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Selection Principle of RAFT Chain Transfer Agents and Universal RAFT Chain Transfer Agents

Hang Yin , Zhi Li , Xiaofeng Guo , Anchao Feng , Liqun Zhang , San Hoa Thang

Progress in Chemistry ›› 2022, Vol. 34 ›› Issue (6) : 1298 -1307.

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Progress in Chemistry ›› 2022, Vol. 34 ›› Issue (6) : 1298-1307 DOI: 10.7536/PC210736
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Selection Principle of RAFT Chain Transfer Agents and Universal RAFT Chain Transfer Agents
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Abstract

Reversible Addition-Fragmentation Chain Transfer Polymerization (RAFT Polymerization) is currently one of the most common “Reversible Deactivation Radical Polymerization”, and it has been widely used by scientists in different directions because of its advantages such as narrow molecular weight distribution, wide range of applicable monomers, and mild reaction conditions. However, when scientists choose RAFT chain transfer agents (also known as RAFT agents) in polymerization, they often donot clearly understand the principle of matching the activity of RAFT chain transfer agents and monomers. Therefore, in the preparation of block copolymers of “more activated” monomers (MAMs) and “less activated” monomers (LAMs), there are problems that the product has a wide molecular weight distribution, a slow polymerization rate, and even the reaction cannot successfully continue. Based on this, we first review the selection principles of RAFT chain transfer agents in polymerization, and then introduce the principle and application conditions of a universal RAFT chain transfer agent (Universal RAFT agent) (including non switchable type and proton switchable type) developed in recent years that is suitable for the polymerization of MAMs/LAMs. Finally, the latest development and application of block copolymers with MAMs and LAMs based on Universal RAFT agents are discussed in depth.

Contents

1 Introduction

2 Structure of RAFT agents and selection principle

3(Non Switchable)Universal RAFT agents

4 Proton Switchable RAFT agents

4.1 Chain transfer kinetics

4.2 Application of Switchable RAFT agents in aqueous solution

4.3 Effect of acids’type and amount on Switchable RAFT agents’activity

4.4 Removal method of end group

4.5 Block copolymer realized with Switchable RAFT agents

5 Conclusions and outlook

Graphical abstract

Keywords

RAFT polymerization / universal RAFT agent / block copolymers / self-assembly

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Hang Yin, Zhi Li, Xiaofeng Guo, Anchao Feng, Liqun Zhang, San Hoa Thang. Selection Principle of RAFT Chain Transfer Agents and Universal RAFT Chain Transfer Agents. Progress in Chemistry, 2022, 34(6): 1298-1307 DOI:10.7536/PC210736

