Bio-Based Functional Styrene Monomers Derived from Naturally

Article pubs.acs.org/Macromolecules

Bio-Based Functional Styrene Monomers Derived from Naturally Occurring Ferulic Acid for Poly(vinylcatechol) and Poly(vinylguaiacol) via Controlled Radical Polymerization Hisaaki Takeshima,† Kotaro Satoh,*,†,‡ and Masami Kamigaito*,† †

Department of Molecular and Macromolecular Chemistry, Graduate School of Engineering, Nagoya University, Nagoya 464-8603, Japan ‡ Precursory Research for Embryonic Science and Technology, Japan Science and Technology Agency, 4-1-8 Honcho, Kawaguchi, Saitama 332-0012, Japan S Supporting Information *

ABSTRACT: A naturally occurring vinylphenolic compound, 4-vinylguaiacol (4VG: 4-hydroxy-3-methoxystyrene), which is derived from naturally occurring ferulic acid via decarboxylation, was used for the synthesis of well-defined bio-based poly(vinylguaiacol) and poly(vinylcatechol) with phenolic functions. Through one-step chemical conversions of 4VG, a series of 4VG derivatives protected with acetyl (Ac4VG), tert-butyldimethylsilyl (TBDMS4VG), or triethylsilyl (TBDMS4VG) groups as well as bis(triethylsilyl)-protected vinyl catechol (TES2VC) were synthesized with high yields (>90%). The controlled radical polymerization of these protected bio-based phenolic styrene monomers successfully proceeded in the presence of appropriate reversible addition−fragmentation chain transfer (RAFT) agents or alkoxyamines with or without radical initiators such as 4,4′azobis(isobutyronitrile) (AIBN) to result in polymers with controlled molecular weights. Bimolecular combination reactions were suppressed even at the high monomer conversions of >95%, especially for TES2VC possessing two bulky substituents, indicating the excellent living character of the polymerization in comparison to other styrene monomers. Deprotection of the silyl groups was easily attained with hydrochloric acid in THF and water to result in well-defined poly(vinylguaiacol) and poly(vinylcatechol) without loss of the RAFT terminals. These bio-based polystyrenes with phenolic functions were soluble in methanol and alkaline solution. The block copolymerization of TES2VC was accomplished with various common vinyl monomers, such as styrene, methyl methacrylate, methyl acrylate, and n-butyl acrylate, in the presence of their prepolymers as macro-RAFT agents, resulting in well-defined catechol-containing block copolymers after deprotection without any damage to the ester substituents and RAFT terminals.

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INTRODUCTION

compounds and are obtained simply by replacing the raw materials with those originating from natural resources, while others have unique structures and different properties originating from the natural compounds. Both types of biobased polymers have been commercialized; they are mostly condensation polymers such as polyesters, polyamides, and polycarbonates. Considering the abundance of natural vinyl compounds such as those observed in terpenoids and phenylpropanoids, we have been developing novel bio-based

Abundant, natural, renewable resources have been attracting much attention in broad areas of chemistry, especially those related to energy and materials from the viewpoint of sustainable development in industry and science.1−17 Another important viewpoint for natural compounds is the judicious use of natural, specific molecular structures for special purposes as functional materials, which often cannot be easily accessed from petroleum-derived chemicals. From both viewpoints, a wide variety of bio-based polymers have been prepared from naturally occurring compounds. Some of these polymers have the same structures as those obtained from petroleum-derived © XXXX American Chemical Society

Received: May 11, 2017

A

DOI: 10.1021/acs.macromol.7b00970 Macromolecules XXXX, XXX, XXX−XXX

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Macromolecules

This study is directed toward the synthesis of novel silylprotected 4VGs, for which the protection and deprotection are facile, and controlled radical polymerization of the protected monomers for the synthesis of well-defined biobased polystyrenes with phenolic functions (Scheme 1). We focused

