PMAA-based RAFT dispersion polymerization of MMA in ethanol: conductivity, block length and self-assembly

Abstract

The reversible addition–fragmentation chain transfer (RAFT) dispersion polymerization of methyl methacrylate (MMA) was carried out in ethanol using polymethacrylic acid (PMAA)–4-cyanopentanoic acid dithiobenzoate (CADB) (degree of polymerization = 30, 122 and 450) as a macro chain transfer agent (CTA) and 2,2′-azobis(2,4-diemthyl valeronitrile) (V-65) as an initiator. In contrast to the random copolymerization systems, a dramatic increase of conductivity during the initial stage of RAFT polymerization was observed. It was confirmed that the conductivity resulted from the charged solvophobic blocks of soluble diblock copolymers, strongly dependent on the chain length of PMAA-CTA and PMMA. Objects of PMAA-b-PMMA were prepared by three methods, i.e. the polymerization-, temperature- and ion-induced self-assembly. The procedure of self-assembly commonly resulted in a dramatic decrease of conductivity. All results indicated that the electrostatic interaction played a role in the process of self-assembly, rather than just the solvophobic interaction of PMMA blocks in ethanol.

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1. Introduction

Random precipitation/dispersion copolymerization in a nonaqueous solution with a thermally degradable initiator is well-known as an approach to prepare monodisperse particles or microgels.¹ For decades, this technique has been the only one used, especially for the preparation of bioapplicable microgels composed entirely of water soluble polymers,² until recently when the template dispersion polymerization of methacrylic acid (MAA) on polyvinyl pyrrolidone (PVP)³ and vinyl pyrrolidone (VP) on PMAA⁴ in aqueous solution was developed. However, the mechanism of random precipitation or dispersion (co)polymerization is still under debate. The mechanism suggested by Fitch et al.⁵ has dominated the discussion of particle formation for decades. Nevertheless, in recent years, more and more experimental facts have been found which contradict its predictions.^6–10 A new mechanism was proposed, namely that the mini-/nano-monomer droplets played the dominant roles in nucleation and monomer transfer, thermodynamically created by the phase-separation of the monomer and solvent^3,4,11,12 or the agitation effects at the interface of the monomer and water in the heterogeneous polymerization system.^6,13,14 Trials of kinetic modeling have been carried out based on this mechanism.^15–18 However, a new challenge appears in RAFT dispersion polymerizations. A major technical breakthrough was made in the self-assembly of diblock copolymers, which was known as polymerization-induced self-assembly conducted in concentrated solution.^19–23 It is interesting because the self-assembly of surfactants and amphiphilic block copolymers conventionally occurred in a very dilute solution (usually <1 wt%).^24–29 It is not unusual to prepare block copolymers in a common dispersion polymerization system but no vesicles or worms were observed.^30,31 All mechanisms suitable for dispersion polymerization give no hints about the self-assembly of nano-objects, though micron-sized particles or microspheres with various shapes and morphologies have normally been prepared by precipitation/dispersion or emulsion polymerization.^32–35 Therefore, a question arises, i.e. what is the difference in the scenario between the normal dispersion copolymerization and RAFT dispersion polymerization?

In our previous paper, an abnormal increase of conductivity was observed in the RAFT polymerization systems of PMAA/PX (X = acrylamide (AAm), styrene (St) and 4-vinyl pyridine (4VP)).³⁶ Reviewing the relevant literature, we found that the abnormal increase of conductivity was also observed in the aqueous soap-free emulsion polymerization of St using KPS³⁷ or azo-type oil-soluble initiators.³⁸ According to these results, we put forward an explanation that the soluble amphiphilic diblock copolymer chains in alcohol³⁶ and the soluble oligomers of polystyrene in the aqueous phase^37,38 might be dipoles where the solvophobic blocks were positively charged as envisaged in Scheme 1.

Scheme 1 Dipole-like diblock copolymer chains and their electrostatic interactions (blue line: solvophilic, red line: solvophobic).

