Controlled Radical Homopolymerization of Representative

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Controlled Radical Homopolymerization of Representative Cationically-Polymerizable Vinyl Ethers Shinji Sugihara, Ayano Yoshida, Taka-aki Kono, Tsuyoshi Takayama, and Yasushi Maeda J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.9b06671 • Publication Date (Web): 09 Aug 2019 Downloaded from pubs.acs.org on August 9, 2019

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Journal of the American Chemical Society

Controlled Radical Homopolymerization of Representative Cationically-Polymerizable Vinyl Ethers

Shinji Sugihara,* Ayano Yoshida, Taka-aki Kono, Tsuyoshi Takayama, Yasushi Maeda

Department of Applied Chemistry and Biotechnology, Graduate School of Engineering, University of Fukui, 3-9-1 Bunkyo, Fukui 910-8507, Japan

*Correspondence to Shinji Sugihara (E-mail: [email protected])

ABSTRACT Facile direct radical homopolymerization of vinyl ethers without a hydroxy group was achieved up to near full conversion. This polymerization was conducted in water suspension in the presence of lithium

hydroxide

using

a

thermally-triggered

azo-initiator

of

dimethyl

2,2’-azobis(2-

methylpropionate). In the polymerization system, appropriate hydrogen bonding and cation-π interactions under basic conditions are keys to the successful direct radical homopolymerization. The hydrogen bonding between water and vinyl ether oxygen reduces the reactivity of the growing radical, thus suppressing unfavorable side reactions such as β-scission. In addition, Li+ interacts with the oxygen and the vinyl group of vinyl ethers. The vinyl ether tends to be “activated” and the polymerization can be facilitated. Based on the results of free radical polymerization of vinyl ethers, controlled polymerization was also accomplished using the appropriate dithiocarbamate RAFT agent in view of the solubilities of the radical leaving group.

INTRODUCTION Radical initiators promote the polymerization of almost any carbon-carbon double bond because the generated radical species are neutral and have little structural restrictions for the propagating radical substituent. Therefore, these initiators have been applied to a vast range of industrial vinyl polymer syntheses. However, few radical homopolymerizations of unactivated α-olefins and vinyl 1 ACS Paragon Plus Environment

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ethers (VEs) are known.1 In the latter case, although VEs can radically and alternatively copolymerize with electron-accepting monomers,2,3 they are believed to be not radically “homo”-polymerizable but cationically-polymerizable monomers. This is because the generated radical species are highly reactive due to their instability derived from the σ-radical,4,5 which leads to unfavorable side reactions such as β-scission and frequent hydrogen abstraction, such that homopolymerization does not proceed or oligomerization ocurrs.4,6,7 If such non-radically “homo”-polymerizable monomers can be polymerized with one active species, no complicated transformations8 will be necessary, and it will be possible to produce various unprecedented functional (co)polymers. Thus, achieving radical homopolymerization of vinyl monomers, which are difficult to polymerize in a radical fashion, is a longstanding challenging task but holds much promise. There have been a few breakthroughs related to the direct radical polymerization of the abovementioned non-radically polymerizable vinyl monomers. For terminal alkenes including α-olefins, Michl et al. reported that the poorly solvated (“naked”) Li+ cation forms a complex with α-olefins, dienes, and alkynes, and then their activated monomers induce radical polymerization using conventional azo-initiators.9 The radical polymerization system has been further developed for highly branched polymers using various simple alkenes at moderate temperatures and pressures.10 For VEs, there is a report of poly(oligoethylene glycol methyl VE)s with the highest Mn ever of 6600 but at very low conversion (~7%).11 We recently succeeded in the (controlled) radical polymerization of hydroxyfunctional VEs with higher conversion, using dimethyl 2,2’-azobis(2-methylpropionate), V-601 azoinitiator, at 70 ºC (Scheme 1A).12 This achievement was due to the hydrogen bonding (HB) between the VE oxygen and the hydroxy group in the pendant of the VE that reduced the reactivity of the growing radical, thus suppressing unfavorable side reactions such as β-scission.7 This method using HB also enabled the radical copolymerization of VEs with and without hydroxy groups (Scheme 1B).13 Furthermore, the rate of polymerization was accelerated by the fluidizing action of the aqueous environment to achieve full monomer conversion ([monomer]0 = 40-60 wt% in water). These results yielded valuable clues to achieving radical homopolymerization of “unactivated” VEs without special functional groups such as hydroxy groups. In this work, we report the highly efficient and facile radical homopolymerization of VEs in water containing lithium hydroxide using the appropriate azo-initiators (Scheme 1C). This system is effective for many radical homopolymerizations of VEs regardless of the presence or absence of hydroxy groups in the monomer. Furthermore, the controlled radical 2 ACS Paragon Plus Environment

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polymerization can progress by the addition of a suitable reversible addition-fragmentation chain transfer (RAFT) agent based on the results of free radical polymerization (Scheme 1D).

