New Degradable Alternating Copolymers from Naturally Occurring

Article pubs.acs.org/Macromolecules

New Degradable Alternating Copolymers from Naturally Occurring Aldehydes: Well-Controlled Cationic Copolymerization and Complete Degradation Yasushi Ishido, Arihiro Kanazawa, Shokyoku Kanaoka, and Sadahito Aoshima* Department of Macromolecular Science, Graduate School of Science, Osaka University, Toyonaka, Osaka 560-0043, Japan S Supporting Information *

ABSTRACT: Three naturally occurring conjugated aldehydes, (1R)-(−)-myrtenal, (S)-(−)-perillaldehyde, and βcyclocitral, were cationically copolymerized with isobutyl vinyl ether using the EtSO3H/GaCl3 initiating system in the presence of 1,4-dioxane as an added Lewis base. Alternating copolymerization proceeded exclusively via 1,2-carbonyl addition of the aldehydes. In addition, controlled alternating copolymerization was achieved under appropriate reaction conditions, producing copolymers with controlled molecular weights and narrow molecular weight distributions. The relationships between the copolymerization behaviors and the cyclic side group structures of the aldehydes suggested that conjugated and bicyclic structures were important factors for controlled alternating copolymerization. However, too much bulkiness around the carbonyl group resulted in termination of copolymerization. The resulting alternating copolymers were stable under neutral and basic conditions. In sharp contrast, mild acidic conditions degraded the alternating copolymers almost selectively to conjugated aldehydes with low molecular weights as nearly single products.

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INTRODUCTION Polymer synthesis from naturally occurring renewable resources has been driven by the urgent need to conserve petroleum resources, leading to a “carbon neutral” and sustainable society.1 In fact, various biomass-based polymers have recently been synthesized as alternatives for petroleum-based polymers.2−7 Naturally occurring compounds often contain specific structures such as mono- or bicyclic structures, olefinic moieties, or chirality, some of which are not easily obtained from petroleum-derived molecules. Typical examples are terpenes, the polymers of which exhibit various characteristics such as a very high glass transition temperature, high transparency, and/or unique monomer sequences.8−10 Lactic acid,11 its derivatives,12 and some lactones13 have also drawn attention as biomass-based materials for degradable polymers. The recent success of controlled/living polymerizations of such monomers has allowed additional functionalization by block copolymerization or chain-end modification.14−18 Potential natural resources for cationic polymerization are aldehydes, which are abundant in nature. For example, benzaldehydes (BzAs) are found in almond19 or cherry,20 and p-methoxyBzA (also called anisaldehyde) can be extracted from anise21 and fennel oils.22 However, the cationic polymerization of these compounds has rarely been studied due to difficulty in their polymerization as reported decades ago.23,24 We recently reported the successful controlled cationic copolymerization of BzAs with alkyl vinyl ethers (VEs) using weak Lewis base-assisting initiating systems for the first time.25,26 Nearly alternating copolymers with controlled © 2012 American Chemical Society

molecular weights and narrow molecular weight distributions (MWDs) were obtained from the appropriate combinations of monomers, i.e., VEs with lower reactivity and aldehydes with larger basicity (Scheme 1).27 The resulting alternating Scheme 1. Alternating Copolymerization of Benzaldehydes (BzAs) with Vinyl Ethers (VEs) and Selective Acid Hydrolysis of Product Copolymers27

copolymers possessed acetal linkages in their main chains at regular intervals; thus, acid hydrolysis selectively yielded cinnamaldehyde derivatives (CinAs), used in perfumes, which can also be copolymerized with a VE monomer. Received: March 8, 2012 Revised: April 19, 2012 Published: May 1, 2012 4060

dx.doi.org/10.1021/ma3004828 | Macromolecules 2012, 45, 4060−4068

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Scheme 2. Cationic Copolymerization of Naturally Occurring Aldehydes with Isobutyl Vinyl Ether (IBVE) and Acid Hydrolysis of Resulting Copolymers