In this paper, for the purpose of investigating the relationship between the length of blocks, the conductivity of polymerization systems and the self-assembly behavior, RAFT dispersion polymerization of MMA in ethanol using PMAA–CADB as a macro chain transfer agent was selected. The chain length of each block in the PMAA-b-PMMA diblock copolymer can be adjusted over a relatively wide range. Although the self-assembled structure of the PMAA-b-PMMA block copolymer has been studied by Li et al. and the self-consistent field theory was used in their work to describe the self-organized structures in the aqueous phase,³⁹ the in situ self-assembly and effects of conductivity in ethanol were not considered.

2. Experimental

2.1 Chemicals

Methacrylic acid (MAA) and methyl methacrylate (MMA) were purchased from Aladdin Industrial Co. Ltd. After purification by vacuum distillation, they were stored under refrigeration. The chain transfer agent, 4-cyanopentanoic acid dithiobenzoate (CADB, Aldrich, >97%) and the free-radical initiator, 2,2′-azobis(2,4-diemthylvaleronitrile) (V-65, J&K, 97%) were used without any treatment. Anhydrous ethanol (HPLC grade) was bought from Sino Pharmaceuticals Co. Ltd.

2.2 Synthesis of PMAA–CADB

Three PMAA precursor blocks were synthesized with targeted degrees of polymerization (DPs) of 25, 100, and 500. For example, the synthesis of PMAA₂₅ was performed as follows: MAA (1.6142 g, 18.75 mmol), CADB (209.5 mg, 0.75 mmol), ethanol (4.6331 g) and V-65 (37.3 mg, 0.15 mmol) were added to a 50 ml Schlenk flask with a magnetic bar. The mixture was degassed three times using freeze–evacuate–thaw cycles. The flask was then placed in an oil bath preheated to 65 °C. After polymerization for 6 h, the reaction mixture was cooled down to room temperature using ice-water. The product was purified twice as follows: the polymer solution was poured into excess anhydrous ether to precipitate the polymer, successively centrifuged and dried in the vacuum oven at room temperature. The resultant product was stored in the refrigerator (0 °C) until use. The conversion of MAA was determined gravimetrically. PMAA₁₀₀ and PMAA₅₀₀ were obtained by adjusting the molar ratio of MAA/CADB.

2.3 Preparation of PMAA-b-PMMA

In order to control the variation of initial conductivity, the mass concentration of PMAA–CADB was kept constant at 2.5 wt% and the molar ratio of [CADB end groups]/[V-65] was 5.0.

The RAFT polymerization was conducted in a four-necked 25 ml flask equipped with a thermometer, a condenser (also an outlet of N₂), an inlet of N₂ gas, and an inlet for the insertion of an electric conductometer probe. After all the ingredients except for V-65 were charged into the flask, the reaction mixture was degassed by purging nitrogen at room temperature for 30 min. After the temperature of the system was elevated to 70 °C, V-65 (dissolved in ethanol) was injected into the flask with a syringe. This moment was regarded as the start of polymerization. The polymerization was performed for 20 h.

2.4 Preparation of MAA/MMA random copolymer

The procedure for the polymerization is common and is described elsewhere.³⁶

2.5 Characterization

All proton nuclear magnetic resonance (¹H NMR) spectra were recorded using a Bruker HW500 MHz spectrometer (AVANCE AV-500) with DMSO or CDCl₃ as the protonized solvent. Conductivity was recorded with a digital electric conductometer (DDS-308A). The conductivity change for the system was recorded automatically throughout the polymerization. The morphology of the particles or beads was investigated using scanning electron microscopy (SEM). The measurements and preparation of the samples were described in our previous paper.³⁶