Scheme 1. Radical Polymerization of “Activated” VEs.

RESULTS AND DISCUSSION Radical homopolymerization of MOVE (R2 = CH2CH2OCH3) or IBVE (R2 = CH2CH(CH3)2) without a hydroxy group in organic solvent such as toluene using the V-601 azo-initiator resulted in oligomerization or almost no polymerization (entries 1, 2, and 19 in Table 1), as expected. The resulting oligomer did not have the aldehyde group, which was determined by the 1H NMR spectrum 3 ACS Paragon Plus Environment


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of the polymerization mixture (Figures S1 and S2 in Supporting Information). In addition, the MALDI-TOF-MS and 13C NMR results indicated that conventional polymerization (oligomerization) including chain transfer to solvent, backbiting reaction7 and/or post-addition of water to the disproportionated fraction with the poly(VE) end occurred (Figures S3-S6). On the other hand, when polymerization was conducted using methanol or water capable of HB with VE, radical polymerization proceeded more efficiently than when using toluene. However, selectivity of the vinyl polymerization decreased due to the formation of the acetal from methanol and VE under slight acidic conditions and the decomposed aldehyde product from the hemiacetal by adventitious water and VE (entry 3). When radical polymerization was conducted using basic N,N-dimethyl-4-aminopyridine (DMAP) in methanol (entry 4), acetal and aldehyde formation was suppressed and the selectivity of the vinyl polymerization was improved (~100%). As a result, the number-average molecular weight (Mn) was increased up to 7100. The solubilities of VEs in water at 25 ºC are 8.8 g and 0.07 g against 100 mL water for MOVE and IBVE, respectively.14 At 50 wt% monomer concentration in the polymerization mixture, these solubilities were not as high, such that the polymerization system was a suspension system, which differs from toluene and methanol as solution polymerization systems. The obtained Mn of poly(MOVE) was larger than that in other solvents but the selectivity of the vinyl polymerization was not perfect at 26% (entry 5). In the case of IBVE, which is barely soluble in water, water had little effect on the Mn, which only accelerated the polymerization rate. The polymerization scarcely progressed, producing 7% of aldehyde and 3% of acetal (entry 20). Thus, appropriate HB between VE (HB-acceptor) and water (HB-donor) influences the radical polymerization of VE. In practice, the chemical shifts of the vinyl protons of 1 wt% MOVE in D2O were shifted downfield due to HB (Figure S7). When the pH was varied in [MOVE]0/[V-601]0 = 20 (molar ratio), the selectivity of the vinyl polymerization of MOVE was improved in alkaline water, with an optimum pH of 12 or more (Figure S8). When 1-butanol was used instead of methanol in the presence of DMAP for basic condition, the resulting Mn decreased (entry 6 vs. entry 4). In addition, using 2,2,2-trifluoroethanol (TFE), which is higher in acidity than these alcohols, improved the resultant conversion and Mn (entries 7 > 4 > 6). Hence, it is revealed that the use of higher HB-donor solvents such as water, TFE, and methanol under basic conditions promoted free radical polymerization of VE. 4 ACS Paragon Plus Environment

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Table 1. Free Radical Homopolymerization of VE Monomers in the Presence of Additivesa

additiveb

[monomer]0: [additive]0

solvent (pH)c

temp (ºC)

time (h)