vinyl polymers to obtain special properties and functions using radical and cationic polymerization for renewable vinyl monomers with unique molecular structures.18−29 Ferulic acid is an abundant natural phenolic compound that belongs to a major class of aromatic phytochemicals, i.e., phenylpropanoids, which are composed of phenyl (C6) and propyl (C3) fragments.30−33 Ferulic acid is found in many plant cell walls, such as in cereal, rice, and wheat brans; corn hulls and kernels; and seeds of coffee, artichoke, peanut, apple, and orange and is thus one of the most abundant antioxidants in our diet. Although their contents in those plants are at most several grams per 1 kg of the sources, it can also be derived from huge renewable resources, i.e., lignin and its derivatives such as vanillin. The chemical structure of ferulic acid features a skeleton of phenylpropenoic or cinnamic acid along with phenolic functions similar to other naturally occurring hydroxycinnamic acids such as coumaric, caffeic, and sinapic acids. Their reduced forms, hydroxycinnamyl alcohols, are biologically polymerized in plant cells via oxidative coupling into lignin,34,35 which is a natural cross-linked phenolic polymer and the second most abundant natural product after cellulose. Recently, various ferulic acid-based polyesters, polyamides, and polycarbonates have been prepared using both hydroxyl and carboxylic acid groups in the hydroxycinnamic acid skeletons, mainly via condensation polymerizations.36−48 These polymers have a wide range of thermomechanical properties due to the mainchain aromatic moiety and additional components as building blocks. Some of these polymers mimicked the structure of poly(ethylene terephthalate) and exhibited similar properties.40 These studies indicated that ferulic acid is a promising natural resource for producing renewable aromatic polyesters, polyamides, and polycarbonates, while phenolic functions are basically lost in the resulting polymers due to condensation reactions. Another way to convert ferulic acid into polymers may be addition polymerization of the vinyl group in the cinnamic acid skeleton. Although the conjugated carbon−carbon double bond can be regarded as a part of either styrene or acrylic acid, it can barely be polymerized via radical polymerization due to the inherently bulky 1,2-disubsituted vinyl group as well as the antioxidant phenolic group. Direct radical copolymerization of ferulic acid with methacrylic acid has been investigated for producing antioxidant materials, although the resulting polymer structure was not clarified.49−51 Alternatively, after protection of both the carboxylic acid and phenol moieties, the monomer was radically copolymerized, while the content of the ferulic acid unit was low due to the steric hindrance around the vinyl group.52 Via decarboxylation, ferulic acid can be converted into a monosubstituted styrene derivative, i.e., 4-hydroxy-3methoxystyrene, known as 4-vinylguaiacol (4VG).53,54 This styrene derivative also occurs in nature and is contained in many beverages such as beer, wine, sake (rice wine), and juice as a flavor or off-flavor because it is biologically metabolized from ferulic acid by enzymes and microorganisms.55−58 Direct radical homopolymerization of 4VG without protecting the phenolic functions has been attempted, resulting in low molecular weight oligomers (Mn ∼ 2 × 103).59,60 The acetoxy-protected monomer was prepared and radically polymerized into high molecular weight polymers (up to Mn ∼ 2 × 105), which were subsequently deprotected into poly(4VG).61,62 However, there are almost no studies on other protected monomers derived from 4VG as well as their controlled polymerizations for the well-defined poly(4VG).

Scheme 1. Synthesis of Bio-Based Functional Styrene Monomers Derived from Naturally Occurring Ferulic Acid Followed by Controlled Radical Polymerization for Precision Synthesis of Poly(vinylcatechol), Poly(vinylguaiacol), and Their Block Copolymers

especially on the one-pot synthesis of disilyl-protected vinylcatechol from 4VG and the synthesis of well-defined poly(vinylcatechol) and its block copolymer via the RAFT polymerization followed by simple deprotection under mild conditions without any damage to other substituents and the RAFT terminal. Catechol moieties are also widely found in natural products, such as urushiols produced in the lacquer tree and 3,4-dihydroxy-L-phenylalanine (DOPA) in the adhesive proteins of mussels.63−65 Since the 1,2-dihydroxybenzene (1,2benzenediol) unit shows many interesting properties such as adhesive, curable, reductive, and antioxidative properties, this group has been introduced into many synthetic polymers to mimic the properties of natural polymers or to grant special functions. For vinyl polymers, several protected vinylcatechols B