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Since CinAs could be copolymerized with VEs into alternating copolymers, other naturally occurring cyclic conjugated aldehydes became candidates as comonomers for the synthesis of new selectively degradable copolymers with VEs. Three terpenoids, (1R)-(−)-myrtenal (myrtenal), (S)(−)-perillaldehyde (PA), and β-cyclocitral (CC), are the most readily available (Scheme 2). Myrtenal, a main component of the essential oils of asteraceae plants, is a chiral bicyclic conjugated aldehyde.28 PA, a chiral monocyclic conjugated aldehyde with an olefinic moiety, is abundantly extractable from perilla.29 CC is another monocyclic conjugated aldehyde with three methyl substituents in the proximity of the polymerizable carbonyl group and is convertible from citral,30−32 an acyclic conjugated aldehyde existing in lemongrass oil.33 However, there are no examples of the addition polymerization of these monomers, although a polycondensation using myrtenal was recently reported.34 In this study, we investigated cationic copolymerizations of the above-mentioned naturally occurring aldehydes, myrtenal, PA, and CC, with isobutyl VE (IBVE) using the ethanesulfonic acid (EtSO3H)/GaCl3 initiating system in the presence of weak Lewis bases (Scheme 2). The effects of the side group structures such as mono- or bicyclic structures, bulkiness around carbonyl groups, and olefin structures on the copolymerization behaviors are of great interest. Addition modes of the enal (α,β-unsaturated carbonyl) moiety, i.e., 1,2-, 1,4-carbonyl, and vinyl additions, may vary depending on the catalyst and reaction conditions,35,36 while complete control of the addition mode in conjugated aldehyde (co)polymerizations has never been achieved except for exclusive vinyl addition (co)polymerization via a radical process.37 Thus, we also focused on the microstructure of the product copolymers as well as the optimization of the reaction conditions for controlled cationic copolymerization of the three aldehydes bearing conjugated cyclic structures with VEs. The present study not only develops degradable polymers from naturally occurring compounds but also establishes a new strategy for polymerization of conjugated aldehydes. Under appropriate reaction conditions, the controlled alternating copolymerizations successfully proceeded, yielding copolymers with narrow MWDs. Surprisingly, NMR analysis of the product copolymers revealed the exclusive 1,2-carbonyl addition reactions of the aldehydes. The effects of the side group structures of the aldehydes on the copolymerization behavior were also examined. Moreover, the resulting alternating copolymers could be completely degraded via acid hydrolysis to single aldehydes with extended conjugated structures without any oligomeric residues.

EXPERIMENTAL SECTION

Materials. Isobutyl VE (IBVE; TCI, >99.0%) was distilled twice over calcium hydride before use. Toluene-d8 (Wako, 99.6%) was distilled once over calcium hydride before use. (1R)-(−)-Myrtenal (myrtenal; Aldrich, >97%), (S)-(−)-perillaldehyde (PA; Aldrich, >92%), β-cyclocitral (CC; Aldrich, >90%), cyclohexanecarbaldehyde (CHA; TCI, >98%), and 2,6-di-tert-butylpyridine (DTBP; Aldrich, >97%) were distilled over calcium hydride at least twice under reduced pressure before use. Diethyl ether (Wako, >99.5%) and 1,4-dioxane (Wako, >99.5%) were distilled over calcium hydride and then lithium aluminum hydride. Toluene and hexane were dried by passage through solvent purification columns (Glass Contour). The adduct of IBVE with HCl [IBVE−HCl; CH3CH(OiBu)Cl] was prepared from addition reaction of IBVE with HCl according to literature methods.38 Ethanesulfonic acid (EtSO3H; Aldrich, 95%) and an EtAlCl2 solution (Wako, 1.0 M in hexane) were used as received. For FeCl3, a stock solution in diethyl ether was prepared from commercial anhydrous FeCl3 (Aldrich, 99.99%). For GaCl3, a stock solution in hexane was prepared from commercial anhydrous GaCl3 (Aldrich, 99.999+%). Hydrochloric acid (Nacalai Tesque) and tetrahydrofuran (THF; Wako, >99.5%) were used as received. All reagents except for toluene, hexane, THF, and hydrochloric acid were stored in brown ampules under dry nitrogen. Polymerization Procedure. Polymerization was carried out under a dry nitrogen atmosphere in a glass tube equipped with a three-way stopcock. The tube was carefully dried using a heat gun (Ishizaki; PJ206A; blow temperature ∼450 °C) under dry nitrogen before polymerization reactions. A prechilled initiator solution (0.40 mL) was added to a prechilled mixture solution of a monomer, an added base, and a solvent (3.20 mL) at 0 °C by a dry medical syringe. The polymerization reaction was started by the addition of a prechilled Lewis acid solution (0.40 mL) to the prechilled mixture (3.60 mL) at −78 °C. The reaction mixture was stirred with a magnetic stir bar throughout the polymerization. The polymerization was quenched with prechilled methanol containing a small amount of aqueous ammonia solution (2.50 mL, 0.1%). The quenched reaction mixture was diluted with dichloromethane and then washed with water to remove the initiator residues. The volatiles were evaporated under reduced pressure, and the residue was vacuum-dried for at least 6 h at 60 °C. For copolymerization with CC, the quenched mixture was analyzed directly by NMR and gel permeation chromatography (GPC) due to the instability of the product copolymers to the purification process. The monomer conversion was determined by the gravimetric method and/or 1H NMR analysis. Hydrolysis. Obtained copolymers were purified by reprecipitation in alcohols at least twice to remove low molecular weight oligomers. The purified copolymer (30 mg) was dissolved in THF (5.0 mL), and then the reaction was started by the addition of an aqueous HCl−THF (1.0 M, 5.0 mL) solution at 30 °C. The reaction mixture was stirred with a magnetic stir bar throughout the hydrolysis. The hydrolysis was quenched with aqueous sodium hydroxide (1.0 M). The quenched reaction mixture was diluted with dichloromethane, washed with water to remove the resulting salt, and evaporated under reduced pressure. The residue was vacuum-dried at room temperature. 4061