convn (%)d

selectivity (%)e

Mnf

Mw/Mnf

1

-

-

toluene

60

48

37

100

1700

2.15

2

-

-

cyclohexanone

70

24

36

100

700

1.25

3

-

-

methanol

60

48

24

64

6100

1.55

4

DMAP

5000:1

methanol

60

48

36

100

7100

1.61

5

-

-

water (5.3) g

60

48

99

26

4500

1.58

6

DMAP

5000:1

1-butanol

60

48

31

100

5400

1.40

7

DMAP

5000:1

TFE

60

48

43

98

7400

1.57

8

KPF6

10:1

toluene

70

48

33

99

5600

1.20

KPF6

2:1

toluene

70

24

52

99

10400

1.24

10g

KPF6

1:1

toluene

70

24

52

99

10800

1.27

11f

KPF6

1:1

cyclohexanone

70

24

51

99

6400

1.23

12

LiClO4

1:1

acetone

40

48

30

96

3400

1.45

13

LiClO4

1:1

DME

70

24

71

96

5600

1.86

14

LiClO4

1:1

DME

r.t.h

90

10

99

18300

1.65

15

LiClO4

1:1

methanol

60

36

100

63

7500

1.73

16

LiOH

500:1

water (12.4)

60

89

90

100

12300

1.67

17

NaOH

500:1

water (12.5)

60

72

49

99

10000

1.77

18

KOH

500:1

water (12.5)

60

72

44

99

4500

2.50

19

-

-

toluene

60

72

20

100

2000

1.40

20

-

-

water (6.8)

60

24

43

90

1600

2.68

21

LiOH

500:5

water (13.0)

60

96

30

100

10000

1.46

LiOH

500:2

water (12.8)

60

96

31

100

9700

1.52

23

LiOH

500:1

water (12.4)

60

96

41

100

8400

1.62

24

NaOH

500:2

water (13.0)

60

96

36

100

9600

1.53

25

KOH

500:2

water (13.1)

60

96

40

100

9200

1.58

26

LiOH/TEMPO

500:1/1

water (12.4)

60

48

0

-

-

-

27

TEA

500:1

water (12.1)

60

96

39

100

7400

1.62

entry

monomer

9 MOVE

22

IBVE

a [monomer] :[V-601] = 500:1, and [monomer] = 50 wt%. 0 0 0 b DMAP: N,N-dimethyl-4-aminopyridine; TEMPO: 2,2,6,6-tetramethylpiperidine

1-oxyl; and TEA: triethylamine. c Initial pH of aqueous phase of polymerization mixture. d Calculated from the residual monomer by 1H NMR spectroscopy. e Polymer products against all products such as polymer, acetal and acetaldehyde, measured by 1H NMR spectroscopy and gas chromatography. An example of calculation for selectivity by 1H NMR spectrum is shown in Figure S10. f By GPC (PSt standards). g[MOVE] = 35 wt%. 0

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h By

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UV-LED (365 nm) as a light source instead of heating.

Next, we aimed to further lower the electron density on the vinyl group by cation-π interactions in order to “activate” VE, and increase the stability of the σ-radical, thereby promoting radical polymerization up to full conversion. In order to reduce the influence of the counteranion, radical polymerization of MOVE was conducted with K+ and Li+ with noncoordinating anions such as KPF6 and LiClO4 dissolved in various organic solvents (entries 8-15). In the presence of any cation, the polymerization of MOVE was promoted (vs. entry 1). However, when Lewis basic solvents such as cyclohexanone and acetone (entries 11 and 12, respectively) were employed or when the amount of additive was reduced (entry 8 vs. entries 9 and 10), the conversion and Mn were also reduced. Furthermore, when using UV-LED as a light source at room temperature instead of heating, the resulting poly(MOVE) had a much higher molecular weight (Mn = 18300) even at low conversion (10%) in 1,2-dimethoxyethane (DME) (entry 14 vs. entry 13). This is likely to be due to enhanced cation-π interactions at decreased temperature. However, since the fluidizing action as seen in an aqueous environment is reduced by the increased interaction, the polymerization rate slows down, resulting in low conversion. In entries 8-14, cationic polymerization is also possible.15,16 Therefore, the polymerization was conducted in methanol, which is a terminator for cationic polymerization (entry 15, Figure 1A and B). In the presence of LiClO4 in methanol, the polymerization rate was clearly increased, and full conversion was reached in 36 hours. In the polymerization system, quasi 3:1 and 2:1 complexes were likely to be formed between MOVE and LiClO4 (i.e., Li+). This model was estimated by a Job’s plot using 1H NMR spectra (Figure S9). In addition, on the basis of longitudinal relaxation time (T1) measurements of

13C

NMR signals for MOVE and MOVE with LiClO4, it was found that Li+

interacted with the vinyl group and oxygen atom in methanol (Figure 2A). However, the resulting poly(MOVE) after 36 h polymerization contained 37% aldehyde and acetal, thus the selectivity was 63% (Figure S10).


