Functional poly(carbonate-co-ether) synthesis from glycidyl methacrylate/CO₂ copolymerization catalyzed by Zn–Co(III) double metal cyanide complex catalyst

Abstract

Polycarbonates with pendant functional groups have attracted much attention due to their capability for further chemical modification and post-polymerization. This work describes the synthesis of a poly(carbonate-co-ether) with massive pendant acrylate groups from the copolymerization of glycidyl methacrylate (GMA) with carbon dioxide (CO₂), using a nanolamellar zinc-cobalt double metal cyanide complex (Zn–Co(III) DMCC) catalyst. The carbonate linkage content (F_CO2) of the poly(carbonate-co-ether) could be varied from 42.2 to 68.0% by changing the polymerization conditions. Of importance, 4-methoxyphenol was applied for regulating the copolymerization. It could not only act as an inhibitor for completely depressing the self-polymerization of GMA via free radical polymerization of the double bond, but also modulate the molecular weight of the resultant copolymers. The obtained copolymer had two terminal hydroxyl groups, which were confirmed by the electrospray ionization-tandem mass spectrometry (ESI-MS) technique. A new thermoset with high glass transition temperature (T_g: 105 or 120 °C) and massive carbonate units as well as hydroxyl (or carboxylic) groups was prepared by the curing reaction of the GMA–CO₂ copolymer with allyl alcohol or acrylic acid in the presence of 2,2′-azobisisobutyronitrile (AIBN).

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Introduction

The carbon dioxide (CO₂)/epoxides copolymerization into valuable polycarbonates has attracted growing attention in the last 40 years.¹ Since the innovative work of Inoue and co-workers² by using a diethyl zinc/water catalyst in 1969, much progress has been made in the development of more efficient and selective catalysts for the CO₂–epoxides copolymerization.³ However, the epoxides selected for the copolymerization with CO₂ were very limited, and considerable amounts of studies were focused on the few aliphatic epoxides such as propylene oxide (PO) and cyclohexene oxide (CHO).⁴

As the polymer properties are generally governed by the structure of backbones and side chains,⁵ several works^6–12 aimed to improve the performance of the CO₂-based polycarbonate have been reported. For example, some new epoxides with electron-withdrawing groups or aromatic backbones such as styrene oxide,⁶ epichlorohydrin,⁷ indene oxide,⁸ [2-(3,4-epoxycyclohexyl)ethyl]-trimethoxysilane,⁹ 3,4-cyclohexene-oxide-1-carboxylic acid methyl ester,¹⁰ 3,4-cyclohexene-oxide-1-carboxylic acid phenyl ester,¹⁰ limonene oxide,¹¹ 4-vinyl-1-cyclohexene-1,2-epoxide¹² have been applied for CO₂ copolymerization. As expected, the corresponding polycarbonates possessed improved thermal or mechanical performances. Moreover, if the produced polycarbonates had pendant functional groups, a sequential post-modification could also improve the properties. For example, our group has reported a well-defined degradable polymer brush that was synthesized by firstly introduction of side hydroxyl groups into the copolymer from 4-vinyl-1-cyclohexene-1,2-epoxide with CO₂, and then ring-opening copolymerization of ε-caprolactone.¹²

From this viewpoint, the glycidyl ethers are of particular interest because various pendant functional groups could be available. However, few works have been reported for the selective copolymerization of glycidyl ethers and CO₂. Tominaga¹³ disclosed the production of alternating copolymers of CO₂ with glycigyl ether monomers possessing phenyl, n-butyl, t-butyl and methoxyethyl by using zinc glutarate as catalyst. Frey¹⁴ reported the alternating copolymerization of CO₂ with ethoxy ethyl glycidyl ether, benzyl glycidyl ether and isopropylidene (glyceryl glycidyl ether) by ZnEt₂/pyrogallol catalyst. Recently, Lu¹⁵ reported the production of stereo-regular benzyl glycidyl ether–CO₂ copolymer catalyzed by a salen Co(III) complex. All of the above obtained polycarbonates could have massive hydroxyl groups by the deprotection of pendant alkoxy groups. A polycarbonate with vinyl groups was also produced by the copolymerization of CO₂ with allyl glycidyl ether,¹⁶ by using the ZnEt₂/pyrogallol catalyst. However, no works were reported for the copolymerization of CO₂ with glycidyl esters, which are less active than the aforementioned glycidyl ethers due to strong electron-withdrawing ester group.