3. Results and discussion

3.1 Characterization of PMAA–CADB and the block copolymers

The actual degrees of polymerization (DPs) of PMAA–CADB and PMAA-b-PMMA are calculated from the ¹H NMR spectra by employing a common method as reported before.^19–23 However, for samples of PMAA₁₂₂-b-PMAA₁₂₂₀, PMAA₁₂₂-b-PMMA₂₄₄₀ and the PMAA₄₅₀-b-PMMA_x series (x = 45–4500, where x stands for the theoretical DP of a PMMA block prepared using PMAA₄₅₀ macro-CTA), a deuterium solvent suitable to dissolve the block copolymers was not found, possibly due to the long chains of two blocks. Therefore, these block copolymers were not characterized with NMR. And also, no solvent was suitable for the measurement of size exclusion chromatography.

Typical ¹H NMR spectra for PMAA–CADB and copolymers are shown in Fig. 1. As shown in Fig. 1a, the proton signals of CADB agree well with its structure. Multipeaks, j′′–l′′, are assigned to the protons of the phenyl group. Peaks a′′ and b′′ indicate the protons of the methylene groups next to the carboxyl group. Peak c′′ belongs to the protons of the methyl group. In order to calculate the actual DP of PMAA–CADB (Fig. 1b), the integral areas of peaks j′–l′ (5 aromatic protons of CADB) and e′ (3 methyl protons of MAA) were selected as the representative numbers of CADB and MAA units, respectively. Similarly, for the block copolymer, the integral area of peak i (3 methyl protons of the MMA ester) was selected to calculate the number of MMA repeat units (Fig. 1c). Conversions of monomer were determined gravimetrically. All polymerization data are summarized in Table 1.

Fig. 1 Typical ¹H NMR spectra for (a) CADB, (b) PMAA–CADB and (c) PMAA-b-PMMA.

Table 1 Number of units in PMAA–CADB and PMAA-b-PMMA

MAA (theory)^a	MAA (NMR)^b	MMA (theory)^a	MMA (NMR)^b	Monomer conversion (%)
a The theoretical number.b Number of units calculated using NMR.
25	30			93.1
100	122			98.0
500	450			93.3
30		75	67	91.3
30		100	98	99.4
30		200	188	93.7
122		80	76	95.0
122		200	178	94.7
122		250	230	89.6

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As shown in Table 1, the calculated number of monomer units is close to the theoretical number, which is indicative of good polymerization control in the PMAA/MMA RAFT dispersion polymerization.

3.2 Electric conductivity of RAFT polymerization systems

The abnormal increase of electric conductivity in RAFT polymerization systems was contradictory to the common knowledge. Various factors including the effects of CADB, V-65, water and temperature were checked in our previous paper.^12,36,40 The conclusion was drawn that the abnormal increase of conductivity resulted from the soluble diblock copolymers. In order to further elucidate the mechanism of high conductivity, the electric conductivity of the RAFT polymerization of PMAA–CADB and MMA was investigated for the first time.

The variation of conductivity was traced during the RAFT dispersion polymerization of MMA using PMAA–CADB as a macro-transfer agent and V-65 as an initiator. The results are shown in Fig. 2. As a comparison, the variation of conductivity during the random copolymerization of MAA/MMA is also exhibited in Fig. 2d. A common phenomenon was obvious that, in RAFT polymerization systems, the conductivity dramatically increased soon after the polymerization started, regardless of the reaction components. The random and diblock copolymers of MAA/MMA were checked using cyclic voltammetry (CV). No difference was found in the CV diagrams of the random and diblock copolymers of MAA/MMA, and no peak was found at least in the voltage range from −1.0 V to +1.0 V. This result suggested that no redox reaction occurred on the electrodes of the conductometer. In fact, if polymerization induced by electron transfer took place on the electrodes, the conductivity should decrease since the layer of copolymer covering the electrode was neither an electron nor ion conductor. In ethanol, the dissociation degree of PMAA was very low. The conductivity continuously increased with the evolution of the polymerization, thus the conductivities obtained at the end of the polymerization were much higher than those of the random copolymerization systems (Fig. 2d). However, some exceptions were observed, e.g. curve d in Fig. 2b, and curves d–g in Fig. 2c. The increase in the conductivity for these samples was insignificant, thus the final conductivities were close to those of the random copolymerization systems. Reminiscent of experimental phenomena, it was observed that the solution became turbid during the polymerization. The so-called polymerization-induced self-assembly occurred for the reactions with a high MMA content. This result further confirmed that, as reported previously,³⁶ the high conductivity was attributed to the soluble PMAA-b-PMMA, rather than the precipitated species.