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1 Introduction

Reversible Addition-Fragmentation Chain Transfer Polymerization (RAFT) was first proposed by Ezio Rizzardo at the 37th International Polymer Congress in 1998.It is a Reversible Deactivation Radical Polymerization, also known as "living"/controlled Radical Polymerization, which has developed into one of the most versatile and powerful polymerization techniques for the synthesis of polymer systems with complex topological structures due to its mild reaction conditions, wide range of monomer choices, and strong molecular design capabilities[1][2]. Compared with the traditional free radical polymerization, the RAFT polymerization not only retains the versatility of the traditional free radical polymerization, but also solves the problem that the molecular weight and macromolecular structure can not be well controlled by the traditional radical polymerization, so the polymer prepared by the RAFT polymerization has the advantages of controllable molecular weight, narrow molecular weight distribution and easy realization of chain end functionalization[2]. The main principle of RAFT polymerization is shown in Figure 1. The RAFT reagent as a Chain Transfer Agent (CTA) is added to the free radical polymerization to protect the free radicals that are easy to terminate by Chain Transfer, so that most of the free radicals in the polymerization reaction are converted into dormant free radicals, and the dormant chain segment and the active chain segment coexist in the reaction process.And continuously perform rapid mutual switching through a dynamic reversible reaction, so that only a few polymer chains exist in the form of active chains and grow at any time,Finally, the growth probability of each polymer chain segment is approximately equal, and the characteristics of living polymerization are shown (the molecular weight increases linearly with the conversion rate, the molecular weight distribution is narrow, and the block copolymer can be prepared)[3,4].
In the more than 20 years since its birth, RAFT polymerization has received extensive attention. The research has changed from the initial focus on clarifying the polymerization mechanism to the polymer structure and functional materials available in the polymerization process, as well as more recent application-driven research[2,5]. As a polymerization method, RAFT polymerization has good compatibility with the introduction of external stimuli, so the reaction kinetics can be controlled and adjusted by the effect of external stimuli on initiators and chain transfer agents, such as light, microwave, oxygen, pH value, etc[6][7][8][9][10,11]. In terms of application, RAFT polymerization is mainly used to synthesize well-defined homopolymers, gradient copolymers, multi-block polymers, star polymers, dendrimers, polymer brushes and other rich microstructures[12][13][14][15,16][17][18]. The RAFT polymerization process can tolerate a variety of functional groups, and the end groups of the product can be functionalized, so it can be used as a means of polymer multi-functionalization and material assembly, and has a wide range of applications in the direction of polymerization-induced self-assembly[19][20,21][13,22⇓⇓ ~25]. In addition, with the in-depth study of RAFT polymerization, the scope of its research is constantly expanding, from polymer structure, such as the application of RAFT polymerization in random polymers, to polymerization methods, such as RAFT emulsion polymerization, and even to polymerization mechanism, such as RAFT stepwise polymerization, RAFT polymerization continues to show great research value[26][27][28].
At the same time, the application of RAFT aggregation as a powerful research tool in the scientific community is also expanding, including 3D printing technology, nanotechnology and biomedicine[29][30][30,31].
As a polymerization method with obvious advantages, RAFT polymerization has been widely used in research by more and more researchers. However, RAFT polymerization requires the selection of the corresponding RAFT reagent according to the activity of the monomer used. Although RAFT polymerization has been widely used as a scientific tool, there are few detailed Chinese reviews on RAFT reagents and their selection principles in recent years. We hope to simplify the process of selecting RAFT reagents through this review, and solve the problem that many researchers find that RAFT reagents do not match the activity of monomers in the process of selecting RAFT reagents, resulting in the failure to complete the control of molecular weight and molecular weight distribution, and may even hinder the reaction. Based on this, we would like to first review the selection principles of RAFT reagents in polymerization, so as to provide a reference for researchers to select RAFT reagents according to monomer activity. At the same time, the action principle and application conditions of a class of universal RAFT reagents (including non-switching and acid-base switching) developed in recent years, which are suitable for the polymerization of high/low activity monomers, are introduced, and the progress and application of block copolymers based on universal RAFT reagents for high/low activity monomers are emphatically discussed.