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Figure 1. Free radical homopolymerization of MOVE or IBVE using V-601 with various Li+ additives at 60 ºC in methanol or water. (A) Incremental monomer conversion (closed circles) and selectivity (squares) and (B) Mn (open circles) or Mw/Mn (triangles) vs. conversion for polymerization of MOVE in methanol with (brown) and without (red) LiClO4: [MOVE]0/[V-601]0/[LiClO4]0 = 500/1/500 or 0, [MOVE]0 = 50 wt%. (C) Incremental monomer conversion (selectivity ~100%) and (D) Mn or Mw/Mn vs. conversion for polymerization of MOVE (blue) or IBVE (green) in water with LiOH: [VE]0/[V601]0/[LiOH]0 = 500/1/1. [VE]0 = 50% (suspension).

Therefore, radical polymerization of MOVE in the presence of LiOH was investigated in water (entry 16, Figures 1C and D) in order to construct a novel polymerization system with HB between VE and water, under basic conditions, and including cation-π interactions. As a result, Li+ interacted with the oxygen and vinyl groups of MOVE and reduced all the T1s of the carbons in MOVE (Figure 7 ACS Paragon Plus Environment


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2B). The radical polymerization of MOVE proceeded up to near full conversion while maintaining the selectivity at 100% (Figure 1C), and the resulting Mn was over 12300 (Figure 1D). The obtained poly(MOVE) is obviously V-601-initiated polymer without the end group by β-scission, which was determined by 1H NMR analysis in Figure 3 (the 13C NMR spectrum is shown in Figure S11). In addition, the resulting poly(MOVE) had exactly the same structure by 13C NMR analysis except for the steric structure [racemo (dyad) = 54.4% vs. 47.8% for entry 16 vs. poly(MOVE) via cationic polymerization at 0 ºC, respectively (Figure S11). For poly(IBVE), see Figure S12]. Furthermore, even in the polymerization of hydrophobic IBVE, the radical homopolymerization proceeded to a higher Mn up to 8400 at the same feed ratio of [IBVE]0:[LiOH]0 = 500 : 1 (entry 23 and Figure 1D). Although IBVE barely dissolves in water, it interacts with aqueous LiOH at the interface between the VE layer and water, which was confirmed by the slight decrease of T1s only for the vinyl group (Figure 2C). Therefore, as the LiOH concentration increased, IBVE tended to be “activated” and the polymerization was facilitated (entries 21-23). At [IBVE]0:[LiOH]0 = 500 : 5 in entry 21, the radical homopolymerization of IBVE proceeded to the highest Mn up to 10000. However, the rate of polymerization (conversion at 96 h) was decreased as the LiOH concentration increased.

Figure 2. T1 measurements of the carbons for (A) 90 wt% MOVE with LiClO4 in CH3OH: [MOVE]/[LiClO4] = 2/1, (B) 98 wt% MOVE and (C) 98 wt% IBVE with LiOH in water: [VE]/[LiOH] 8 ACS Paragon Plus Environment

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= 500/1. T1 means T1 with Li+ additive minus T1 without Li+, indicating the mobility of the monomer in the solvent used.

Figure 3.

1H

NMR spectrum in CDCl3 of the purified poly(MOVE) prepared by radical

polymerization (entry 16). *: solvents.

Likewise, aqueous KOH and NaOH water systems also interacted (based on T1 measurements of the carbons of MOVE with KOH or NaOH in Figure S13), but LiOH with the strongest cation-π interaction allowed the fastest polymerization rate of MOVE (Figure 4 and entries 16-18). However, in the case of IBVE with a small contact area with the water interface, there was little or no effect of the type of cation on the polymerization rate (entries 24 and 25 vs. entry 22). The amount of cation had some effect on the Mn in the same manner as LiOH. Indeed, when 1H NMR analysis of the polymerization mixture was conducted immediately after polymerization, the remaining vinyl group was shifted downfield as the amount of LiOH increased (Figure 5). Hence, it was revealed that the radical polymerization was promoted by the cation-π interaction. As a matter of course in radical polymerization, such polymerizations of VE in LiOH water do not proceed 9 ACS Paragon Plus Environment


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at all in the presence of a sufficient amount of radical polymerization inhibitor TEMPO (entry 26). Furthermore, Mn is lower under basic conditions using TEA that does not have cation-π interactions (entry 27).

