Conjugate substitution and addition of α-substituted acrylate: a highly efficient, facile, convenient, and versatile approach to fabricate degradable polymers by dynamic covalent chemistry

Yasuhiro Kohsaka *, Takumi Miyazaki and Keito Hagiwara
Faculty of Textile Science and Technology, Shinshu University, 3-15-1 Tokida, Ueda, Nagano 386-8567, Japan. E-mail: kohsaka@shinshu-u.ac.jp; Tel: +81-268-21-5488

Received 22nd December 2017 , Accepted 27th December 2017

First published on 28th December 2017


Polycondensation of bis[α-(halomethyl)acrylate] and dithiols, dicarboxylic acids, primary monoamines, and bisphenols via conjugate substitution yielded a series of poly(conjugated ester)s with acryloyl groups in the backbones. In the case of a dithiol monomer, a polar solvent promoted the conjugate addition (Michael addition) of a mercapto chain end into the acryloyl group in the resulting backbone to cause gelation. Therefore, kinetic control of the polymerization by the selection of nonpolar solvents and end capping with the mercapto group in the case of polar dithiols was necessary to prepare linear polymers. The monomer, bis[α-(halomethyl)acrylate], was also effective in achieving chain extension of common polyesters with phenolic hydroxyl end groups. These poly(conjugated ester)s underwent main chain scission when they were treated with benzyl mercaptan in the presence of a basic catalyst through a thiol exchange reaction via conjugate addition and substitution.


Introduction

Allyl halide derivatives bearing electron-withdrawing groups that can form conjugate structures with the double bond, such as acyl,1 oxycarbonyl,2–4 cyano,2–5 and sulfonyl2 groups, undergo a specific nucleophilic reaction known as conjugate substitution. The reaction involves performing a nucleophilic attack on the vinyl group, rearrangement of the double bond, and subsequent elimination of the halogen atom (SN2′ mechanism). For example, α-(halomethyl)acrylate (A)6 accepts a quantitative conjugate substitution reaction even with weak nucleophiles, such as carboxylic acids,7 amines,8–10 and phenols,11,12 in the presence of excess base under ambient conditions (Scheme 1). In addition, the products, α-substituted acrylates, B, are attractive compounds for polymer chemists. In fact, the conjugate substitution reaction is often used to prepare new α-substituted acrylates that function as monomers,13–17 chain-transfer agents,16,18 and functional initiators.19,20 The conjugate substitution reaction is also effective in terminating the stereospecific living anionic polymerization of (meth)acrylates, while simple allyl halides cannot terminate the polymerization owing to the weak nucleophilicity of the living polymer anion.21–24
image file: c7py02114c-s1.tif
Scheme 1 Synthesis and dynamic covalent chemistry of α-substituted acrylates.

Importantly, the α-substituted acrylates B resulting from conjugate substitution are also active against further conjugate reaction. For example, α-(acetoxymethyl)acrylate allows the conjugate addition (Michael addition) of thiols to afford the adduct C,25 while amines26 and alcohols27 lead to subsequent elimination to afford other α-substituted acrylates D. These conjugate addition and substitution reactions are equilibrium processes, and their selectivity appears to be dependent on not only the nucleophiles and the leaving group but also the reaction conditions. In this decade, chemically degradable polymers based on dynamic covalent chemistry28 have garnered much attention from the perspective of macromolecular engineering.29,30 Therefore, the aforementioned dynamic features of α-substituted acrylate derivatives are useful for preparing degradable polymers.

Recently, we reported on an efficient polycondensation technique based on a tandem reaction involving conjugate substitution and addition [Scheme 2(a)];31 the conjugate substitution reaction of ethyl α-(bromomethyl)acrylate (1) and 1,10-decanedithiol (2a) afforded intermediate 3, an acrylate-bearing mercapto group, and the subsequent conjugate addition resulted in polysulfide P1/2a. Since P1/2a has a similar skeleton to that of C in the backbone, it would allow chain scission by treatment with excess monothiol via a thiol exchange reaction. Hence, we are interested in the dynamic structure of polymers composed of basic skeletons B and C.


image file: c7py02114c-s2.tif
Scheme 2 (a) Synthesis of polysulfides containing skeleton C. (b) Polymerization of bis[α-(chloromethyl)acrylate] and various nucleophilic monomers to prepare polymers with skeleton B.