2 Structure of RAFT Reagent and Its Selection Principle

The structure of the RAFT reagent can be considered to consist of a dithioester with an active group (Z group), a leaving group (R group) (Fig. 2).
The type of Z group is closely related to the activity of RAFT reagents, so RAFT reagents are usually classified according to the type of Z group, and different types of RAFT reagents have different activities. Common RAFT agents include certain Dithioesters, Trithiocarbonates, Xanthates, dithiocarbamates, and others (Figure 2).
RAFT agents in which the Z group is an alkyl, aryl, or aralkyl group are known as dithioesters, and dithiobenzoates and similar dithioesters in which the Z group is an aryl group are generally the most active RAFT agents,It provides optimal control for highly reactive monomers (MAMs), almost completely inhibits the polymerization of low reactive monomers (LAMs), and has general applicability in the polymerization of acrylic acid (AA), styrene (St) and acrylamide monomers. Trithiocarbonate, with the Z group as a sulfanyl group, is slightly less active than dithioester RAFT reagents, but is still able to control the polymerization of acrylate monomers and St well. The Z group of dithiocarbamate RAFT reagent is amino, which has good control effect on vinyl monomers such as vinyl acetate (VAc), N-vinyl pyrrolidone (NVP), N-vinyl carbazole (NVC), and has relatively low activity in the polymerization of St and acrylic monomers. RAFT agents with alkoxy and aryloxy groups in the Z group are xanthates, which have been widely used in the RAFT polymerization of VAc, NVP, and other vinyl monomers, and are generally inefficient (low chain transfer coefficient) in the polymerization of St and acrylate monomers, failing to control methacrylate polymers[32]. Therefore, in the practical application of RAFT polymerization, it is very important to correctly select the Z group to adapt its activity to the whole reaction. The effect of common Z groups on the activity of RAFT reagents is shown in Figure 3 (B). In addition, the Z group also has a certain influence on the nucleation step in emulsion RAFT polymerization[33].
The R group acts as a leaving group for RAFT agents, in most cases a group linked through a carbon[34]. In RAFT polymerization, the ability of the S — R bond of the RAFT reagent to promote living radical polymerization depends largely on the leaving ability of the R group. Reasonable selection of the R group is also very important for more efficient initiation of polymerization. The selection principle of common R groups is shown in Figure 3 (a). In addition, the choice of R group also has a significant impact on the amount of RAFT reagent[34,35].
In addition to common RAFT reagents, there are some RAFT reagents with other elements, such as fluorine-containing RAFT reagents, phosphorus-containing RAFT agents, etc[36,37][38].
In RAFT polymerization, the selection of RAFT reagents should be based on the activity of monomers, and the corresponding RAFT reagents should be selected according to the activity of monomers. In radical polymerization, MAMs and LAMs are usually divided according to the activity of monomers[39]. MAMs refer to monomers whose double bond is bonded to AN aromatic ring (e.g., St, 4-vinylpyridine), a carbonyl group (e.g., methyl methacrylate (MMA), methyl acrylate (MA), acrylamide (AM)) or a cyano group (e.g., acrylonitrile (AN)). LAMs are monomers whose double bond is attached to a saturated carbon (such as Dimethyldiallylammonium chloride), an oxygen or nitrogen lone electron pair (such as VAc, NVP), or a heteroatom on a heteroaromatic ring (such as NVC)[40]. Compared with LAMs, MAMs are more easily attacked by free radicals, and the free radicals formed have good stability and low activity, which leads to the relatively high activity of RAFT reagents suitable for controlling the polymerization of MAMs in RAFT polymerization, which will inhibit or strongly delay the polymerization of LAMs. However, RAFT reagents suitable for controlling the polymerization of LAMs have low reactivity to radicals with terminal MAMs units, so they can not effectively control the polymerization reaction. Therefore, in RAFT polymerization, the leaving group generated by the conversion of the dormant species structure to the active species structure must be a free radical with an activity similar to that of the free radical formed by monomer polymerization. However, in the synthesis of MAMs and LAMs block copolymers, the activities of the two monomers are different, and a single RAFT agent cannot match the activities of the two monomers respectively, so it is impossible to prepare MAMs and LAMs block copolymers with low molecular weight distribution by continuous monomer addition[41]. For most polymerizations, there are two ways to solve this problem: the first is to prepare polymers with different active monomers separately, and the block copolymers are prepared by end-group coupling; Another method is post-modification, in which after the preparation of the first monomer segment, the RAFT end group is modified to change its activity, and then the second monomer is polymerized. Although the preparation of high/low reactivity monomer block copolymers can be accomplished by these two methods, the reaction process is very cumbersome[42]. Therefore, before the Universal/Switchable RAFT Agent was discovered and defined by Moad et al. In 2009, the preparation of MAMs and LAMs block copolymers with low dispersion using a conventional RAFT Agent was a major problem in RAFT polymerization[14].