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Macromolecules

Figure 1. 1H NMR spectra (CDCl3 at rt) of 4VG and a series of protected 4VGs: 4VG (A), Ac4VG (B), TBDMS4VG (C), TES4VG (D), and TES2VC (E).

catechols that can be easily deprotected under mild conditions, especially from the viewpoint of material designs based on welldefined polymer structures. Renewability is an additional key factor for these developments. In this paper, we examined various protections on 4VG using acetyl and silyl groups to synthesize a series of protected vinylguaiacols and vinylcatechol and investigated RAFT polymerization for the synthesis of welldefined poly(vinylguaiacol) and poly(vinylcatechol) as well as their block copolymers.

such as 3,4-dimethoxy- and 3,4-methylenedioxy- and related alkyl-protected styrenes have been prepared from their aldehyde precursors, mainly via the Wittig reaction.66−72 However, the deprotection of these alkyl groups requires harsh conditions using HBr or BBr3 and may give some damage to some other units if present, such as block copolymers. Recently, bis-tert-butyldimethylsilyl-protected vinylcatechol was prepared from 3,4-dihydroxybenzaldehyde via protection of the dihydroxy groups followed by the Wittig reaction with a yield of approximately 20%.73 It is thus attractive to develop simpler and more effective synthetic method for protected vinylC


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RESULTS AND DISCUSSION Synthesis of Protected 4-Vinylguaiacols. A series of protected 4VGs were prepared from commercially available naturally occurring 4VG as well as that obtained via decarboxylation of ferulic acid. The acetyl- (Ac4VG) and monosilyl-protected 4VGs with tert-butyldimethylsilyl (TBDMS4VG) and triethylsilyl (TES4VG) were synthesized almost quantitatively (98%) via general protection reactions of alcohols or phenols with acetic anhydride61 and trialkylsilyl chlorides (Figure 1). The one-pot double silylation of both hydroxy and methoxy groups was investigated with a stoichiometric amount of triethylsilane in the presence of a catalytic amount of B(C6F5)3.65,74 The disilylation proceeded almost quantitatively without any possible polymerization of the styrene unit and resulted in the bis(triethylsilyl)-protected vinylcatechol (TES2VC) in high yield (90%). Alternatively, TES2VC was synthesized via two steps from 4VG, i.e., single silylation of the phenolic group in 4VG with Et3SiCl followed by substitution of the methoxy group in the obtained TES4VG with Et3SiH in the presence of B(C6F5)3. TES2VC is a novel protected vinylcatechol, which is easily and almost quantitatively derived from naturally occurring compounds, 4VG and ferulic acid. Controlled Radical Polymerization of Protected 4Vinylguaiacols. RAFT polymerizations of all the synthesized protected 4VGs, as well as unprotected 4VG, were examined using cumyl dithiobenzoate (CDB) as the RAFT agent, which is generally effective for controlling the radical polymerization of styrenes,75 in the presence of AIBN in toluene at 60 °C (Figure 2). Although the polymerization of unprotected 4VG