Fig. 2 Variation of conductivity of MAA/MMA during (a–c) RAFT dispersion polymerization and (d) random copolymerization.

In the case of PMAA₃₀ and PMAA₁₂₂, the conductivity increased as the block length of PMMA increased, whereas, for the PMAA₄₅₀ series, conversely it decreased. It is shown that the conductivity depended on the block length of both PMAA and PMMA. In principle, the conductivity is determined by two factors, i.e. the charge number (or charge density for colloidal particles) and the mobility of the species. Accordingly, a rational explanation was that the charge number of the diblock copolymer increased along with the chain extension of the PMMA block. It was hard to consider that the mobility increased as the PMMA block increased the length of the diblock copolymer, if PMMA was considered as a neutrally charged block. Additionally, the high conductivity was not observed in the pure solution of PMAA–CADB because all the conductivities of PMAA–CADB were almost constant as the temperature increased, regardless of the chain length. And also, upon mechanically mixing PMAA–CADB and PMMA, the conductivity of mixture was still low. As shown in Fig. 2, the final conductivity was ordered as follows: PMAA₁₂₂ series > PMAA₃₀ series > PMAA₄₅₀ series. The mobility of the diblock copolymer decreased as the PMAA block length increased. The longer soluble PMAA chain resulted in a bigger solvated coil. This was also the reason that particles or beads gave a very low conductivity. The balance of the charge number and the size effect resulted in the above order of conductivity.

Why did the solvophobic block of PMMA cause high conductivity? The reason is still unclear, but it was a fact that the soluble diblock copolymer was charged. Fig. 3 shows the tests of the soluble diblock copolymer in a DC field at room temperature. It was clear that the soluble PMAA₄₅₀-b-PMMA_x (x = 45, 90, 225 and 450 units) moved to the negative side of the carbon electrode and deposited onto it. This meant that the soluble diblock copolymers were positively charged. In addition, the electro-deposition was reversible because the diblock copolymers re-dissolved after unloading the DC field. The electro-deposition of pure PMAA–CADB was not found on any electrode, which suggested that PMAA–CADB was electrically neutral. It should be noted that PMAA–CADB can be irreversibly deposited on the cathode (negative side) due to the insoluble PMAA potassium salt formed when the KCl/ethanol solution is added. The PMAA potassium salt precipitated to form charged colloidal particles. However, the difference was obvious that PMAA₄₅₀-b-PMMA_x (x = 45, 90, 225 and 450 units) was soluble in ethanol at room temperature and its electro-deposition was reversible. Hence, a unique rational explanation was that the PMMA blocks were positively charged. According to the ions possibly existing in the system, a deduction was that PMMA blocks adsorbed H⁺ dissociated from the carboxylic acid. This result is coincident with that observed in the PMAA-b-AAm and PMAA-b-PS systems denoted in Scheme 1.³⁶

Fig. 3 Behaviors of PMAA₄₅₀-b-PMMA_x block copolymers in a DC field at room temperature for (a) x = 45, (b) x = 225, and (c) x = 450.

Additionally, it should be pointed out that such a phenomenon was not observed in the systems of PMAA₃₀-b-PMMA_y (y = 67, 98, 188) and PMAA₁₂₂-b-PMMA_z (z = 76, 178, 230), even though, as shown in Fig. 2, the conductivities of these systems were high at the polymerization temperature, 70 °C. This result was not contradictory to the above one because as discussed below, the high conductivity led to the thermo-induced self-assembly of the diblock copolymers at low temperature. Finally, as previously reported,³⁶ this phenomenon was not found in the random copolymerization systems, possibly due to the random distribution of charges along the copolymer chain.