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Figure 1. (A) Time−conversion curves, (B) Mn and Mw/Mn for polymer peaks, and (C) molecular weight distribution curves of products obtained by copolymerization of (1R)-(−)-myrtenal with isobutyl vinyl ether (IBVE) {[(1R)-(−)-myrtenal]0 = 0.60 M, [IBVE]0 = 0.60 M, [EtSO3H]0 = 4.0 mM, [GaCl3]0 = 4.0 mM, [1,4-dioxane] = 1.0 M, in toluene at −78 °C; Mn: number-average molecular weight, Mw/Mn: polydispersity ratio (weightaverage molecular weight/number-average molecular weight), by GPC (polystyrene calibration)}. aFor polymerization. bFor polymer. cCyclic trimer ratio in products.

Table 1. Cationic Copolymerization of (1R)-(−)-Myrtenal and Isobutyl Vinyl Ether (IBVE) Using Various Lewis Acids and Initiatorsa entry

Lewis acid

initiator

time (h)

conv (%)

Mn

Mw/Mn

polymer/oligomer

aldehyde content (%)

1 2 3 4 5

GaCl3 GaCl3 GaCl3 FeCl3 EtAlCl2

EtSO3H IBVE−HCl none EtSO3H EtSO3H

2 2 2 48 48

92 86 17 20 6

22100 16300 10700 4100 2200

1.12 1.61 1.74 1.81 1.51

98/2 97/3 94/6 94/6 81/19

48 47 48 45 −

a [(1R)-(−)-myrtenal]0 = 0.60 M, [IBVE]0 = 0.60 M, [initiator]0 = 4.0 mM, [Lewis acid]0 = 4.0 mM, [1,4-dioxane] = 1.0 M, in toluene at −78 °C. Mn: number-average molecular weight, Mw/Mn: polydispersity ratio (weight-average molecular weight/number-average molecular weight), by GPC (polystyrene calibration).