As mentioned previously, the equilibrium among B, C, and D should be dependent on the nucleophile and the leaving group. However, the strategy described in Scheme 2(a) was limited to primary dithiol monomers affording polysulfides because the conjugate addition of other nucleophilic monomers is not quantitative. Therefore, a new monomer, 1,4-butylene bis[α-(halomethyl)acrylate] (4) was designed to prepare a variety of polymers containing skeleton B with different leaving groups. In this report, we have described the polymerization of 4 and the dynamic covalent chemistry of the resulting polymers as well as the degradation of P1/2a.

Results and discussion

Monomer synthesis

Compound 4 was prepared in three steps from tert-butyl acrylate (8) (Scheme 3). The Baylis–Hillman reaction of 8 and formaldehyde yielded 9 (yield: 57.2%), and chlorination of the hydroxyl group and t-butyl ester was simultaneously performed with thionyl chloride (yield: 72.3%). The chloride was reacted with 1,4-butanediol to afford 4 (yield: 40.0%) as a needle crystal. The structure of 4 was confirmed by 1H NMR [Fig. S2(a)], 13C NMR [Fig. S3(a)], and FTIR spectroscopy.32
image file: c7py02114c-s3.tif
Scheme 3 Synthesis of 3. *DABCO: 1,4-diazabicyclo[2.2.2]octane.

Polymerization with dithiols: kinetic control to avoid crosslinking

We first investigated the polymerization of 4 using 1,10-decanedithiol (2a) as a simple dithiol monomer. In our previous work, 1 accepted sequential double nucleophilic attacks of dithiol monomers, i.e., conjugate substitution and subsequent conjugate addition [Scheme 2(a)].31 Therefore, 4, which is the dimer of 1, is potentially capable of accepting four nucleophilic attacks. Consequently, the polymerization of 2a and 4 may result in the fabrication of a crosslinked polymer. In fact, the polymerization in CH3CN (Table 1, Run 1) and N,N-dimethylformamide (DMF, Run 2) resulted in gelation, thus indicating crosslinking by the conjugate addition of a mercapto end group to the acryloyl moiety in the backbone (Scheme 4).
image file: c7py02114c-s4.tif
Scheme 4 Crosslinking reaction via conjugate addition catalyzed by Et3N.
Table 1 Polymerization of 2 and 3 in the presence of Et3N under ambient conditions
Runa M Solvent Base (equiv.) Polymer code Yield/% M n[thin space (1/6-em)]c Đ Solubilityd T d5/°C T m/°C
CHCl3 THF H2O
a 4: 0.40 mmol, [4]0/[M]0/[Et3N]0 = 1/1.0/2.5, solvent: 1.0 mL. b M: Comonomer (nucleophilic monomer). c Determined by SEC (THF, 40 °C, polystyrene standards). d Soluble (S), partly soluble (PS), and insoluble (IS) (1 mg mL−1). e The samples might be wet owing to the high hygroscopicity. f THF soluble fraction. g The reaction mixture was treated with 11 (0.14 mmol) after 1 h. h 1,8-Diazabicyclo[5.4.0]undec-7-ene. i 4: 0.41 mmol, [4]0/[CoM]0/[base]0 = 1/1.0/2.2, CH2Cl2: 0.80 mL, H2O: 1.5 mL, 25 °C, 24 h. Benzyltriethylammonium chloride (20 mol%) was used as a phase transfer catalyst.
1 2a CH3CN Et3N (2.5) P2a/4-gel Gelation IS IS IS
2 2a DMF Et3N (2.5) Gelation IS IS IS
3 2a CHCl3 Et3N (2.5) P2a/4 79 58[thin space (1/6-em)]000 2.27 S S IS 263 75
4 2b CHCl3 Et3N (2.5) P2b/4 91 12[thin space (1/6-em)]000 1.43 PS PS IS (82)e
5 2c CHCl3 Et3N (2.5) P2c/4 87 12[thin space (1/6-em)]000 1.84 S S IS 251
6 2d CHCl3 Et3N (2.5) P2d/4 79 9000 1.84 S S IS 150 66
7 2e CHCl3 Et3N (2.5) P2e/4-branch 80 (20[thin space (1/6-em)]000)f (5.77)f PS PS IS
8g 2e CHCl3 Et3N (2.5) P2e/4 84 17[thin space (1/6-em)]000 2.08 S S IS 259
9 5 DMF Et3N (2.5) P4/5 53 14[thin space (1/6-em)]000 1.76 S S IS 331 54
10 6 Dioxane Et3N (2.5) 85 1000 1.86 S S PS
11 6 Dioxane DBUh (2.5) P4/6 90 1900 1.92 S S PS (106)e
12i 7 H2O/CH2Cl2 NaOH (2.2) 74 2800 1.56 S S IS
13 7 CH3CN K2CO3 (2.5) 98 19[thin space (1/6-em)]000 1.95 S S IS
14 7 CHCl3 Et3N (2.5) P4/7 93 32[thin space (1/6-em)]000 1.98 S S IS 260