3 (Non-switching) universal RAFT chain transfer agent

Xanthates and dithiocarbamates are less effective than dithioesters and trithiocarbonates in MAMs polymerization, so they are rarely used in MAMs polymerization. However, some of them have shown the ability to control the polymerization of both MAMs and LAMs separately, and these intermediate RAFT agents can effectively control the reaction when polymerizing monomers with different activities[2,43]. However, when they control the polymerization of different active monomers, the control effect on one or two monomers is often not good. Although the molecular weight of the block copolymer can be controlled, the MAMs segment of the product often has a high molecular weight distribution, or plays a certain role in retarding the polymerization of LAMs in the polymerization process[44,45].
Girard et al. Used methyl 2-mercaptopropionate-o-ethyl dithiocarbonate (Fig. 4, 1) as a RAFT reagent to control N, N-dimethylacrylamide (DMA) first, and then control vinylidene fluoride for chain extension to synthesize amphiphilic block copolymers with fluorine-containing backbone, which provided an interesting direction for the development of new polymer surfactants[46]. Dayter et al. Prepared styrene-vinyl acetate block copolymers by continuous monomer addition using N, N-diphenyldithiocarbamate malonate (Fig. 4, 2) as a RAFT reagent[47]. The RAFT agent exhibits good overall control over the preparation of block copolymers formed from MAMs and LAMs. If the molecular weight distribution of the target polymer needs to be further reduced, the Z group of the RAFT reagent needs to be adjusted. For example, cyanomethyl (3,5-dimethyl-1-H-pyrazole) -carbodithioate (fig. 4, 3) has a good control effect on the polymerization of many LAMs and MAMs, such as MA, DMA and St, which provide excellent control of the molecular weight distribution at a very low level (D- < 1.1), and VAc, which provides good control in the polymerization of LAMs (D- < 1.3), but not methacrylates. Gardiner et al. Finally controlled the molecular weight distribution of methyl methacrylate at a lower level (D- < 1.5) by modifying the Z group and the R group with 2- (4-chloro-3,5-dimethyl-1-H-pyrazole-1-carbodithioate) -2-cyanobutane (Fig. 4, 4). [44]
This kind of RAFT agent is called Universal RAFT agent because it can control the polymerization of some different active monomers. When Universal RAFT reagent is used to prepare block copolymers by controlling monomers with different activities, the molecular weight distribution of the product is different from that of the homopolymer of RAFT polymerization, but it is enough to meet the requirements of most industrial applications, and Universal RAFT reagent allows the production of block copolymers of MAMs and LAMs through a simple one-pot method, which significantly simplifies the polymerization process. However, in industrial applications, the high cost of Universal RAFT reagent due to its complex preparation process and purification methods limits its wider application in industry[2]. For example, cyanomethyl (3,5-dimethyl-1-H-pyrazole) -carbodithioate (Figure 4, 3), produced by Boron Molecular, has an official website price of $20,000 per kilogram (December 7, 2021). In addition, Universal RAFT reagent has poor water solubility, so it is difficult to prepare water-based polymers, which limits its application in coatings, adhesives and other aspects[48].

4 Proton-switchable RAFT chain transfer agent

Although (non-switching) Universal RAFT reagents can regulate some MAMs and LAMs in polymerization, it is difficult to broaden their scope of application because of their uncontrollable structure. In order to further explore the preparation of MAMs and LAMs block copolymers, Moad et al. First prepared styrene-vinyl acetate block copolymers using N-methyl-N- (4-pyridyl) dithiocarbamate in 2009, and then proposed Proton Switchable RAFT Agents whose activity can be adjusted by pH.Switchable RAFT reagent), the electron distribution and molecular structure of the pyridine structure on the active group of the RAFT reagent can be adjusted by changing the pH value of the environment, thereby changing the activity of the RAFT reagent as CTA and further broadening the range of applicable high/low activity monomers[49].
The Switchable RAFT reagents found in the current study mainly belong to the dithiocarbamate class, which has a low chain transfer coefficient, and this low activity can be attributed to the typical zwitterionic structure present in these compounds, which significantly reduces the double bond activity of the thiocarbonyl group. Dithiocarbamates are often used as chain transfer reagents for LAMs in RAFT polymerization because the nitrogen lone pair is difficult to shift in the aromatic structure. In the Switchable RAFT system, the polar group on the Z group forms a nitrogen lone pair electron with the hydrogen ion free from the system in the protonic acid or Lewis acid, which can enhance the chain transfer coefficient of the RAFT reagent. In the non-protonated state, the C = S double bond energy in the Switchable RAFT reagent is low due to the influence of the zwitterionic form composed of the positive charge on the nitrogen atom and the negative charge on the sulfur atom, and after the protonation of the conjugated nitrogen,A structure in which the nitrogen lone electron pair is connected to the electron-withdrawing group or the nitrogen lone electron pair is a part of the aromatic ring is formed, so that the bond length of the C = S double bond is reduced and the bond energy is enhanced (structures 2 and 4 in fig. 5), and finally the activity of the compound as a CTA is improved, and the polymerization of a monomer with higher activity can be effectively controlled. Compared with the coupling method and the end group modification method, the Switchable RAFT reagent is more simple and feasible to prepare the high/low activity monomer block copolymer. In the polymerization process, the Z group of a specific RAFT reagent is protonated, so that the Z group of the specific RAFT reagent is mutually converted into a protonated state and a non-protonated state, and the non-protonized state has a lower chain transfer coefficient, so that the polymerization of LAMs can be well controlled; The protonation of the Switchable RAFT reagent is realized by a simple process of adding protonic acid or Lewis acid, and the Switchable RAFT reagent has a higher chain transfer coefficient and can effectively control the polymerization of MAMs with higher relative activity, so that the MAMs/LAMs block copolymer with narrow molecular weight distribution is finally prepared by continuously adding different monomers by using one RAFT reagent in the RAFT polymerization reaction[51].
Subsequently, Moad et al. Further elaborated the mechanism of action of Switchable RAFT reagent system in 2011, and developed new reagents with different R groups[52]. Molecular orbital calculations were used to calculate the energy difference between the highest occupied molecular orbital and the lowest occupied molecular orbital and the change of C = S bond length, and then combined with the efficiency of protonated dithiocarbamate in controlling the polymerization of MAMs, the importance of the presence or absence of nitrogen charge in the RAFT reagent was proved. Relative to the non-protonated form, the protonated RAFT reagent has a shorter C = S bond length, a smaller energy level difference, and a reduced partial charge on the thiocarbonyl sulfur. The protonated dithiocarbamate controls the polymerization of MAMs, the chain transfer coefficient increases, the activity changes, and the charge on the nitrogen atom of the dithiocarbamate plays a decisive role. Based on the above research, scientists have carried out a series of studies on its protonation mode, monomer and RAFT reagent selection, polymerization conditions and so on.