Glycidyl methacrylate (GMA), one of the most common glycidyl esters, contains two reactive groups of vinyl and oxirane. Selective polymerization on vinyl group of GMA will produce the linear poly(glycidyl methacrylate) (PGMA) with numerous pendant oxirane groups, which are considered to be a versatile polymeric building block for post-polymerization modifications,¹⁷ whereas the anionic polymerization of the ring-opening of oxirane groups of GMA could lead to the generation of polyether with vinyl substituents,¹⁸ which is a promising polymer for yielding thermosetting materials.^17a Several attempts focused on the formation of five-membered cyclic carbonates were also reported by the coupling reaction of CO₂ with GMA, PGMA or copolymers with PGMA units.¹⁹ However, to the best of our knowledge, no works were reported on direct copolymerization of CO₂ with oxirane group of GMA, affording an acrylate-functionalized polycarbonate, of which a post-polymerization could be carried out to improve the properties of resultant polycarbonate.

Zinc–cobalt double metal cyanide complex (Zn–Co(III) DMCC) is a typical heterogeneous catalyst, which is often prepared from the precipitation reaction of excess ZnCl₂ and K₃Co(CN)₆ at the presence of organic complexing agents. Traditionally, t-BuOH was employed to be the most effective organic complexing agent for high catalytic activity, and the empirical formula of this heterogeneous catalyst could be noted as Zn₃[Co(CN)₆]₂·xZnCl₂·yt-BuOH·zH₂O.^20,21 The excess ZnCl₂ and an appropriate amount of t-BuOH during preparation were proved to be indispensable for its catalysis. Zn–Co(III) DMCC catalyst could catalyze numerous epoxide-involving reactions, such as epoxides homo-polymerization,^20b,c epoxides/CO₂ copolymerization^20e and the cycloaddition of epoxides and CO₂.^20f,g Recently, a nanolamellar Zn–Co(III) DMCC catalyst was synthesized and carried out for the selective copolymerization of PO and CO₂ at relatively low temperatures of 40–60 °C, exhibiting an improved catalytic activity and carbonate selectivity.^21a The highly excellent performance of this catalyst may be related to its high BET surface areas and nanostructure, which physically ensure a better exposure of the active sites to the reactants.

In the present work, such nanolamellar Zn–Co(III) DMCC catalyst was applied to the selective copolymerization of GMA and CO₂, resulting in poly(carbonate-co-ether)s with pendant acrylate groups. The effects of various polymerization conditions such as temperature, time, pressure and the mass ratio of the inhibitor (4-methoxyphenol)/Zn–Co(III) DMCC on GMA–CO₂ copolymerization were systematically investigated. The self-polymerization of the double bond could be completely inhibited by introducing 4-methoxyphenol into the GMA–CO₂ copolymerization system, the molecular weight (MW) of the resultant poly(carbonate-co-ether)s could be regulated by the amounts of 4-methoxyphenol via chain transfer reaction. Moreover, a post-modification was performed for producing a cross-linked polymer with considerable amounts of carbonate units, hydroxyl (or carboxylic) groups and high glass transition temperature (T_g).

Experimental

Materials

K₃Co(CN)₆ (Yixing City Lianyang Chemical Co., Ltd, China, 99%) was recrystallized in de-ionized water before use. Zinc chloride (ZnCl₂), tert-butanol (t-BuOH), tetrahydrofuran (THF), allyl alcohol and acrylic acid were all analytical grade and used without further purification. Glycidyl methacrylate (GMA) was refluxed over calcium hydride (CaH₂) for three hours and then distilled. Carbon dioxide (CO₂) with a purity of 99.995% was used as received. 2,2′-Azobisisobutyronitrile (AIBN) and 4-methoxyphenol were purchased from Aldrich and used without further purification.