However, importantly, the conjugate substitution reaction is much faster than the following conjugate addition, which is sensitive to solvent effects.33,34 If kinetic control allows a selective conjugate substitution reaction, a linear polymer containing acryloyl in the backbone can be synthesized. Therefore, model reactions of 4 and benzyl mercaptan were conducted under various conditions.32 The results indicated that conjugate substitution was the major reaction, but it competed with conjugate addition in CH3CN, while selective conjugate substitution occurs in CHCl3 (Run 3). Therefore, the polymerization was conducted in CHCl3. The resulting polymer exhibited good solubility in CHCl3 and THF, thus indicating a non-crosslinked structure, which was expected. Fig. S2(b) shows the 1H NMR spectrum of the resulting polymer. Signals b (5.34 ppm) and c (6.19 ppm) clearly indicate the existence of a vinylidene group in the backbone, thus suggesting a selective substitution reaction. All other signals were also assigned to the expected linear polymer structure. In addition, the size exclusion chromatogram (SEC) of the resulting polymer [Fig. 1(a)] exhibited a unimodal peak, which confirmed that there was no branching or crosslinking. Therefore, polymerization in CHCl3 is effective in suppressing branching/crosslinking by conjugate addition and in achieving a selective substitution reaction for the linear polymer. In fact, the obtained polymer, P2a/4, exhibited good solubility in common organic solvents such as CHCl3 and THF, thus indicating the non-crosslinked structure. The chemical structure of P2a/4 was also confirmed by 13C NMR and IR spectra (Fig. S3 and S4, respectively). To optimize the reaction time, the polymerization mixture was sampled and the increase in the molecular weight was determined [Fig. 1(g)]. The Mn of the products became almost constant after 1 h and remained unchanged even after 24 h; no branching was observed.


image file: c7py02114c-f1.tif
Fig. 1 SEC curves of (a) P2a/4, (b) P2b/4, (c) P2c/4, (d) P2d/4, (e) P2e/4-branch, and (f) P2e/4 in Table 1. (g) Growth of the Mn in the polymerization of 2a and 3.

The polymerization was subsequently performed with 2b, a functional dithiol bearing hydroxyl pendants (Table 1, Run 4). The resulting polymer had a unimodal SEC peak [Fig. 2(b), Mn = 12[thin space (1/6-em)]000, Đ = 1.43], while the 1H NMR spectrum [Fig. S2(b)] indicated the expected linear structure. Therefore, the conjugate substitution reaction proceeded selectively on the mercapto group to afford a poly(ester sulfide) bearing hydroxyl pendant groups.


image file: c7py02114c-f2.tif
Fig. 2 1H NMR spectra of (a) prepolymer P7/12 and (b) terpolymer P4/(7/12) (400 MHz, CDCl3, 26 °C). •: CHCl3. Labels for the assignments correspond to those in Scheme 6.

A similar polymerization was conducted with secondary dithiol 2c (Run 5). Despite the steric hindrance, 2c afforded a polymer with Mn > 104. Aromatic dithiol 2d also yielded a polymer with moderate molecular weight (Run 6, Mn = 9000, Đ = 1.84) although the aromatic thiolate anion is stable and is not highly reactive. These results are contrastive to polymerization based on a conjugate addition-type thiol–ene reaction in which secondary and aromatic dithiols are inefficient monomers.31 All the obtained polymers were characterized by 1H NMR [Fig. S2] and they exhibited unimodal SEC peaks [Fig. 1(c) and (d)].