3.3 Self-assembly behavior of block copolymers

In the RAFT dispersion polymerization of PMAA–CADB/MMA, polymerization-induced self-assembly emerged, namely that objects were directly formed during the RAFT polymerization, but only in the cases with a high content of MMA, e.g. PMAA₁₂₂-b-PMMA₂₄₄₀, PMAA₄₅₀-b-PMMA₂₂₅₀ and PMAA₄₅₀-b-PMMA₄₅₀₀. The content of MMA was as much as 5 or 10 times of the MAA content. As shown in Fig. 2, their conductivities were very low at the polymerization temperature because of self-assembly.

The thermo-induced self-assembly was also observed because the solubility of PMMA in ethanol was thermosensitive. The solubility declined as the temperature decreased. For example, a homopolymerization solution of PMMA (M_n = 25 000, 8 wt% solid content in ethanol by V-65) was transparent at 70 °C, but bulk precipitation appeared when it was immersed in an ice bath. The products were massive PMMA blocks. Moreover, the aggregation of PMMA in ethanol was semi-reversible, so when the temperature was elevated up to 70 °C, only a small proportion of the PMMA blocks was re-dissolved. Since the conductivity was very low during the homopolymerization (curve g in Fig. 2d), it was considered that the formation process of massive PMMA blocks was dominated solely by the solvophobic interactions.

However, for diblock copolymers, the thermo-induced self-assembly behavior was observed only in the solutions of PMAA-b-PMMA with high conductivities. For example, the final conductivity of PMAA₃₀-b-PMMA₁₈₈ was ca. 25 μS cm⁻¹, whilst the final conductivities of PMAA₁₂₂-b-PMMA_z (z = 178, 230, 1220) were higher than 30 μS cm⁻¹ (Fig. 2). As shown in Table 2, their transparent solutions became turbid when they were cooled. In contrast, the thermo-induced self-assembly was not observed in the solutions of PMAA₃₀-b-PMMA₆₇ and PMAA₄₅₀-b-PMMA_x (x = 45, 90, 225), for which the final conductivities were lower than 20 μS cm⁻¹. These results indicated that the thermo-induced self-assembly was closely related to the electrical conductivity of the diblock copolymer. The thermo-induced self-assembly only occurred when the conductivity was higher than 20 μS cm⁻¹. However, an exception existed. In the solution of PMAA₄₅₀-b-PMMA₉₀₀, the thermo-induced assembly also emerged, though its conductivity was only about 4 μS cm⁻¹. This may be ascribed to the long block of PMMA. Moreover, the emergence of thermo-induced assembly was also dependent on the total solid content of the copolymers. As shown in Table 2, the thermo-induced assembly of PMAA₄₅₀-b-PMMA₄₅₀ was not observed at 2.2 wt%, but emerged at 4.3 wt%, which was coincident with that reported in ref. 19–23.

Table 2 Phenomena occurring in the solutions of RAFT dispersion polymerization at 70 °C and 0 °C^a

Block copolymers	70 °C	0 °C
a PMAA122-b-PMMA2440, PMAA450-b-PMMA2250 and PMAA450-b-PMMA4500: precipitation during polymerization.
PMAA30-b-PMMAy (y = 67, 98)	Transparent	Transparent
PMAA30-b-PMMA188	Transparent	Turbid
PMAA122-b-PMMA76	Transparent	Transparent
PMAA122-b-PMMAz (z = 178, 230, 1220)	Transparent	Turbid
PMAA450-b-PMMAx (x = 45, 90, 225)	Transparent	Transparent
PMAA450-b-PMMA450 (solid content: 2.2 wt%)	Transparent	Transparent
PMAA450-b-PMMA450 (solid content: 4.3 wt%)	Transparent	Turbid
PMAA450-b-PMMA900	Transparent	Turbid