than 90% conversion in 2 h (Figure 1A).39 The Mns of the product copolymers increased linearly in direct proportion to the myrtenal conversion, and their MWDs were very narrow throughout the reaction (Mw/Mn = 1.09−1.16; Figure 1B). The MWD curves of the copolymers clearly shifted to the higher molecular weight region, indicating the generation of long-lived growing species (Figure 1C). Our previous study showed that the copolymerizations of aldehydes with VEs by the EtSO3H/ GaCl3 initiating system produced not only copolymers but also cyclic trimers (9%−56%) consisting of two aldehydes with one VE.26,27 However, the cyclic oligomer was scarcely produced (1−2%) in the copolymerization of myrtenal (Figure 1C). The copolymerization was also examined using other Lewis acid catalysts and initiators. Although polymers were obtained in each case as shown in Table 1, the Mn values of the product copolymers and their copolymerization behaviors were markedly different depending on the Lewis acid and the initiator. When IBVE−HCl/GaCl3 (entry 2) was used, a copolymer with a MWD broader than that by EtSO3H/GaCl3 (entry 1) was produced. Without any initiator, the monomer conversions were much smaller than that with an initiator, although a copolymer was produced (entry 3). FeCl3, a very active Lewis acid catalyst for the polymerization of VEs,40 showed poor monomer conversion (entry 4) compared to the reaction with GaCl3, although a long-lived growing species existed. With EtAlCl2, a Lewis acid that induced only cyclotrimerization in the copolymerizations of BzAs with VEs,26 the monomer conversion was very low and a polymer with a low molecular weight was obtained (entry 5). These results demonstrate that the proper choice of the Lewis acid and initiator is critical for controlled copolymerization of myrtenal and IBVE.

Characterization. The MWD of the polymers was measured by GPC in chloroform at 40 °C with three polystyrene gel columns (TSK gel G-4000HXL, G-3000HXL, and G-2000HXL; 7.8 mm i.d. × 300 mm; flow rate = 1.0 mL/min or TSKgel MultiporeHXL-M; 7.8 mm i.d. × 300 mm; flow rate = 1.0 mL/min) connected to a Tosoh DP-8020 pump, a CO-8020 column oven, a UV-8020 ultraviolet detector, and an RI-8020 refractive index detector. The number-average molecular weight (Mn) and polydispersity ratio [weight-average molecular weight/number-average molecular weight (Mw/Mn)] were calculated from the chromatographs with respect to 16 polystyrene standards (Tosoh; 292−1.09 × 106, Mw/Mn ≤ 1.1). NMR spectra were recorded at 30 °C in CDCl3 or CD2Cl2 using a JEOL ECA 500 spectrometer (500.16 MHz for 1H, 125.77 MHz for 13C and DEPT). Differential scanning calorimetry (DSC EXSTER-6000, Seiko Instruments Inc.) was used to determine the glass transition temperature (Tg) in the range from −100 to 120 °C for product copolymers. The heating and cooling rates were 10 °C/min. The Tg of the copolymers was defined as the temperature of the midpoint of a heat capacity change on the second heating scan. Optical rotations were measured in hexane with a 10 mm quartz cell on a JASCO J-720WO spectropolarimeter and calculated on the basis of the aldehyde units.

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RESULTS AND DISCUSSION 1. Precision Synthesis and Selective Acid Hydrolysis of Alternating Copolymer from Myrtenal with IBVE. 1.1. Controlled Copolymerization of Myrtenal and IBVE. Cationic copolymerizations of myrtenal with IBVE were first carried out using the EtSO3H/GaCl3 initiating system in the presence of 1,4-dioxane as an added Lewis base in toluene at −78 °C ([myrtenal]0 = 0.60 M, [IBVE]0 = 0.60 M, [EtSO3H]0 = 4.0 mM, [GaCl3]0 = 4.0 mM, [1,4-dioxane] = 1.0 M). These were the most appropriate conditions for the copolymerizations of BzAs with VEs.26,27 The copolymerization proceeded smoothly, and each monomer was consumed to reach more 4062


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1.2. Structure Analysis of Product Copolymers. 13C NMR measurements of the product copolymers were conducted after cyclic oligomers were removed by reprecipitation from a methanol/ethanol mixture. As shown in Figure 2, the resonance

Resonance integral ratios of the olefin peaks (g) and the acetal peaks (b) were almost the same as in the copolymerization of IBVE and myrtenal. These results support that the olefin moieties in the polymer side groups did not react with propagating chain ends at all through the copolymerization. This is also confirmed in that no shoulder peaks resulting from cross-linking reactions were observed in the higher molecular weight regions in the MWD curves (Figure 1C). This is probably because the olefin moieties in the side groups are sterically hindered and possess less electron density compared with the carbon−carbon double bond of IBVE and the carbonyl group of myrtenal.