However, in the case of the polymerization with 2e, which is a disulfide carrying an oligo(ethylene glycol) moiety, the resulting polymer included a fraction insoluble in THF and CHCl3 (Run 7). The SEC of the THF-soluble fraction [Fig. 1(e)] shows a broad peak. As described above, branching/crosslinking through conjugate addition occurs if the polymerization is conducted in a polar solvent such as CH3CN or DMF. Therefore, we suspect that the polar local environment around the mercapto chain end created by the oligo(ethylene glycol) moiety might lead the Michael addition toward branching/crosslinking. Thus, the polymerization was repeated under similar conditions, but the reaction time was shortened from 24 h to 1 h, and the reaction mixture was treated with methyl α-(chloromethyl)acrylate (11) to cap the mercapto end (Scheme 5, Table 1, Run 8). The resulting polymer was readily soluble in CHCl3 and THF, and the SEC peak [Fig. 1(f)] was unimodal with an almost ideal molecular weight distribution (= 2). The 1H NMR spectrum [Fig. S2(f)] also indicated an unbranched structure. Consequently, end capping of the mercapto group seems to be effective in suppressing branching/crosslinking, and the polymerization can be controlled even when the monomer has high polarity that might induce conjugate addition.


image file: c7py02114c-s5.tif
Scheme 5 End capping of the mercapto group with 1a after the polymerization.

Polymerization with dicarboxylic acids, primary monoamines, and bisphenols

As described in the introduction, α-(halomethyl)acrylates can react with carboxylic acids,7 amines,8–10 and bisphenols11,12 in the presence of excess base. Therefore, different types of poly(conjugated ester)s with different backbones could be prepared through the polycondensation of 4 with nucleophilic monomers.

Polymerization with adipic acid (5) was conducted (Table 1, Run 9) in the presence of Et3N. In the polymerization with dithiols, polar solvents such as CH3CN (Run 1) and DMF (Run 2) promoted the conjugate addition to lead to crosslinking; thus, CHCl3 was used as a suitable solvent. However, CHCl3 was not suitable for the polymerization of 5 owing to its poor ability to dissolve 5, while DMF could be used as a reaction solvent since the carboxylate anion does not cause conjugate addition. The resulting polymer exhibits 1H NMR signals that confirm the expected structure [Fig. S8(a)], and it has a high molecular weight (Mn = 14[thin space (1/6-em)]000, Đ = 1.76), thus indicating high conversions of the carboxylate anion and α-(chloromethyl)acryloyl groups.

The α-(halomethyl)acrylates undergo dimerization via the SN2′ reaction with a half equimolar amount of a primary amine.8–10 Therefore, the reaction with 4 and n-propylamine (6) should afford the poly(amino ester). The polymerizations were conducted in 1,4-dioxane in the presence of excess Et3N (Run 10) and 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU, Run 11). Although the Mn values of the resulting products were moderate (Mn < 2000), the 1H NMR spectrum [Fig. S8(b)] indicated the proceedings of polymerization. The polymer, P4/6, was soluble in THF and CHCl3, but it was insoluble in H2O and conc. aq. HCl.

The polymerization of 4 and bisphenol A (7) was also investigated. Interfacial polymerization in the presence of benzyltriethylammonium chloride as a phase transfer catalyst afforded the corresponding polymer although the molecular weight was moderate (Run 12, Mn = 2800, Đ = 1.56). Polymerization in CH3CN in the presence of K2CO3 was effective in achieving a high molecular weight (Run 13, Mn = 19[thin space (1/6-em)]000, Đ = 1.95). Moreover, solution polymerization in a homogeneous system, i.e., the polymerization in CHCl3 in the presence of Et3N, resulted in the highest molecular weight (Run 14, Mn = 32[thin space (1/6-em)]000, Đ = 1.98). The structures of the obtained polymers were confirmed by 1H NMR spectrometry [Fig. S8(c)].

Synthesis of terpolymer with dicarbonyl chloride and bisphenol A

As 4 can react with bisphenol, it is effective as a chain-extension reagent for a common polyester with phenol ends. For example, prepolymer P7/12 was prepared from the polycondensation of adipoyl chloride (12) and a slight excess of 7 (Scheme 6). The 1H NMR spectrum of P7/12 [Fig. 2(a)] shows the signals of the phenol end group [signals b(t) and c(t) at 7.04 and 6.69 ppm, respectively], while those of adipic acid end groups [signals d(t) and e(t) at 2.56 and 1.76 ppm, respectively] were rarely observed. Therefore, most of the end groups of P7/12 were phenolic hydroxyl groups. Subsequently, P7/12 was reacted with 4 to extend the polymer chain. The 1H NMR spectrum of the resulting terpolymer P4/(7/12) [Fig. 2(b)] showed the expected signals, while an increase in the molecular weight from Mn = 750, Đ = 1.95 to Mn = 6420, Đ = 1.93 was observed. These results indicated the formation of a terpolymer. This example suggests that 4 is effective as a chain-extension reagent to incorporate the acryloyl moiety into the backbone of a conventional polyester. As 4 can react with mercapto, amino, carboxylate, and phenol groups, the monomer can be applied to a variety of common polymers.
image file: c7py02114c-s6.tif
Scheme 6 Terpolymerization of bisphenol A (7), adipoyl chloride (12), and 4.