4.1 Chain transfer kinetics

Keddie et al. Reported the effect of structural changes in Switchable RAFT reagents on their reactivity[50,53]. Switchable RAFT reagents with different Z groups were synthesized by modification of dithiocarbamate, and used to control monomers such as MA, NVC and VAc.It was found that the apparent chain transfer coefficient increased with the decrease of thiocarbonyl electron density, and the chain transfer coefficient of dithiocarbamate for MA monomer was related to its Hammett coefficient (acidity coefficient), indicating a predictable relationship between the structure of RAFT agent and its controlled polymerization activity.
Different Z-based and R-based Switchable RAFT reagents are shown in fig. 5, and the naming convention described later is R '1R1 for the R'=H,R=CH2CN in the RAFT reagent, and so on. R '1R1-R' 7R1 in the protonated state has a good control effect for MA, R '2R1, R' 3R1 and R '4R1 for VAc, and R' 5R1 or R '6R1 for NVC. Dithiocarbamates with more electron-withdrawing substituents were finally proved to be more active[50].
Later, Keddie et al. Continued to study the effect of the Z group on the RAFT polymerization of NVP controlled by dithiocarbamate[54]. Since the effect of the final control of polymerization in RAFT polymerization is determined by both the monomer and the RAFT agent, the study of the activity of the RAFT agent needs to be discussed in conjunction with the corresponding monomer. Keddie et al. Continued to study several Switchable RAFT reagents with different substituents (R), ranked their activities and used them to control the polymerization of NVP. RAFT reagents with relatively low activity (R '2R1 and R' 7R1) can achieve excellent control of NVP, and RAFT reagents with moderate activity (R '3R1 and R' 4R1) can also achieve a certain degree of control of NVP, while the polymerization of NVP is not controlled by R '5R1 or R' 6R1 due to the inhibition of polymerization by the high activity of RAFT reagents. When R is methoxy (R '2R1), the polymerization control effect of NVP is the best. By continuing the chain extension of NVP, it was proved that the RAFT end group in the polymer was well preserved, and it was also proved that the RAFT reagent (R '2R1) with R as methoxy group could achieve the best control of NVP. This work further demonstrates that structural differences alter the activity of RAFT reagents and significantly affect the control of polymerization.

4.2 Application of Switchable RAFT reagent in aqueous phase

Aqueous RAFT polymerization has been an important research direction because of its potential environmental benefits, the limited solubility of monomers with cationic, anionic, zwitterionic and neutral polar groups in organic media and the need for RAFT polymerization under physiological conditions[50]. Keddie et al. First used Switchable RAFT reagent to successfully realize the RAFT polymerization of DMA with high activity in water phase, and obtained PDMA with narrow distribution and high end group retention rate, and then used it as macromolecular RAFT reagent and used NVP, VAc and NVC for chain extension, respectively, to prepare narrow distribution block copolymers P (DMA-co-NVP), P (DMA-co-NVC) and P (MDA-co-VAc)[48].
Most polymers of LAMs prepared by RAFT polymerization are not stable in the aqueous phase (especially NVP). This is due to the instability of the macromolecular RAFT reagent formed after the polymerization of LAMs, especially under acidic conditions, which is the inevitable result of the protonation regulation of Switchable RAFT reagents. Therefore, the current aqueous Switchable RAFT polymerization is usually used to prepare MAMs segments, and the resulting macromolecular RAFT reagents are further extended in the organic phase[55].