Preparation of nanolamellar Zn–Co(III) DMCC catalyst

A typical nanolamellar Zn–Co(III) DMCC catalyst was synthesized according to our previous work:²¹ 8.0 g ZnCl₂ were dissolved into the mixed solvent of de-ionized water (10 ml) and t-BuOH (10 ml). Aqueous solution of K₃Co(CN)₆ (6.6 g in 10 ml de-ionized water) were added drop-wisely into ZnCl₂ solution over 30 min at room temperature under vigorous stirring. The reaction mixture was then heated to 75 °C and agitated greatly for 3 h. The resulting white precipitates were separated by pressure filtration and reslurried in the mixed t-BuOH–de-ionized water (v/v = 1/1) with vigorous stirring over 2 h. Once again the precipitates were isolated and reslurried in the mixed t-BuOH–de-ionized water (v/v = 1/1). With increasing volume proportion of t-BuOH over water, the precipitate was washed several times to remove potassium ions. Finally the precipitates were reslurried in neat t-BuOH to remove water, and separated and dried at 70 °C under vacuum to a constant weight. The elemental analysis result of the Zn–Co(III) DMCC catalyst was: Co: 12.48; Zn: 27.29; N: 16.57; C: 23.34; H: 2.27; Cl: 9.50. The structure characterization of this catalyst can be seen in our recent works.^6e

Copolymerization of GMA and CO₂

A 10 ml autoclave combined with a small magnetic stirrer was pre-dried at 120 °C for 3 h and then cooled down to room temperature in desiccators. Appropriate amounts of the catalyst, 4-methoxyphenol and 3.0 ml GMA were transferred into the autoclave. The reactor was then heated and pressured to the desired temperature and CO₂ pressure. After the reaction, the autoclave was cooled with ice-water bath and the CO₂ pressure was vented. A small amount of crude resultant was removed for ¹H NMR spectroscopy and GPC analysis to determine the carbonate linkage selectivity and molecular weight. The remained products were dissolved with dichloromethane and then precipitated by excess methanol. The precipitated copolymers were collected and dried at 60 °C under vacuum to a constant weight.

Preparation of the cross-linked polymers

The detailed experiment of the cross-linked polymers production is as follows: 2.0 g GMA–CO₂ poly(carbonate-co-ether) and 0.01 g 2,2′-azobisisobutyronitrile (AIBN) were dissolved in 20 ml tetrahydrofuran (THF) at 25 °C under agitation, and then the temperature was raised to 70 °C. Allyl alcohol (or acrylic acid) solution (0.1 mol monomer sample dissolved in 10 ml THF) was added drop-wisely into the polymer/AIBN solution. After 7 h, the cross-linked polymer was obtained by filtration and drying under vacuum at 60 °C for 24 h.

Characterization

Infrared spectra were measured by using a Brucker Vector 22 FT-IR spectrophotometer. Elemental analyses were performed by using an ICP-AES instrument (Leeman Labs) and a Vario MACRO instrument (Elementar). ¹H NMR spectra of the products were recorded on a Bruker Advance DMX 400 MHz spectrometer using tetramethylsilane as an internal reference. MW and molecular weight distribution (M_w/M_n) were determined by using a PL-GPC 220 chromatograph (Polymer Laboratories Ltd.) equipped with a HP 1100 pump from Agilent Technologies. The gel permeation chromatography (GPC) columns were eluted with tetrahydrofuran at 1.0 ml min⁻¹ at 35 °C. Calibration was performed using mono-dispersed polystyrene standards covering the MW ranging from 580 to 460 000 Da. Differential scanning calorimetric (DSC) tests were conducted on a TAQ200 instrument (New Castle, DE) with a heating rate of 10 °C min⁻¹ under N₂ atmosphere, and the data from the second heating curve were collected. ESI-MS analysis was performed on an Esquire3000 plus mass spectrometer by using a mixture of dichloromethane and methanol as the solvent for the copolymer.

Result and discussion

The GMA–CO₂ copolymerization was successfully catalyzed by the nanolamellar Zn–Co(III) DMCC catalyst. Note that a vinyl homo-polymerization inhibitor was required for this copolymerization, or a slight cross-linked copolymer could be obtained because the products were partly insoluble in dichloromethane and THF. Herein, 4-methoxyphenol was selected as an effective inhibitor without poisoning the catalyst, which could also act as an efficient chain transfer agent for the copolymerization (Scheme 1).

Scheme 1 CO₂–GMA copolymerization catalyzed by Zn–Co(III) DMCC catalyst.