Figure 2. Overlaid DEPT 135 (upper) and 13C NMR (lower) spectra of poly[(1R)-(−)-myrtenal-co-isobutyl vinyl ether] [Mn(GPC) = 2.23 × 104, Mw/Mn(GPC) = 1.12, aldehyde content: 48%; 125.77 MHz in CDCl3 at 30 °C].

Figure 3. 1H NMR spectrum of poly[(1R)-(−)-myrtenal-co-isobutyl vinyl ether] [Mn(GPC) = 2.23 × 104, Mw/Mn(GPC) = 1.12, aldehyde content: 48%; 500.16 MHz in CDCl3 at 30 °C].

peaks derived from the myrtenal units were observed. The absence of a carbonyl resonance ruled out polymerization through vinyl addition with myrtenal. Furthermore, olefin carbons adjacent to the oxygen atom, which would arise from 1,4-addition, were also not observed, and 15 resonance peaks were consistent with the structures derived from 1,2-carbonyl addition. These results demonstrated the incorporation of myrtenal into the copolymer through only the 1,2-carbonyl addition mode. This highly selective addition mode is attributed to the following two effects: electron densities of the polymerizable groups and steric hindrance around the double bonds. The carbonyl group in the enal moiety withdraws electrons strongly from the adjacent carbon−carbon double bond; hence, the growing carbocation reacts with the electron-rich carbonyl group selectively. In addition, the carbon−carbon double bond of the conjugated aldehyde is trisubstituted in a bulky sixmembered ring, whereas the carbonyl group is monosubstituted. Therefore, the vinyl addition reaction is also unfavorable in terms of steric hindrance. These effects are most likely to contribute to the exclusive 1,2-carbonyl addition reaction. The 1H NMR spectra of the product copolymers were then measured to determine the contents of the myrtenal units (Figure 3). The contents of the myrtenal units were calculated to be 47−48% based on the resonance integral ratios of the acetal peaks (b) and the aliphatic proton peaks (e, j, k, n). These values and no homopolymerizability of myrtenal indicate that the resulting copolymers had nearly alternating structures. Similar to the 13C NMR spectrum, there were no resonance peaks in the 1H NMR spectrum assignable to the olefin protons adjacent to the oxygen atom or the aldehyde proton, which again indicates that the copolymers had no 1,4-carbonyl and vinyl addition structures.

1.3. Acid Hydrolysis and Characteristic Properties of Resulting Alternating Copolymers. As described above, the resulting copolymers had alternating structures, which means that the acetal linkages of the IBVE−myrtenal sequences existed in the polymer backbone at regular intervals. The acid hydrolysis of poly(myrtenal-co-IBVE) was conducted in a THF−aqueous HCl mixture at 30 °C for 2 h. The reaction mixture was homogeneous throughout the reaction. The MWD curves of the original copolymer and the hydrolysis product are shown in Figure 4A. After the hydrolysis, the peak of the copolymer disappeared quantitatively and a sharp peak was observed in the low molecular weight region instead. The 1H NMR spectrum of the hydrolysis product, measured without further purification, showed that the major product was an aldehyde with an extended conjugation system [Figure 4B; the structure of the aldehyde was also confirmed by 13C NMR analysis (Figure S1)]. The almost exclusive production of an aldehyde with an extended conjugated structure without oligomeric products also supports the alternating structures of the copolymers resulting from the 1,2-additions of myrtenal (Scheme 3). We further examined the characteristics of the new alternating copolymer obtained in this study. The polymer was stable in the bulk state and in neutral and basic solutions, although it was easily degraded under acidic conditions. The polymer did not decompose at all after storage in a freezer for one year. The glass transition temperature (Tg) of the copolymer was 56 °C, which was higher than those of alternating copolymers from BzAs with VEs. 26,27 The copolymer showed a specific optical rotation derived from the chiral side groups of the myrtenal units {[α]D = 52 (at 25 °C); Mn(GPC) = 1.9 × 104, Mw/Mn(GPC) = 1.09, aldehyde 4063


