our laboratory.

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Supplementary material related to this article canbefound, in the online version, at doi:https://doi.org/10.1016/j.cclet.2019.10.026.