Figure 4. Effect of added MOH (M = Li, Na, and K) on free radical polymerization of MOVE in water at 60 ºC: [MOVE]0/[V-601]0/[MOH]0 = 500/1/1, [MOVE]0 = 50 wt%. (A) Time-conversion curves and (B) representative GPC traces of the resulting poly(MOVE)s (entries 16-18).


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Figure 5.

1H

NMR spectra for IBVE polymerization mixtures (entries 21-23) just after termination.

The organic layer of the polymerization mixture was directly measured using a coaxial system for external locking, i.e., using a sample tube with a capillary for the external reference (reference: CDCl3 with 1.0% TMS).

Since the direct radical polymerization of MOVE and IBVE has been achieved in water with LiOH for the first time, RAFT radical polymerizations and subsequent block copolymerization using vinyl acetate (VAc) were conducted. Radical polymerization involving the RAFT process, i.e., RAFT radical polymerization, is based on conventional free radical polymerization. The RAFT process is generally accomplished by performing a radical polymerization in the presence of a thiocarbonylthio compound [RS(C=S)Z, RAFT agent].17-19 Since free radical polymerization of MOVE in water with LiOH is a suspension system, CTA 1-3 with different solubilities of R and Z in water were prepared (Scheme 2).



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Scheme 2. CTAs for RAFT radical polymerization of VEs.

The use of CTA 1 as a RAFT agent is known to provide good control and little or no retardation of the RAFT radical homopolymerizations of hydroxy-functional VEs.12,13,20 Thus, polymerization using either CTA 1 or 2 in which the radical generating site is dissolved in the organic phase of VE was conducted. Figure 6 shows the polymerization results using CTA 1 (and Figure S14 for CTA 2). The polymerization rate was slower than that without the RAFT agent. The molecular weight distribution (MWD) of poly(MOVE) clearly shifted toward higher molecular weights as polymerization time progressed from 0 to 100 h. The detailed chemical structure of the poly(MOVE) was determined by 1H NMR (Figure 7A). The main polymers exhibited signals attributable to the methylene protons (a) due to the RAFT ends derived from the free radical leaving group (i.e., α-end), the methyl (g), and phenyl (h) groups on the terminal RAFT end (i.e., ω-end) and the protons due to MOVE repeating units (b–f). In addition, the MALDI-TOF-MS spectrum (Figure S15) showed almost three series of peaks for the repeating unit of MOVE. The detailed structures are estimated to be poly(MOVE) with ω-end decomposition of the hemiacetal dithioester, poly(MOVE) with the aldehyde group due to hydrolysis in the MALDI experimental process,8c,12 and V-601 initiated poly(MOVE) with the -end decomposition moiety. By comparison of the three structures, the ratios of -ends of RAFT and V601 are 0.70 and 0.30, respectively, which are close to the values determined in Figure 7A. The 12 ACS Paragon Plus Environment

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distance between individual peaks which was 102.1 mass units also corresponds to the molar mass of the MOVE repeating unit. The detailed structures for each peak are also shown in Figure S15.

Figure 6. GPC traces of poly(MOVE) (24-100 h polymerization) obtained by RAFT radical polymerization using CTA 1 in water with LiOH at 60 ºC: [MOVE]0/[CTA 1]0/[V-601]0/[LiOH]0 = 200/1/0.4/0.4, [MOVE]0 = 50 wt%, and poly(MOVE)-b-poly(VAc) obtained by RAFT bulk polymerization of VAc at 70 ºC using the purified poly(MOVE) macro-CTA obtained at 48 h polymerization: [VAc]0/[poly(MOVE) macro-CTA]0/[AIBN]0 = 500/1/1.

Furthermore, radical polymerization of VAc using the resulting poly(MOVE) obtained at 48 h polymerization as a macromolecular RAFT agent (macro-CTA) also occurred smoothly. The resulting MWD clearly shifted toward a higher molecular weight relative to poly(MOVE) macro-CTA, indicating the preparation of a block copolymer composed of poly(MOVE) and poly(VAc) obtained by radical polymerization alone. The formation of the block copolymer was also confirmed using 1H 13 ACS Paragon Plus Environment


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NMR spectroscopy (Figure 7B). However, the water-soluble CTA 3 was ineffective in homopolymerization of VE in water with LiOH at 60 ºC: [MOVE]0/[CTA 3]0/[V-601]0/[LiOH]0 = 200/1/0.4/0.4, [MOVE]0 = 50 wt%. Thus, the polymerization likely was initiated in the organic VE phase, and the VE monomer was activated via HB with water and cation-π interactions with Li+ at the interface between VE and the aqueous phase. The propagating radical including HB between ether and water grew through the RAFT process by suppressing unfavorable side reactions such as βscission.