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Figure 4. (A) Molecular weight distribution (MWD) curves of original poly[(1R)-(−)-myrtenal-co-isobutyl vinyl ether] (upper) and the hydrolysis product (lower) and (B) 1H NMR spectrum of the hydrolysis product [500.16 MHz in CDCl3 at 30 °C; hydrolysis conditions: 0.50 M aqueous HCl−tetrahydrofuran (THF) at 30 °C for 2 h, 0.33 wt % polymer solution; original copolymer: Mn(GPC) = 2.23 × 104, Mw/Mn(GPC) = 1.12, aldehyde content: 48%; #: peaks of THF and its ring-opening product; Mn: number-average molecular weight, Mw/Mn: polydispersity ratio (weightaverage molecular weight/number-average molecular weight), by GPC (polystyrene calibration)].

structure. Cyclohexanecarbaldehyde (CHA), a petroleum-based unconjugated aldehyde possessing a similar structure with the three naturally occurring aldehydes, was also copolymerized with IBVE. 2.1. Copolymerization of PA with IBVE. The copolymerization of PA with IBVE using EtSO3H/GaCl3 gave polymers with relatively high molecular weights and narrow MWDs. This system was examined in detail and the addition of 2,6-di-tertbutylpyridine (DTBP), a proton trap,41 was found to be crucial for controlling the copolymerization of PA with IBVE (Figure 5 and Figure S2; reactions with and without DTBP, respectively).42 The Mns of the product copolymers increased in direct proportion to PA conversion, and their MWDs were relatively narrow (Figure 5B). This is in sharp contrast to the reaction without DTBP, in which the Mn values clearly decreased and the MWD became broader in the later stage of the copolymerization due to frequent occurrences of proton transfer reactions (Figure S2). Another noticeable difference in the case of myrtenal is the larger amount of the cyclic trimer byproduct as shown in Figure 5C. The structure of the product copolymer was examined by 13 C NMR analysis. No resonance peaks derived from 1,4carbonyl or vinyl addition reactions were observed, and the

Scheme 3. Controlled Alternating Copolymerization of (1R)-(−)-Myrtenal with Isobutyl Vinyl Ether and Selective Acid Hydrolysis of the Product Copolymers

content: 47%}. The solubility of the copolymer was also examined for 1.0 wt % solutions [Mn(GPC) = 2.23 × 104, Mw/ Mn(GPC) = 1.12, aldehyde content: 48%]. The copolymer showed good solubility in hexane, toluene, dichloromethane, chloroform, diethyl ether, and THF. Water, methanol, ethanol, and dimethyl sulfoxide did not dissolve the copolymer. 2. Cationic Copolymerization of Other Aldehydes with IBVE: Effects of Cyclic Side Group Structures. Copolymerizations of other naturally occurring aldehydes, PA and CC, with IBVE were examined to elucidate the relationship between the copolymerization behavior and the aldehyde

Figure 5. (A) Time−conversion curves, (B) Mn and Mw/Mn for polymer peaks, and (C) molecular weight distribution curves of products by copolymerization of (S)-(−)-perillaldehyde (PA) with isobutyl vinyl ether (IBVE) {[PA]0 = 0.60 M, [IBVE]0 = 0.60 M, [EtSO3H]0 = 4.0 mM, [GaCl3]0 = 4.0 mM, [1,4-dioxane] = 1.0 M, [2,6-di-tert-butylpyridine] = 4.0 mM, in toluene at −78 °C; Mn: number-average molecular weight, Mw/ Mn: polydispersity ratio (weight-average molecular weight/number-average molecular weight), by GPC (polystyrene calibration)}. aFor polymerization. bFor polymer. cCyclic trimer ratio in products. 4064


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selective 1,2-carbonyl addition reaction was confirmed as in the case with myrtenal (Figure S3). The PA contents of the resulting copolymers calculated by 1H NMR analysis were 44− 46% (Figure S4). The cyclohexene and the isobutylene moieties in the side group also remained intact throughout the copolymerization. 2.2. Copolymerization of CC with IBVE. The monomer conversions of the copolymerization of CC with IBVE were very low in a reaction time similar to those for myrtenal and PA. With longer reaction time, the conversions increased slightly, but leveled off after 48 h (Figure 6A), and oligomers