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Furthermore, the thermo-induced assembly was reversible. An example was PMAA₁₂₂-b-PMMA₁₇₈ which self-assembled at 0 °C but the self-assembled objects re-dissolved at room temperature. Fig. 4a shows a typical variation in the conductivity during the processes of RAFT dispersion polymerization and cooling down (PMAA₄₅₀-b-PMMA₄₅₀, 4.3 wt%). Point A refers to the moment of adding V-65; B is the end of polymerization at 70 °C; C is at 0 °C; and D is at room temperature. It can be seen that the conductivity greatly decreased along with the self-assembly of soluble copolymers (B to C). The process was reversible. As shown in Fig. 4b, when the above solution containing the precipitated objects was heated up, the conductivity greatly increased as the temperature rose from room temperature (point A) to 70 °C (point B). Meanwhile, the precipitated objects gradually dissolved. When the solution was cooled down in an icy water bath (point C), the conductivity decreased drastically and the self-assembly occurred. These results re-confirmed that, at first, the soluble diblock copolymers did give rise to the high conductivity. When the soluble diblock copolymers were incorporated into the self-assembled objects, the charges disappeared, thus the conductivity of the solution was very low. This explained the results obtained in the DC field (Fig. 3), in that the samples with high conductivities were not electro-deposited on the electrodes at room temperature, and indicated the important role of the electrostatic force in the process of thermo-induced self-assembly.

Fig. 4 Variation of conductivity in the process of (a) RAFT polymerization and (b) heating up.

3.4 Morphologies of self-assembled objects

Armes et al.^19–21 reported that the morphologies of polymerization-induced self-assembly objects were affected by the length of the solvophobic block and the total solid content of the RAFT polymerization system. With the increase of the solvophobic chain length or solid content, the morphologies of the objects evolved from spheres to worms, and then to vesicles. However, in the present system, as discussed above, the polymerization-induced assembly only emerged in three systems, i.e. PMAA₁₂₂-b-PMMA₂₄₄₀, PMAA₄₅₀-b-PMMA₂₂₅₀ and PMAA₄₅₀-b-PMMA₄₅₀₀, where the content of MMA was at least 5 times as much as that of MAA. In Fig. 5e and f, we observed objects with a predominant spherical morphology with a wide size distribution and an approximate size of ca. 0.2 μm to 5 μm. Since the samples were prepared and observed at room temperature, it was inadvisable to exclude the possibility of the emergence of thermo-induced self-assembly during the cooling process of the polymerization system. Hence, it was hard to determine which objects in Fig. 5e were formed by polymerization-induced assembly. However, all were spheres, except for the beads. As shown in Fig. 5f, some concave–convex morphologies were found on the surface of the beads. This implies that the initial big beads contained some volatile liquid or the distribution of the PMAA was not uniform on the surface. Therefore, if the smaller particles (Fig. 5e) resulted from the thermo-induced self-assembly of soluble diblock copolymers, it might be concluded that the vesicles (beads) were formed by the polymerization-induced self-assembly. However, according to the mini/micro-monomer droplets particle formation mechanism,^13,36 these beads were prepared by the polymerization within the monomer droplets. It might be a virtue of polymerization-induced self-assembly.

Fig. 5 Representative SEM micrographs of self-assembled objects formed by a series of PMAA-b-PMMA block copolymers for (a) PMAA₃₀-b-PMMA₁₈₈, (b) PMAA₁₂₂-b-PMMA₂₃₀, (c) PMAA₁₂₂-b-PMMA₁₂₂₀, (d) PMAA₄₅₀-b-PMMA₉₀₀, and (e) PMAA₄₅₀-b-PMMA₂₂₅₀; (f) shows a magnified image of the PMAA₄₅₀-b-PMMA₂₂₅₀ beads in (e).