Figure 7.

1H

NMR spectra of (A) poly(MOVE) obtained at 48 h RAFT radical polymerization using

CTA 1 and (B) poly(MOVE)-b-poly(VAc) (x:y = 32:405, determined from f and n) obtained using the purified poly(MOVE) macro-CTA. For the polymerization condition: see Figure 6. Endfunctionalities can be determined from the ratio a vs. k for α-end and h vs. a and k for ω-end.


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CONCLUSIONS In conclusion, direct radical homopolymerizations of VEs were performed in water in the presence of LiOH. Appropriate HB and cation-π interactions under basic conditions were keys to the successful direct radical polymerization of VEs. This facile free radical polymerization system was also expanded to a controlled polymerization via the RAFT process. The polymerization enabled the preparation of poly(VE)-b-poly(VAc) by radical polymerization alone. This discovery not only makes possible the radical polymerization of VE monomers, which had been regarded as impossible, it will also have great influence on various radical addition reactions and polymerizations of electron-donating vinyl compounds.

EXPERIMENTAL SECTION Materials. 2-Methoxyethyl vinyl ethers (MOVE, Maruzen Petrochemical >99%) and IBVE (TCI; >99.0%) were washed with aqueous alkaline solution and water and then distilled twice over calcium hydride. Vinyl acetate (VAc, Wako; >98.0%) was distilled over calcium hydride. Cyanomethyl methyl(phenyl)carbamodithioate (CAT 1, Sigma-Aldrich; 98%) and 2-cyanopropan-2-yl N-methylN-(pyridin-4-yl)carbamodithioate (CTA 2, Sigma-Aldrich; 97%) as a RAFT agent was used as received. Dimethyl 2,2’-azobis(isobutyrate) (V-601, Wako; >97.0%) was recrystallized from methanol. 2,2’-azobis(isobutyronitrile) (AIBN, TCI; >98.0%) was recrystallized from diethyl

ether. For the aqueous solvent, ultrapure water (Wako) was used. For organic solvent, super dehydrated solvents of toluene, methanol, and acetone (water 99.0%), cyclohexanone (Wako; >99.0%), and 1,2-dimethoxyethane (DME, Wako; >99.0%) were used as received. Synthesis of CTA-3. CTA 3 was synthesized from bis(methyl phenyl thiocarbamoyl)disulfide (BMPTD). CHCl3 (125 mL), N-methylaniline (35.36 g, 0.33 mol), CS2 (10.0 mL, 0.17 mol), and iodine (21.40 g, 0.17 mol) were added to a 300-mL round bottomed flask, and the mixture was stirred for 24 h at 0 ºC until the iodine was completely consumed. The reaction mixture was poured into cold methanol to precipitate the crude BMPTD. The precipitate was washed with methanol and dried in vacuo overnight to yield the yellowish BMPTD (39.1 g, 65% yield). Subsequently, a solution of 4,4'15 ACS Paragon Plus Environment


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azobis(4-cyanovaleric acid) (5.840 g, 21 mM) and BMPTD (5.104 g, 14 mM) in ethyl acetate (80.0 mL) was heated at reflux for 16 h. After removal of the volatiles in vacuo, the crude product was subjected to column chromatography (Wakogel C-200, 100-200 mesh) using n-hexane/ethyl acetate (1:1) as the eluent to separate the carboxylic acid of CTA 3 (CTA-COOH). The eluent was removed by evaporation under reduced pressure and dissolved in ultrapure water. The aqueous solution including CTA-COOH was neutralized by 0.8 N aqueous LiOH solution, filtered off, and lyophilized (89% from CTA-COOH and 3.04 g, 69% yield from BMPTD), and the product was stored in a freezer. 1H