Figure 7. Molecular weight distribution (MWD) curves of (A) original poly((S)-(−)-perillaldehyde-co-isobutyl vinyl ether) (upper) and hydrolysis products (lower) and (B) original poly[β-cyclocitral-coisobutyl vinyl ether] (upper) and hydrolysis products (lower) {hydrolysis conditions: 0.50 M aqueous HCl−tetrahydrofuran at 30 °C for 2 h, 0.33 wt % polymer solution; original copolymer: (A) Mn(GPC) = 1.39 × 104, Mw/Mn(GPC) = 1.21, aldehyde content: 46%, (B) Mn(GPC) = 3.4 × 103, Mw/Mn(GPC) = 1.17, aldehyde content: 41%}; Mn: number-average molecular weight, Mw/Mn: polydispersity ratio (weight-average molecular weight/number-average molecular weight), by GPC (polystyrene calibration).

under similar conditions (Figure 7B). The copolymer was degraded into oligomers with various molecular weights due to its partially random sequence. As a matter of course, the alternating sequence of a copolymer is crucial for the selective degradation to conjugated aldehydes. 2.4. Relationships between Copolymerization Behaviors and Side Group Structures. To study the effects of the conjugated structures of aldehydes on copolymerization, CHA was used as a comonomer for IBVE. The copolymerization of CHA with IBVE with EtSO3H/GaCl3 was almost completed in 4 h (>90% total monomer conversion; Figure 8). While a small amount of polymers was produced, low molecular weight compounds were the major components. The 1H NMR spectrum of the polymer after the removal of the oligomeric parts by preparative GPC was similar to that of a homopolymer of IBVE but also included some weak resonance peaks derived from CHA units (∼9%, Figure S9). The copolymerization behaviors of the aldehydes with IBVE and the properties of the product copolymers are summarized in Table 2 to illustrate the effects of the side group structures on the copolymerizations. Three naturally occurring conjugated aldehydes were incorporated into the product copolymers with higher aldehyde contents (38−48%) than CHA (9%), an aliphatic aldehyde. This is probably caused by the difference in stability among the intermediate cations. Judging from the copolymerization results using myrtenal and PA, aldehydes with a bulkier side group were favorable for

Figure 6. (A) Time−conversion curves and (B) molecular weight distribution curves of products by copolymerization of β-cyclocitral (CC) with isobutyl vinyl ether (IBVE) {[CC]0 = 0.60 M, [IBVE]0 = 0.60 M, [EtSO3H]0 = 4.0 mM, [GaCl3]0 = 4.0 mM, [1,4-dioxane] = 1.0 M, in toluene at −78 °C; Mn: number-average molecular weight, Mw/Mn: polydispersity ratio (weight-average molecular weight/ number-average molecular weight), by GPC (polystyrene calibration)}. aFor polymerization. bFor polymer.

with a broad MWD were obtained [Mn(GPC) = 1.7 × 103, Mw/ Mn(GPC) = 1.49]. This indicates that side reactions in the copolymerization of CC were not transfer, but termination, reactions in contrast to the copolymerization of PA. The 13C NMR spectra of the products purified by reprecipitation from methanol showed that repeating CC units in the copolymer chains formed via the exclusive 1,2-addition reactions of the enal group (Figure S5). The 1H NMR analysis revealed that the resulting copolymers had 38−41% CC content (Figure S6). 2.3. Acid Hydrolysis of Poly(PA-co-IBVE) and Poly(CC-coIBVE). Acid hydrolysis of poly(PA-co-IBVE) was examined under the conditions similar to those for poly(myrtenal-coIBVE) (Figure 7A). The acid hydrolysis yielded a nearly single compound, a conjugated aldehyde, as shown in Figure 7A (see Figures S7 and S8 for the identification), although slight amounts of products derived from undegraded VE−VE sequences also existed. Poly(CC-co-IBVE) was also hydrolyzed 4065


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suppresses both addition reactions: a CC monomer to an IBVE growing chain end and a CC-derived growing cation to IBVE.44