The objects formed purely by thermo-induced assembly are shown in Fig. 5a–d. Since the samples were prepared through freeze-drying at −15 °C and then studied by means of SEM at room temperature, fusion of the objects seemed to happen. This may be a feature of aggregation induced by the electrostatic interactions. Nevertheless, the species shown in Fig. 5a seemed assignable to the aggregates of worms, whilst those in Fig. 5d could be assigned to a mixture of spheres and worms.

Finally, since the diblock copolymers were charged, the ion-induced self-assembly was conducted for those solutions of PMAA-b-PMMA where neither polymerization- nor thermo-induced self-assembly emerged. These were samples of PMAA₃₀-b-PMMA_y (y = 67, 98), PMAA₁₂₂-b-PMMA_z (z = 76) and PMAA₄₅₀-b-PMMA_x (x = 45, 90, 225). A saturated solution of KCl/ethanol was added to these solutions of diblock polymers at room temperature (1 to 1 in volume), and then the samples were put into the DC field. Generally, block copolymers were deposited on the negative electrode and formed a membrane (Fig. 6a). This meant that the copolymers were positively charged due to the generation of a PMAA potassium salt. However, PMAA₁₂₂-b-PMMA₁₇₈ was special. As shown in Fig. 6b, discuses were observed on the positive electrode, so were negatively charged objects. This result indicated that the discuses probably come from the deformation of vesicles. Abnormally, PMMA may dominate the surface of the discuses rather than PMAA. This implies that the ion-induced self-assembly of PMAA-b-PMMA was complicated.

Fig. 6 PMAA₁₂₂-b-PMMA₁₇₈ objects induced by KCl on the electrodes: cathode (negative, a); anode (positive, b).

4. Conclusions

In conclusion, it was confirmed that the conductivity abnormally and dramatically increased soon after the RAFT dispersion polymerization of PMAA–CADB/MMA started. The high conductivity was caused by the soluble diblock copolymers. The increase of conductivity was dependent on the block length of PMAA and PMMA. For the PMAA₃₀ and PMAA₁₂₂ series, the conductivity increased with the increase of the PMMA block length, whereas for the PMAA₄₅₀ series, it was reverse. As a comparison, the final conductivity was ordered as follows: PMAA₁₂₂ series > PMAA₃₀ series > PMAA₄₅₀ series. It was considered that an increase of both PMAA and PMMA block length decreased the mobility of the diblock copolymer, thereby decreasing the conductivity. However, an increase in the PMMA block length increased the charge number of PMAA-b-PMMA. Hence, the final conductivity was the balanced result of the mobility and the size effect. The soluble PMAA-b-PMMA was positively charged, indicating that the PMMA block had positive charges. Therefore, the electro-deposition of diblock copolymers was reversible.

Three types of self-assembly were observed in the RAFT dispersion polymerizations, i.e. polymerization-, thermo- and ion-induced self-assembly. Polymerization-induced self-assembly emerged only in three systems, i.e. PMAA₁₂₂-b-PMMA₂₄₄₀, PMAA₄₅₀-b-PMMA₂₂₅₀ and PMAA₄₅₀-b-PMMA₄₅₀₀, where the content of MMA was at least 5 times as much as that of MAA. The main products possibly were big beads with concave–convex morphologies on the surface, though it was not distinguishable due to the existence of possible thermo-induced self-assembly objects. Thermo-induced self-assembly was reversible, and was only observed in the systems with final conductivities higher than 20 μS cm⁻¹. The products were spheres or worms. As for ion-induced self-assembly, negatively charged discuses were observed in the PMAA₁₂₂-b-PMMA₁₇₈ solution. The procedure of self-assembly commonly resulted in the dramatic decrease of conductivity.

Acknowledgements

This project is supported by Natural Science Foundation of China (NSFC 51541302) and the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD). All the experiments and data analysis were conducted by Ms Junxiu Liu.

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