A typical GMA–CO₂ copolymer was obtained at 50 °C and 4.0 MPa within 13 h, with a Zn–Co(III) DMCC loading of 20 mg for 3 ml GMA monomer in the presence of 4-methoxyphenol. This copolymer was characterized by FT-IR and ¹H NMR, respectively. Fig. 1 shows the FT-IR spectra of the crude and purified GMA–CO₂ copolymers. The peak at 1720 cm⁻¹ was derived from the stretching vibration of ester structure of the GMA unit. The peaks at 1800 and 1750 cm⁻¹ were from cyclic carbonate and carbonate units, respectively. The peak at 1640 cm⁻¹ was ascribed to the pendant C=C bond, indicating that the vinyl groups of GMA were retained after the copolymerization.

Fig. 1 FT-IR spectra of the crude (line 1) and purified (line 2). GMA–CO₂ copolymer from GMA–CO₂ copolymerization catalyzed by Zn–Co(III) DMCC catalyst.

Fig. 2 shows the ¹H NMR spectra of the crude and purified GMA–CO₂ copolymers. All protons in the spectra were clearly assigned. The chemical shifts at 6.2 ppm and 5.6 ppm were attributed to the protons of the pendant C=C bond. The chemical shifts at 5.2 ppm and 5.1 ppm were ascribed to the methine protons of carbonate linkages in the copolymer. Herein, the chemical shift at 5.1 ppm was caused by the random distribution of ether linkages.^7c The isolated triple peak at 4.6 ppm was assigned to a proton of the five-membered carbonate group of cyclic carbonate (i, curve 1 in Fig. 2), and the peaks of two other protons (j, k) were covered by peaks from other protons, as seen in Fig. 2. The wide peaks from 3.5 ppm to 3.8 ppm (curve 2 in Fig. 2) were mainly resulted from the protons of ether linkages in the GMA–CO₂ copolymer. Based on the ¹H NMR spectra, the molar fraction of carbonate unit in the copolymer (F_CO2) and the weight percentage of cyclic carbonate in the crude copolymer (W_cc, wt%) could be calculated. The productivity of the catalyst was defined to be the g polymer (excluding the cyclic carbonate by-product) per g zinc in the Zn–Co(III) DMCC catalyst and was calculated based on the yield of the purified copolymer.

Fig. 2 ¹H NMR spectra of the crude (line 1) and purified (line 2) GMA–CO₂ copolymer.

Table 1 summarizes the effects of polymerization temperatures and CO₂ pressures on GMA–CO₂ copolymerization catalyzed by Zn–Co(III) DMCC catalyst. As is well known, the formation of five-membered cyclic carbonate and polyether is usually considered to be thermodynamically favourable.^21a Therefore, the relatively higher temperatures would result in lower polymer selectivity and carbonate selectivity. As shown in entries 1–4 in Table 1, the lowest F_CO2 of 42.2% and massive cyclic carbonate content of 28.7% were obtained at the highest copolymerization temperature of 80 °C. Decreasing the temperature from 80 °C to 50 °C led to clear increases of F_CO2 from 42.2 to 66.6%, and MW of the copolymer from 4.4 to 7.9 kg mol⁻¹. The productivity of catalyst increased from 366.5 to 687 g polymer per g Zn with decreasing reaction temperatures from 80 °C to 60 °C. Since the cyclic carbonate was derived from the intramolecular elimination of the propagating chains through a back-biting mechanism,^6e,7c the lower content of cyclic carbonate is consistent with the higher MW of GMA–CO₂ copolymer and productivity of the catalyst. A slight decrease in MW and productivity from 60 °C to 50 °C might be attributed to the slower polymerization rate at lower temperature in a given time (entries 3 and 4, Table 1). However, no product was found when the reaction temperature was lowered to 45 °C (entry 5, Table 1), even in a longer time of 24 h (entry 6, Table 1). In addition, higher CO₂ pressure was favorable to the production of polycarbonate, F_CO2 of 68.7% for entry 7 copolymer was achieved at a CO₂ pressure of 5.0 MPa (Table 1), while the F_CO2 value decreased to 61.8% when 2.0 MPa was applied (entry 9, Table 1). Note that the ring-opening reaction of GMA took place easily in the presence of Zn–Co(III) DMCC catalyst. A polyether was obtained at 50 °C within 13 h (entry 10, Table 1), which means the rate of GMA homo-polymerization was faster than that of GMA–CO₂ copolymerization, caused low alternating degree of GMA–CO₂ copolymer.