(>90%), probably due to the bimolecular combination reaction between the propagating polymer radicals, which is usually observed for styrene radical polymerization. Meanwhile, it is noteworthy that there were no shoulders for the SEC curves of the polymers of TES2VC, showing narrow molecular weight distributions (MWDs) (Mw/Mn < 1.1) even at a high conversion (95%). This indicates that the termination reaction was suppressed by the two bulky substituents resulting from the monomer. The number-average molecular weights (Mn) increased in direct proportion to monomer conversions in all cases. The Mn values based on the calibration by polystyrene standards in SEC were slightly lower than the calculated values (Table 1), assuming that one molecule of the RAFT agent generates one polymer chain. The lower values are ascribed to differences in the hydrodynamic volumes of the polymers with two bulky substituents. Indeed, the molecular weights measured by a multiangle laser light scattering (MALLS) detector equipped with the SEC were much closer to the calculated values (Figure S1). The RAFT polymerization of TES2VC in the presence of 2cyano-2-propyl dithiobenzoate (CPDB) with the same Z group in the RAFT agent (R-SC(S)Z) also gave controlled polymers with similarly narrow MWDs (Mw/Mn < 1.1) (entry 9 in Table 1 and Figure S2). However, 2-cyano-2-propylethyl trithiocarbonate (CPETC) (entry 10) and 2-cyano-2-propylpyrrole dithiocarbamate (CPPyDC) (entry 11) resulted in controlled molecular weights but slightly broader MWDs (Mw/Mn = 1.1− 1.2 and 1.2−1.3, respectively) due to a slower RAFT process with trithiocarbonate and dithiocarbamate for styrene monomers.75 Notably, there were also no shoulders for poly(TES2VC) obtained with CPDB, CPETC, and CPPyDC, even at high monomer conversions (>95%). These results indicate that all these protected monomers prepared from 4VG were polymerized in a controlled fashion via RAFT polymerization in the presence of an appropriate RAFT agent and AIBN. The control of thermal polymerization without using a radical initiator was investigated for these bio-based styrene monomers at 110 °C in bulk in the presence of CDB (Table 1). In all cases, the polymerization proceeded, resulting in polymers with similarly controlled molecular weights with narrow MWDs (Mw/Mn = 1.1−1.2) (Figure S3). In addition, the thermal polymerization of TES2VC was investigated in the presence of alkoxyamine (St-TIPNO; N-tert-butyl-N-(2-methyl1-phenylpropyl)-O-(1-phenylethyl)hydroxylamine), which is generally effective for controlling styrene polymerization via a dissociation−combination mechanism,75 under similar conditions without a radical initiator (entries 14−16 in Table 1 and Figure S4). The obtained polymers had controlled molecular weights and narrow MWDs (Mw/Mn ∼ 1.1) without any shoulder peaks. These bio-based styrene monomers were thus thermally polymerized without using any radical initiators, resulting in polymers with controlled molecular weights in the presence of appropriate RAFT or alkoxyamine agents. Living Radical Polymerization of Protected 4-Vinylcatechol. The absence of high-molecular-weight shoulder peaks in the RAFT polymerization of TES2VC suggests the excellent living nature of the polymerization in comparison to those of general styrene monomers. To confirm the “living” character in the RAFT polymerization of TES2VC, a monomer addition experiment was performed: when the conversion of the initial feed of monomer reached 92%, the same amount of the second feed of monomer was added. Even after the monomer addition, the polymerization continued to yield

Figure 2. Time−conversion curves for the RAFT polymerization of 4VG and protected 4VGs: [M]0/[CDB]0/[AIBN]0 = 100/1.0/0.25 in toluene (for protected 4VGs) or CH3OH (for 4VG) at 60 °C, [Ac4VG]0 = 4.0 M, [TBDMS4VG]0 = [TES4VG]0 = [4VG]0 = 3.0 M, [TES2VC]0 = 2.0 M.

stopped at approximately 20% as expected due to the weak inhibition by the free phenolic group, the polymerization of the other protected 4VGs occurred almost quantitatively, for which the polymerization rate depended on the substituents. These substituents are thus effective for protecting the phenolic group and maintaining stability during the RAFT radical polymerizations. As shown in Figure 3, all the size-exclusion chromatography (SEC) curves of the obtained polymers were narrow throughout the reactions and shifted to high molecular weights as the polymerizations proceeded. The SEC curves of the polymers obtained from Ac4VG, TBDMS4VG, and TES4VG showed a small shoulder at the high monomer conversions D


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Figure 3. Mn, Mw/Mn, and SEC curves of the polymers obtained in the RAFT polymerization of protected 4VGs: [M]0/[CDB]0/[AIBN]0 = 100/ 1.0/0.25 in toluene at 60 °C, [Ac4VG]0 = 4.0 M, [TBDMS4VG]0 = [TES4VG]0 = 3.0 M, [TES2VC]0 = 2.0 M.