4.3 Effect of Acid Type and Amount on the Activity of Switchable RAFT Reagent

In the present study, the switching of different activities of Switchable RAFT reagents was achieved by regulating pH. Moad et al. Found that the type and concentration of acid is important for Switchable RAFT systems[52]. In this system, the acid used to form the protonated RAFT reagent should be an appropriate amount of strong acid, and ideally the molar ratio of equivalents of acid to RAFT reagent should be 1:1. The use of excess acid may cause hydrolysis of the RAFT reagent, while the use of less acid or the use of a weak acid (e.g., acetic acid) provides poor control of the system, as evidenced by a wide range of product molecular weight distributions. This conclusion was also successfully demonstrated by the synthesis of PMMA-b-PVAc and PMA-b-PNVC. Additional aprotic Lewis acid (E. G., aluminum triflate) may also provide efficient conversion. Keddie et al. Introduced a Lewis acid (scandium triflate) into the RAFT copolymerization of MA with VAc controlled by Switchable RAFT reagent[57]. The addition of Lewis acid significantly reduced the polymerization rate, changed the copolymerization rate of MA and VAc, and greatly improved the control effect, the molecular weight distribution decreased from 1. 30 ~ 1.40 to 1. 10 ~ 1.30.
The end-group retention of all RAFT copolymers was high using Lewis acid control, demonstrating that the use of pH control did not significantly affect the dithioester of the RAFT reagent. The effects of the type and concentration of acid on the activity of Switchable RAFT reagent were determined by 1H-NMR[13]. The effect of different concentrations of trifluoromethanesulfonic acid (TfOH) on the activity of Switchable RAFT reagent was first determined. The use of less than one RAFT reagent stoichiometry of acid results in incomplete protonation of the RAFT reagent, whereas more than one RAFT reagent stoichiometry of acid results in RAFT reagents showing poor control in dispersibility. In addition, different kinds of acids also have effects on the activity of Switchable RAFT reagent. The effect of three acids with different strengths on the activity of Switchable RAFT reagent was studied at equimolar concentrations of the RAFT reagent, including TfOH, p-toluenesulfonic acid (TsOH), and trifluoroacetic acid (CF3COOH), and the results showed that the TfOH group gave the lowest dispersion of the polymer when polymerizing MAMs. Based on the results, it is concluded that the acid determining the protonation of the RAFT reagent should be strong acid (TfOH), and the optimal ratio of the amount of acid to the amount of RAFT reagent should be equimolar.

4.4 End Group Removal Method

Keddie et al. Have also studied the end group removal of block copolymers controlled by Switchable RAFT agents[21]. Switchable RAFT reagent and its prepared polymers are structurally more stable compared to other RAFT reagents. For example, its high hydrolytic stability allows aqueous polymerization to be carried out in strongly acidic solutions (e.g., pH = 2) without significant loss of end groups. It was found that thermal decomposition could completely desulfurize the polymer synthesized by Switchable RAFT reagent. The decomposition temperature for the removal of the dithioester structure in homopolymers of MAMs and LAMs is strongly influenced by whether the macromolecular R group species is protonated with the Z group pyridine group. The thermal decomposition temperature of the dithioester structure in the protonated RAFT PMAMs is lower, which is consistent with the results of Zhou et al[58]. In addition, the end group decomposition temperature of PLAMs is higher than that of PMAMs, and the thermal stability of the prepared polymer is relatively high for the control process of first polymerizing MAMs and then polymerizing LAMs when using Switchable RAFT reagent, which is of great significance for whether Switchable RAFT reagent can be widely used in the future. At the same time, although PLAMs need higher temperature to remove the end group than PMAMs-H+ or PMAMs, it can be effectively achieved only by heating, which shows that thermal decomposition is a simple and effective method to remove the end group of Switchable RAFT.