NMR (D2O, 500 MHz): δ 1.59 [s, C(CH3)CN], 2.15 [br t, C(=O)CH2CH2], 2.27 [br t,

CH2C(CH3)CN], 3.54 (s, 3H, N-CH3), 7.19 and 7.36 (m, 5H, ArH). 13C NMR (CD3OD, 125 MHz): δ 26.0 [C(CH3)CN], 35.0 [C(=O)CH2], 37.6 [CH2C(CH3)CN], 46.4 [C(CH3)CN], 48.5 (NCH3), 122.7 (CN), 128.9, 131.4, 131.9, 146.3 (Ar), 180.7 (C=O), 194.3 (C=S). IR (KBr, cm-1): 1576, 1420, 1351, 1256, 1085. Polymerization Procedure. Typical free radical polymerization was carried out at 60-70 °C under a dry nitrogen atmosphere. All reagents and a magnetic stir bar were added to a Schlenk tube. After three freeze−pump−thaw cycles, the tube was filled with nitrogen and immersed in a preheated oil bath. In the case of the photopolymerization with UV-LED irradiation instead of heating, a DT-365C UV-LED inspection flashlight (365 nm, 4000 μW/cm2 at 40 cm from the light source) was held against the 1.5 cm Schlenk tube from the side. For an example, in entry 16 in Table 1, 2.0 g of MOVE (19.6 mmol), 9.0 mg of V-601 (3.92 × 10-5 mol), and water (1.99 g) with 1.6 mg of lithium hydroxide monohydrate (3.92×10-5 mol) were used in this order. The polymerization conditions were [MOVE]0/[V-601]0/[LiOH]0 = 500/1/1 and [MOVE]0 = 50 wt%. The reaction was conducted with stirring at 60 °C. After the desired time, the reaction was quenched via rapid cooling in an ice bath and exposure to air. The solvents and volatiles were directly removed under reduced pressure, and the product was verified by GPC analysis. In the cases of poly(MOVE), the product was then purified by dialysis against deionized water using semipermeable cellulose tubing (SPECTRA/POR, corresponding to a molecular weight cutoff of 1000 Da) with six changes of deionized water, followed by lyophilization. All the monomer conversions (convn) of the resulting polymers were determined by 1H NMR analysis of the just quenched mixture. The selectivity of the vinyl polymerization was determined by yields of acetal and aldehyde using either 1H NMR analysis or gas chromatography (GC). The molecular weight distributions (MWDs) of the resulting polymers were assessed by gel 16 ACS Paragon Plus Environment

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permeation chromatography (GPC) before dialysis. Characterization. MWDs were assessed by gel permeation chromatography (GPC) using tetrahydrofuran (THF) eluent at 40 °C using polystyrene gel columns (TSK gel G-MHHR-MX × 3; flow rate 1.0 mL/min) connected to a Tosoh SD-8022 degasser, AS-8020 auto sampler, CCPM-II pump, RI-8020 refractive detector, and UV-8020 UV detectors, respectively. The RI detector was used for determination of the number-average molecular weight (Mn) and polydispersity index (Mw/Mn) using polystyrene standards. 1H and

13C

NMR spectra to determine the detailed structures were

recorded on JEOL JNM-ECX500II [500 (1H) and 125 (13C) MHz] spectrometers. When obtaining an accurate chemical shift value or longitudinal relaxation time (T1), a coaxial insert (Wilmad-LabGlass) was used with the NMR tube for external locking (reference: CDCl3 with 0.1% TMS). 1H NMR analysis and gas chromatography (GC) were used to determine the selectivity of the vinyl polymerization [GC-2014 (Shimadzu) with DB-1 capillary column (Agilent) from 50 to 250 ºC; eluent He; 0.95 mL/min].

SUPPORTING INFORMATION Polymerization results, 1H NMR,

13C

NMR, MALDI-TOF-MS spectra including Job’s plot and T1

measurement.

ACKNOWLEDGMENT This study was supported in part by a JSPS Grant-in-Aid for Scientific Research (C) 16K05791, (B) 19H02762, and the Eno Scientific Foundation. We acknowledge Maruzen Petrochemical Co., Ltd., for supplying monomers and providing useful suggestions.

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Chiefari, J.; Chong, Y.

T.; Meijs, G. Polymerization

F.;Moad, C. by

K.; Ercole, F.; Krstina, J.; Jeffrey, J.; Le, T. L.; Moad, G.; Rizzardo, E.

Reversible

Addition-Fragmentation

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Transfer:

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