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CONCLUSION

Cationic copolymerizations of naturally occurring aldehydes, myrtenal, PA, and CC, with IBVE were examined using the EtSO3H/GaCl3 initiating system with 1,4-dioxane as an added Lewis base. Myrtenal and PA were successfully copolymerized into nearly alternating copolymers with narrow MWDs under appropriate reaction conditions, although copolymerization of CC did not reach quantitative conversion and produced only oligomers. Interestingly, the three aldehydes reacted with the growing carbocations selectively via 1,2-carbonyl addition. By comparison with the case of CHA, an unconjugated aldehyde, it was found that a conjugated structure was crucial for achieving higher aldehyde contents in the product copolymers and that a bulkier side group was also preferred for controlled copolymerization. In contrast, increased bulkiness around the carbonyl groups resulted in slower polymerization and termination reactions. The obtained alternating copolymers have unique properties such as highly selective acid-degradability. The copolymer of myrtenal with an almost perfect alternating sequence can be degraded quantitatively and selectively to an aldehyde with an extended conjugated structure. Furthermore, the copolymers have the potential not only as selectively degradable polymers but also as recyclable polymers, since the conjugated aldehydes obtained as the hydrolysis products are also polymerizable. These features will contribute to the industrial use of these copolymers as photoresistors or temporary adhesive materials with low environmental loads.

Figure 8. Molecular weight distribution curve of product by copolymerization of cyclohexanecarbaldehyde (CHA) with isobutyl vinyl ether (IBVE) {[CHA]0 = 0.60 M, [IBVE]0 = 0.60 M, [EtSO3H]0 = 4.0 mM, [GaCl3]0 = 4.0 mM, [1,4-dioxane] = 1.0 M, in toluene at −78 °C; Mn: number-average molecular weight, Mw/Mn: polydispersity ratio (weight-average molecular weight/number-average molecular weight), by GPC (polystyrene calibration)}. aFor polymerization. bFor polymer. cCyclic trimer ratio in products.

controlled reactions. The controllability of the copolymerization of myrtenal, an aldehyde with a bicyclic side group, was better than the copolymerization of PA, a monocyclic aldehyde. In the latter case, DTBP was required for suppressing protontransfer reactions. The bulky cyclic side groups both in monomers and polymer chains probably suppress the access of basic counteranions and/or carbonyl groups to β-hydrogen atoms of the propagating carbocation. Since a monocyclic side group has less steric bulk, the proton elimination reactions seem to occur more frequently in the copolymerization of PA without DTBP compared to the case of myrtenal. CC also has a bulky side group with three methyl substituents at the βpositions of its carbonyl carbon. The copolymerization behavior of CC, however, was distinctly different from those of myrtenal and PA. In past research, it was found that alkyl substitutions on the α-carbon of aldehydes prevented their polymerization,43 most likely due to the steric hindrance of the substituents. Thus, the steric bulk of the three methyl substituents of CC

Table 2. Copolymerization Behaviors of Aldehydes with Isobutyl Vinyl Ether and Properties of Product Copolymers

Mearsured in hexane at 25 °C. bMn(GPC) = 1.90 × 104, Mw/Mn(GPC) = 1.09, aldehyde content 47%. cMn(GPC) = 1.91 × 104, Mw/Mn(GPC) = 1.20, aldehyde content 45%.

a

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ASSOCIATED CONTENT

S Supporting Information *

Figures showing time−conversion curves, Mn and Mw/Mn− conversion plots of copolymerization, MWD curves, and NMR spectra. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research was supported in part by a Grant-in Aid for Scientific Research (No. 22107006) on Innovative Areas of “Fusion Materials: Creative Development of Materials and Exploration of Their Function through Molecular Control” (No. 2206), and a Grant-in-Aid for JSPS Fellows (No. 231924) for Y. Ishido from the Ministry of Education, Culture, Sports, Science and Technology, Japan (MEXT). Y. Ishido thanks The JSPS Research Fellowships for Young Scientists and The Global COE Program “Global Education and Research Center for Bio-Environmental Chemistry” of Osaka University. We thank Prof. T. Inoue group (Osaka University) for DSC experiments and Prof. T. Sato group (Osaka University), especially Dr. K. Terao and Mr. T. Yoshida, for optical rotation measurements and helpful discussions.

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Macromolecules

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slower reaction is probably due to the steric hindrance of CC around the carbonyl group.

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New Degradable Alternating Copolymers from Naturally Occurring

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