Table 1. Controlled Radical Polymerization of Bio-Based Functional Styrene Monomersa entry

monomer

initiating system

[M]0 (M)

temp (°C)

time (h)

convb (%)

Mn(calcd)c

Mnd

Mw/Mnd

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16

4VG Ac4VG Ac4VG TBDMS4VG TBDMS4VG TES4VG TES4VG TES2VC TES2VC TES2VC TES2VC TES2VC TES2VC TES2VC TES2VC TES2VC

CDB/AIBN CDB/AIBN CDB CDB/AIBN CDB CDB/AIBN CDB CDB/AIBN CPDB/AIBN CPETC/AIBN CPPyDC/AIBN CDB CPETC St-TIPNO St-TIPNO St-TIPNO

3.0 4.0 5.6 3.0 3.5 3.0 3.5 2.0 2.0 2.0 2.0 2.5 2.5 2.5 2.5 2.5

60 60 110 60 110 60 110 60 60 60 60 110 110 110 90 90

328 48 12 110 90 204 60 36 36 42 66 48 48 56 378 1086

20 93 53 89 56 91 58 95 93 92 98 62 74 91 85 90

3300 18100 10500 23800 15100 24300 15700 34900 34200 33800 36000 22900 27100 33500 31200 65900

2200 13900 6900 19500 9600 18800 10300 20800 21100 21700 20200 14900 15200 21500 17000 41500

1.59 1.23 1.06 1.13 1.11 1.12 1.11 1.08 1.08 1.16 1.28 1.15 1.32 1.11 1.12 1.13

a

Polymerization condition: [M]0/[RAFT]0/[AIBN]0 = 100/1.0/0.25 in toluene (for entries 2, 4, 6, and 8−11) or CH3OH (for entry 1), [M]0/ [RAFT]0 = 100/1.0 in bulk (for entries 3, 5, 7, 12, and 13), [M]0/[St-TIPNO]0 = 100/1.0 (for entries 14, 15) or 200/1.0 (for entry 16) in bulk. b Determined by 1H NMR. cMn(calcd) = MW(monomer) × ([M]0/[RAFT]0 or [M]0/[St-TIPNO]0) × conv + MW(RAFT or St-TIPNO). d Determined by SEC.

monomer conversion. These results indicate that the RAFT polymerization of TES2VC can be incredibly well controlled for the radical polymerization of styrene. To further evaluate the controllability of the polymerization, the feed ratio of monomer to RAFT agent was varied from 50 to 1000 by changing the concentration of the RAFT agent while maintaining that of the monomer. In all cases, the polymerization smoothly proceeded to yield polymers with narrow MWDs (Mw/Mn < 1.2) and highly controlled molecular weights up to nearly 2 × 105 at [M]0/[CDB]0 = 1000, although the molecular weights were based on the polystyrene calibration by SEC (Figure 5). These results indicate that the RAFT polymerization of the disilyl-protected vinylcatechol was highly controlled due to the bulky substituents, which prevent the bimolecular termination reaction via combination between the propagating radical chain ends. Deprotection of Silyl Groups. Deprotection of the acetyl and silyl groups was then investigated to synthesize welldefined poly(vinylguaiacol) and poly(vinylcatechol). The triethylsilyl group was easily deprotected by hydrochloric acid under mild conditions in tetrahydrofuran (THF) at room

polymers with similarly narrow MWDs (Mw/Mn < 1.1) and without any remaining first polymer or shoulder peaks (Figure 4). The Mn values further increased in direct proportion to the

Figure 4. Mn, Mw/Mn, and SEC curves of poly(TES2VC) obtained in monomer addition experiment in the RAFT polymerization: [TES2VC]0/[TES2VC]add/[CDB]0/[AIBN]0 = 2000/2000/40/10 mM in toluene at 60 °C. E


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Furthermore, the RAFT terminal (peaks x, y, and z) was almost intact during the deprotection, which was indicated by the presence of the dithiobenzoate terminal as well as the good agreement of the Mn obtained from the NMR spectrum (Mn(NMR) = 6300 or 8800) with the calculated value (Mn(calcd) = 6500 or 8200). In addition, the methyl proton at the α-end of the RAFT terminal became visible along with the disappearance of the overlapped Et3Si group via the deprotection reaction. These results indicate that well-defined poly(VC) was successfully obtained, without any damage at the RAFT terminal of the polymers, via mild deprotection of the protected Et3Si group of poly(TES2VC). Properties of Bio-Based Polystyrenes. The thermal properties and solubilities of the protected and deprotected biobased polystyrenes were then analyzed. As shown in the differential scanning calorimetry (DSC) curves, all the polymers showed a glass transition temperature (Tg), which varied widely from 20 to 190 °C depending on the substituents (Figure 7).