Degradation of polysulfide (P1/2a) via a thiol exchange reaction

Conjugate addition is a reversible reaction; thus, the addition of product C is chemically interchangeable with the elimination of product D (Scheme 1). Since P1/2a carries the skeleton of C in the backbone, the main chain could be cleaved by treatment with a basic catalyst via elimination to generate skeleton D (Scheme 7). In the presence of excess benzyl mercaptan (13), the generated acrylate moiety at the chain end would be capped through conjugate addition. Overall, the reactions raise the degradation of P1/2a to oligomeric products. Therefore, P1/2a (Mn = 10[thin space (1/6-em)]700, Đ = 1.89) was reacted with 13 in a CHCl3/DMF cosolvent in the presence of a DBU catalyst at ambient temperature for 24 h (Table 2, Run 1). SEC profiles observed before and after the reaction are shown in Fig. 3(a). The decrease in molecular weight (Mn = 2100, Đ = 1.83) suggests that main chain scission occurred via a thiol exchange reaction. A similar experiment was performed using Et3N instead of DBU as the catalyst (Run 2), but the product had a large molecular weight (Mn = 6200, Đ = 1.70).
image file: c7py02114c-s7.tif
Scheme 7 Main chain scission of P1/2avia a thiol exchange reaction.

image file: c7py02114c-f3.tif
Fig. 3 SEC curves of (a) P1/2a, (b) P2a/4, (d) P4/5, and (e) P4/7 before and after main chain scission. (c) Changes of Mn and Đ during the reaction.
Table 2 Main chain scission by treatment with 13 in the presence of a basic catalyst
Run Polymer Base (equimolar)a 13 (equimolar)a Temp. (°C) Time (h) Before After Exomethylene contentc (%)
M n[thin space (1/6-em)]b Đ M n[thin space (1/6-em)]b Đ
a Equimolar with respect to the repeating units of polymers. b Determined by SEC (THF, 40 °C, polystyrene standards). c Ratio of the remaining exomethylene group determined by 1H NMR spectroscopy (400 MHz, CDCl3, 26 °C).
1 P1/2a DBU (0.10) 5.0 25 46 10[thin space (1/6-em)]700 1.89 2100 1.83
2 P1/2a Et3N (0.10) 5.0 25 54 10[thin space (1/6-em)]700 1.89 6200 1.70
3 P2a/4 Et3N (0.50) 4.0 25 24 16[thin space (1/6-em)]000 2.04 4300 1.57 <0.1
4 P2a/4 Et3N (0.50) 0 25 24 16[thin space (1/6-em)]000 2.04 14[thin space (1/6-em)]900 2.11 <0.1
5 P2a/4 Et3N (2.50) 20.0 25 72 16[thin space (1/6-em)]000 2.04 4700 1.92 <0.1
6 P2a/4 Et3N (0.50) 5.0 50 70 16[thin space (1/6-em)]000 2.04 1800 1.81 <0.1
7 P4/5 Et3N (0.50) 5.0 25 24 33[thin space (1/6-em)]000 1.76 480 1.25 64
8 P4/7 Et3N (0.50) 5.0 25 24 16[thin space (1/6-em)]000 1.84 780 1.56 <0.1
9 P2a/4-gel Et3N (0.10) 5.0 25 46 2000 2.12 <0.1


Degradation of polyesters by conjugate substitution reaction

Since P2a/4 contains skeleton B, main chain scission would be achieved by treatment with 13 in the presence of a basic catalyst through the dynamic equilibrium among skeletons B, C, and D (Scheme 8). Therefore, P2a/4 (Mn = 16[thin space (1/6-em)]000, Đ = 2.04) was reacted with 13 in the presence of Et3N as a basic catalyst at ambient temperature for 24 h (Table 2, Run 3). The 1H NMR spectrum of the product (Fig. S10) indicated the complete disappearance of the exomethylene group, while the other backbone structures, such as ester and sulfide linkages, remained. A decrease in molecular weight (Mn = 4300, Đ = 1.57) was observed by SEC [Fig. 3(b)]. Fig. 3(c) shows the time vs. Mn plots during the reaction. A decrease in Mn started just after the addition of 13 and became almost constant after 1200 min. The virgin polymer was a white powder, but the product recovered after the degradation was a viscous liquid. A similar experiment was conducted in the absence of 13 as a control, but the molecular weight barely changed (Run 4). These results suggest main chain scission in the mechanism shown in Scheme 8. The use of a large excess of 13 (10 equiv. per acrylate moiety, Run 5) afforded a result similar to that of Run 3; thus, its use was ineffective. The reaction at 50 °C (Run 6) resulted in a lower molecular weight than that in Run 3 probably owing to an increase in the entropy effect in the main chain scission at the high temperature.
image file: c7py02114c-s8.tif
Scheme 8 Main chain scission of polymers of 4via a thiol exchange reaction.