4.5 Switchable RAFT polymerization of realized MAMs/LAMs block copolymer

Since the appearance of Switchable RAFT reagent, scholars have used it to prepare a variety of MAMs and LAMs block copolymers, and its mechanism research and selection principles have been gradually improved, which provides a revolutionary change for the preparation of high/low activity block copolymers with low molecular weight distribution by RAFT polymerization. Among the block copolymers completed so far, MAMs include a variety of important monomers in industrial synthesis, such as St, MMA, MAA, etc., while LAMs mainly include VAc, NVC, etc. Table 1 collates block copolymers that have been successfully implemented using this method to date for ease of reference.
In addition, Switchable RAFT reagents have begun to make progress in the field of application. Our research group developed a method for the synthesis of block copolymer PMAA-b-PNVP with wide application value by using Switchable RAFT reagent (Fig. 6)[56]. The block copolymer can self-assemble to form spherical micelles under acidic conditions. The process of micelle formation, disassembly, and reconstitution can be regulated by pH, and thus has potential value in drug/protein delivery applications. Then our group further described the synthesis method of simple and clear synthesis of block copolymer (PBzMA-b-P (DEAEMA-grad-NVP) -b-PNVP) with CO2stimuli-responsiveness by Switchable RAFT polymerization (fig. 7)[13]. A BzMA-based macromolecular RAFT reagent is prepare by using a protonated RAFT reagent, and aft deprotonation, that macromolecular RAFT reagent is copolymerized with comonomers DEAEMA and NVP with different reaction activities to prepare a gradient chain segment so as to control the gradient and a structural domain, and finally the copolymer with a block and gradient composite structure is prepared. Such copolymers can self-assemble in specific solvents to form assemblies with heterogeneous core-shell structures. Under the stimulation of CO2, the gradient copolymer and the block copolymer in the structure have different degrees of response, resulting in the emergence of obvious heterogeneous and separated shell nanodomains. The material has good biocompatibility and CO2stimulus responsiveness, so that the material has potential in drug delivery application, and the method provides a new direction for designing and realizing precise morphological control.

5 Conclusion and prospect

Free radical polymerization can be divided into high activity monomer and low activity monomer according to the activity of monomer, while RAFT polymerization requires that the activity of RAFT reagent is close to that of monomer in polymerization, so it is difficult to prepare high activity monomer and low activity monomer block copolymer with narrow molecular weight distribution in RAFT polymerization. The development of Universal RAFT reagents and the emergence of Switchable RAFT reagents provide solutions to this problem in different directions. Universal RAFT reagents are usually xanthates or dithiocarbamates, which can control two monomers with certain differences in activity, thus solving the problem of difficult preparation of block copolymers. Switchable RAFT reagents are dithiocarbamates, which can change their chain transfer coefficient in polymerization by protonating their active groups, thus changing their activity. In the polymerization, the protonated/non-protonated state of the Switchable RAFT reagent is switched, and the monomers with different activities are controlled respectively, so that the high-activity/low-activity monomer block copolymer with narrow molecular weight distribution is finally prepared. To date, some progress has been made in the preparation of block copolymers using Switchable RAFT reagents. First of all, there is a unified explanation for the mechanism of protonation and activity change. For the study of its activity, the goal of changing its activity according to needs has also been achieved by modifying its structure. Scholars have successfully prepared a variety of block copolymers with low molecular weight distribution by using this reagent.
In the process of RAFT polymerization from laboratory to application, the sulfur-containing end group has a certain degree of influence on the properties of the polymerization product. For example, in the application field of biomaterials, the sulfur content of materials is often required not to be too high. Therefore, it is very important to remove the terminal RAFT reagent of the material. For polymers prepared using Switchable RAFT reagents, the end groups are more stable, especially the Switchable RAFT end groups on the low active segment, which can be retained or modified to form block, star or other structures for further polymerization operations, providing another advantage for the preparation of multiblock copolymers. When the polymer with the target structure is obtained, it can be desulfurized by simple thermal decomposition. For PSt, PNVP and PNIPAM, the Switchable RAFT end group is easily removed; For PMMA and PVAc, the end groups will be partially removed in the process of thermal decomposition, and the process needs to be further studied.
At present, Switchable RAFT reagents, especially their application in aqueous phase, are promising because of their advantages, but the use of Switchable RAFT reagents is still in the laboratory, and there are still many problems to be solved. First of all, the monomer selection is small, because of the use of pH control, the polymerization occurs under acidic conditions, and the environmental selection of basic monomers is limited. Secondly, when Switchable RAFT reagents switch between the two States, deprotonation is usually carried out by adding salt and base, which introduces and accumulates electrolyte in the system, and ultimately affects the performance of the material. Therefore, exploring new clean and green control means besides pH, such as temperature, light and electromagnetic field, to realize the activity control of RAFT reagents, is a very noteworthy direction.

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Funding

Foundation of State Key Laboratory of Organic-Inorganic Composites, Beijing University of Chemical Technology(oic-202103015)

China Petrochemical Corporation(H2019485)

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