Figure 5. Synthesis of high-molecular-weight poly(TES2VC) by the RAFT polymerization: [TES2VC]0/[CDB]0/[AIBN]0 = 2000/2.0/ 0.50, 2000/4.0/1.0, 2000/10/2.5, 2000/20/5.0, 2000/40/10 mM in toluene at 60 °C.

temperature for 3 h. Figure 6 shows the 1H NMR spectra of poly(TES2VC) obtained in the RAFT polymerization in the presence of CDB before and after deprotection. Before the deprotection (Figure 6A), the Mn value (Mn(NMR)) measured by the peak intensity ratio of the phenyl protons (c) of the TES2VC units to those of the dithiobenzoate group of the RAFT ω-terminal was 16 400, which was close to the calculated value (M n (calcd) = 17 000) and that from MALLS (Mn(MALLS) = 15 400). These results again indicate that the RAFT polymerization of TES2VC successfully occurred in the presence of CDB to produce the polymer possessing welldefined RAFT terminal groups. As shown in Figure 6B, the characteristic Et3Si group signal at approximately 0.5−1.2 ppm for poly(TES2VC) completely disappeared after the reaction with HCl. In addition, a new broad peak appeared at approximately 7.0−7.6 ppm, which can be attributed to catechol protons in poly(VC). A similar spectral change was observed for poly(TES4VG) via the deprotection reaction (Figures S5C and S5D). These results indicate the successful deprotection of the triethylsilyl groups of poly(TES2VC) and poly(TES4VG) into poly(VC) and poly(4VG), respectively.

Figure 7. Differential scanning calorimetry (DSC) curves of the polymers obtained in the RAFT polymerization of protected 4VGs: poly(Ac4VG) (Mn(SEC) = 8500), poly(TBDMS4VG) (Mn(SEC) = 19 500), poly(TES4VG) (Mn(SEC) = 11 000), poly(TES2VC) (Mn(SEC) = 21 700), poly(4VG) (Mn(calcd) = 8200), poly(VC) (Mn(calcd) = 12 900).

The Tg of poly(Ac4VG) was 110 °C, which was almost the same as the reported value.62 For the two silyl-protected 4VGs, both the Tgs were lower than that of the acetoxy-protected one. Furthermore, the Tg of the triethylsilyl-protected 4VG (poly(TES4VG)) was lower than that of poly(TBDMS4VG)

Figure 6. 1H NMR spectra (A: in CDCl3 at 55 °C; B: in (CD3)2CO at 50 °C) of poly(TES2VC) obtained in the RAFT polymerization and poly(VC) obtained after deprotection: [TES2VC]0/[CDB]0/[AIBN]0 = 2000/20/5.0 mM in toluene at 60 °C. F


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Macromolecules Table 2. Solubility of Polymers before and after Deprotectiona

a

polymer

hexane

toluene

CHCl3

THF

Et2O

EtOAc

acetone

CH3OH

poly(Ac4VG) poly(TBDMS4VG) poly(TES4VG) poly(TES2VC) poly(4VG) poly(VC)

− + + + − −

+ + + + − −

+ + + + − −

+ + + + + +

− + + + − −

+ + + − + +

+ + + − + +

− − − − + +

1 M NaOHaq

H2O

+ +

− − − − − −

“+”: soluble; “−”: insoluble, 1 wt %.