Similar experiments were conducted for P4/5. Although 64% of the exomethylene group remained, the Mn of the resulting product was close to that of the monomer (Run 7). The adipoyl moiety in the backbone of P4/5 was not observed in the 1H NMR spectrum (Fig. S11), thus indicating main chain scission at the allyl ester linkage. The signals assignable to the expected byproduct, adipic acid, were also rarely observed, because the byproduct was removed to the aqueous layer with Et3N during purification. The low conversions of conjugate addition would be due to the acidity of the liberating 5, which prevents the formation of the thiolate anion, an active species of conjugate addition, and deactivates the basic catalyst. The higher efficiency of degradation than that of P4/2a could be explained by the stability of the liberating group. P4/7 also afforded decomposed products under similar conditions with the complete disappearance of the exomethylene group (Run 8).

De-crosslinking of P2a/4-gel

As described in Table 1, Run 1, the polymerization of 2a and 4 in CH3CN yielded a crosslinked product (P2a/4-gel) through both conjugate substitution and addition. Since P2a/4-gel has a structure similar to those of P2a/4 and P1/2a in the backbone and crosslinking chain, respectively, the de-crosslinking reaction by treatment with 13 with a basic catalyst was expected. In fact, the swollen gel became a transparent solution after the reaction (Fig. 4). The recovered oligomer exhibited a unimodal SEC peak (Mn = 2000, Đ = 2.12), thus indicating de-crosslinking to form a linear oligomer.
image file: c7py02114c-f4.tif
Fig. 4 Photographs of P2a/4-gel swollen in CHCl3 (a) before and (b) after the de-crosslinking.

Conclusions

The conjugate substitution of α-(chloromethyl)acrylate exhibited excellent performance in preparing a variety of polymers with acrylate groups in the backbones, i.e., the new monomer, 4, underwent polycondensation with dithiols, dicarboxylic acids, primary monoamines, and bisphenols to afford poly(ester sulfide)s, poly(ester ester)s, poly(ester amine)s, and poly(ester ether)s, respectively, although kinetic control and end capping were required for dithiol monomers to suppress the subsequent conjugate addition that led to branching/crosslinking. Moreover, 4 functioned as a chain-extension reagent for prepolymers with phenolic hydroxyl ends.

From the perspective of polymer synthesis, the advantages of the conjugate substitution or the SN2′ reaction of α-(chloromethyl)acrylate over the SN2 reaction of common allyl halide can be summarized as follows: (1) high reactivity: quantitative conversion can be achieved at ambient temperature in air in several hours. Since no heating is required, the reaction can be applied to thermosensitive monomers including α-(chloromethyl)acrylates themselves. (2) Versatility: various nucleophiles, such as thiols, carboxylate anions, amines, and alkoxy anions can be applied in both polar and nonpolar solvents. Consequently, the reaction solvent can be selected according to the solubility of the monomers and polymers. (3) Expandability: the resulting acrylate moieties can be used for further conjugate addition as described in our previous report.20 (4) Dynamicity: the reversibility of conjugate addition led to chemical decomposition. In the current study, main chain scission and de-crosslinking of the polymerization products were achieved by treatment with a monomeric nucleophile, 13. We believe that these features of the conjugate substitution are attractive not only for polymer chemists but also for materials scientists.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This research was supported by the Ogasawara Foundation for the Promotion of Science & Engineering in 2016 (Y. K.). The authors would like to thank Osaka Organic Chemical Industry Ltd and Nippon Shokubai Co, Ltd for supplying the reagent supplements.

Notes and references

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Footnote

Electronic supplementary information (ESI) available: Model reaction to evaluate the kinetic selectivity between the conjugate substitution and addition of thiol and α-(chloromethyl)acrylate. See DOI: 10.1039/c7py02114c

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