Figure 8. SEC curves for the block copolymerization of TES2VC with various vinyl monomers. Synthesis of macro-RAFT agents: (A) [St]0/[CDB]0 = 8700/87 mM in bulk at 110 °C; (B) [MMA]0/[CDB]0/[AIBN]0 = 7000/70/14 mM in toluene at 60 °C; (C) [MA]0/[CDB]0/[AIBN]0 = 4000/ 40/8.0 mM in toluene at 60 °C; (D) [n-BA]0/[CDB]0/[AIBN]0 = 2000/20/4.0 mM in toluene at 60 °C. Block copolymerization of TES2VC using macro-RAFT agents in toluene at 60 °C: (E) [TES2VC]0/[PSt]0/[AIBN]0 = 1000/10/2.0 mM; (F) [TES2VC]0/[PMMA]0/[AIBN]0 = 500/5.0/1.0 mM; (G) [TES2VC]0/[PMA]0/[AIBN]0 = 750/7.5/1.5 mM; (H) [TES2VC]0/[PBA]0/[AIBN]0 = 1000/10/2.0 mM.

(38 vs 71 °C). The Tg was further decreased by disubstitution with the triethylsilyl group (21 °C for poly(TES2VC)). These results indicate that trialkylsilyl groups generally decrease the Tg of the polymers by enhancing the mobility of the polymer chain. However, after deprotecting the silyl groups, the Tgs greatly increased due to the hydrogen-bonding interactions between the phenolic groups, with a much higher Tg observed for poly(VC) (Tg = 190 °C). The solubility of the polymers was examined in various solvents (Table 2). Poly(Ac4VG), i.e., polystyrene with acetoxy and methoxy substituents, was soluble in toluene, chloroform, and THF, as polystyrene is, and was further soluble in ethyl acetate and acetone due to the presence of the polar substituents. The silyl-protected poly(4VG)s, i.e., poly(TES4VG) and poly(TBDMS4VG), became further soluble in n-hexane due to the trialkylsilyl substituents. The bistriethylsilyl-protected polystyrene, i.e., poly(TES2VC), was similarly soluble in n-hexane but became insoluble in ethyl acetate and acetone. After the deprotections, poly(4VG) and poly(VC) became soluble in methanol and aqueous sodium hydroxide (1 M) solution but became insoluble in toluene and chloroform. The solubility of these phenolic polymers is similar to that of poly(vinylphenol), which is used as a photoresist. Block Copolymerization with Vinyl Monomers. The block copolymerization of TES2VC and conjugated vinyl monomers such as styrene (St), methyl methacrylate (MMA), methyl acrylate (MA), and n-butyl acrylate (BA) was investigated to synthesize well-defined block copolymers of

vinylcatechol and common vinyl monomers. A series of prepolymers of these common vinyl monomers, which were prepared using CDB as a RAFT agent, were used as macroRAFT agents with a dithiobenzoate terminal for block polymerization of TES2VC, in the presence of AIBN in toluene at 60 °C. In all cases, TES2VC was smoothly consumed, as in the RAFT homopolymerization. All the SEC curves shifted to high molecular weights while retaining narrow MWDs (Mw/Mn = 1.1−1.2), suggesting the formation of the block copolymers (Figure 8). In addition, all the 1H NMR spectra of the resulting polymers showed the peaks of each monomer unit (Figure 9A,C and Figure S6A,C). The number-average degree of polymerization for each monomer unit was then calculated from the peak intensity ratios of the monomers to the terminal dithiobenzoate moiety (peak x). The numbers of repeating units (n and m) by NMR were in good agreement with the calculated values, assuming that one molecule of one macro-RAFT agent generates one polymer chain. Consequently, the Mn(NMR) was also close to the calculated molecular weight Mn(calcd). These results indicate that block copolymers of TES2VC and the common vinyl monomers were successfully prepared using appropriate RAFT agents. The block copolymerization in the reverse order was also investigated via sequential addition of the second monomer (St, MA, or MMA) in the RAFT polymerization of TES2VC at the high conversion (94%) without isolation of the prepolymer (Figure S7). The SEC curves for the block copolymerization of G


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Figure 9. 1H NMR spectra (A and C: in CDCl3 at 55 °C; B and D: in (CD3)2CO at 50 °C) of block copolymers. (A) PSt-b-poly(TES2VC) obtained by [TES2VC]0/[PSt]0/[AIBN]0 = 1000/10/2.0 mM in toluene at 60 °C. (B) PSt-b-poly(VC) obtained after deprotection. (C) PMMA-bpoly(TES2VC) obtained by [TES2VC]0/[PMMA]0/[AIBN]0 = 500/5.0/1.0 mM in toluene at 60 °C. (D) PMMA-b-poly(VC) obtained after deprotection.

St and MA shifted to high molecular weights, indicating successful blocking from the substituted styrene prepolymer terminal. In the case of MMA, however, the SEC curve showed bimodal distribution with remaining poly(TES2VC) because fragmentation of the styryl terminal is more difficult than that of methacrylate.75 The macro-RAFT approach was similarly effective for St and MA in the reverse order (Figures S8 and S9).

The deprotection of the disilyl groups in all the block copolymers was similarly investigated using HCl in THF at room temperature for 3 h. All the Et3Si groups were completely removed and transformed into catechol without any damage to the ester substituents of the (meth)acrylate units or the dithiobenzoate RAFT terminal (Figure 9B,D and Figure S6B,D). The number-average degree of polymerization for each monomer unit was unchanged via the deprotection and remained in good agreement with the calculated value. Thus, H


Macromolecules well-defined block copolymers of vinylcatechol with styrene, MMA, MA, or BA were successfully obtained via RAFT polymerization using disilyl-protected vinylcatechol as the precursor monomer. Most of the DSC curves of the resulting block copolymers after deprotection showed the presence of high Tg, approximately 160−190 °C, due to the poly(vinylcatechol) segments (Figure S10). More specifically, the block copolymer of styrene and vinylcatechol has two Tgs, i.e., 91 and 175 °C, which can be ascribed to those of the polystyrene and poly(vinylcatechol) segments, respectively, suggesting microphase separation. The block copolymers with BA also showed two Tgs, −42 and 188 °C, which are close to those of poly(BA) (Tg = −49 °C)76 and poly(VC) (Tg = 190 °C), respectively. However, the block copolymers of VC/MMA and VC/MA showed only the one Tg at approximately 160 or 110 °C, suggesting the more-or-less miscible character of the poly(vinylcatechol) segments with the less bulky ester moieties of the poly(methyl (meth)acrylate).

ACKNOWLEDGMENTS

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REFERENCES

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CONCLUSIONS Naturally occurring 4-vinylguaiacol and ferulic acid were easily and quantitatively transformed into novel bio-based styrene monomers, in which the phenolic functions in 4-vinylguaiacol and 4-vinylcatechol were protected with silyl groups. Controlled radical polymerization of these protected phenolic styrenes successfully proceeded in the presence of appropriate RAFT agents or alkoxyamines, where bimolecular termination reactions were significantly suppressed for bis(Et3Si)-protected vinylcatechol due to the steric hindrance of the bulky substituents. The silyl groups were easily deprotected by HCl under mild conditions to result in the well-defined bio-based poly(vinylcatechol) and poly(vinylguaiacol), which were soluble in both methanol and alkaline solutions and had high glass transition temperatures. The block copolymers of vinylcatechol with common vinyl monomers such as styrene, methacrylate, and acrylate were successfully synthesized via RAFT polymerization followed by deprotection of the silyl groups without any damage to the substituents of the ester groups or the RAFT terminals. This synthetic route to biobased polystyrenes can be used for other naturally occurring phenylpropenoic acids, which are now under investigation in our group. In addition, the well-defined block copolymers containing catechol moieties can be applied for novel bio-based functional materials based on their adhesive, curable, reductive, and antioxidative properties. ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.7b00970. Experimental procedures and supplementary data (PDF)

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This work was supported in part by Program for Leading Graduate Schools “Integrative Graduate Education and Research Program in Green Natural Sciences”.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

Masami Kamigaito: 0000-0001-7584-5524 Notes

The authors declare no competing financial interest. I


Article

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Bio-Based Functional Styrene Monomers Derived from Naturally

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