Table 1 The effect of reaction temperatures and pressures on the GMA–CO₂ copolymerization^a

Entry	Temp. (°C)	Pre. (MPa)	FCO2 (%)	Wcc (wt%)	Mn (kg mol^−1)	Mw/Mn	Productivity (g polymer per g Zn)
a Reaction condition: Zn–Co(III) DMCC 20 mg, 4-methoxyphenol 7 mg, GMA 3 ml, time 13 h. The molar fraction of carbonate units (FCO2) and the weight percentage of cyclic carbonate by-product in total product (Wcc, wt%) were calculated by integrating the ^1H NMR peak area: FCO2% = (A5.2–4.9 − A4.6)/(A5.2–4.9 − A4.6 + A3.8–3.5/3), Wcc (wt%) = 186 × A4.6/(186 × A5.2–4.9 + 142 × A3.8–3.5/3). Mn and Mw/Mn of the crude copolymers were determined by GPC at 35 °C, using polystyrene standards for calibration.b The reaction time was 24 h.
1	80	4.0	42.2	28.7	4.4	2.4	366.5
2	70	4.0	54.1	26.1	4.6	2.5	458.2
3	60	4.0	55.5	19.9	9.4	2.5	687.0
4	50	4.0	66.6	10.5	7.9	2.4	412.1
5	45	4.0	ND	ND	ND	ND	ND
6^b	45	4.0	ND	ND	ND	ND	ND
7	50	5.0	68.7	9.7	8.5	2.5	453.6
8	50	3.0	63.2	10.5	7.3	2.5	391.3
9	50	2.0	61.8	10.2	6.3	2.5	300.5
10	50	—	—	—	5.6	2.6	380.2

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With the fixed temperature, pressure, amounts of the catalyst and inhibitor 4-methoxyphenol, the effect of polymerization time on the GMA–CO₂ reaction was investigated, as shown in Table 2. Both F_CO2 and M_n of the copolymers increased from 46.1 to 56.8% and 5.2 to 12.7 kg mol⁻¹, respectively, when the copolymerization times increased from 9 h to 20 h (Table 2). The productivity of the catalyst increased from 366.5 to 802.1 g polymer per g Zn. However, with increasing the reaction time from 12 h to 20 h, W_cc kept nearly the same (18.8–19.4 wt%). This means that the intramolecular elimination reaction of the polymer propagating chains for producing cyclic carbonate was depressed possibly due to high viscous system in the late stage of the copolymerization.

Table 2 The effect of the reaction time on GMA–CO₂ copolymerization^a

Entry	Time (h)	FCO2 (%)	Wcc (wt%)	Mn (kg mol^−1)	Mw/Mn	Productivity (g polymer per g Zn)
a Reaction condition: Zn–Co(III) DMCC 20 mg, 4-methoxyphenol 7 mg, GMA 3 ml temperature 60 °C, pressure 4.0 MPa. The molar fraction of carbonate units (FCO2) and the weight percentage of cyclic carbonate by-product in total product (Wcc, wt%) were calculated by integrating the ^1H NMR peak area: FCO2% = (A5.2–4.9 − A4.6)/(A5.2–4.9 − A4.6 + A3.8–3.5/3), Wcc (wt%) = 186 × A4.6/(186 × A5.2–4.9 + 142 × A3.8–3.5/3). Mn and Mw/Mn of the crude copolymers were determined by GPC at 35 °C, using polystyrene standards for calibration.
1	9	46.1	15.9	5.2	2.5	366.5
2	10	49.6	17.2	5.8	2.8	550.0
3	12	53.2	18.8	8.5	2.6	641.3
4	13	55.5	19.9	9.4	2.5	687.0
5	15	56.1	21.7	10.5	2.6	733.0
6	18	56.6	20.5	11.3	2.7	778.7
7	20	56.8	19.4	12.7	2.5	802.1

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The effect of 4-methoxyphenol on the copolymerization was also investigated in detail. When the copolymerization was carried out without inhibitor, the product showed slight cross-linked property and hard to filter by a 400 nm filter head (entry 1, Table 3). However, when 4-methoxyphenol was used, a polymer with excellent solubility in methylene chloride was obtained. It can be seen from the entries 2–9 in Table 3, the increase of the amounts of 4-methoxyphenol led to the decrease of MW in a linear manner. This suggests that 4-methoxyphenol also acted as the chain transfer agent. The MWs of the resultant GMA–CO₂ copolymers could be adjusted by varying the amounts of 4-methoxyphenol. The F_CO2 and W_cc were almost unaffected by the amounts of inhibitor added. However, the productivity of the catalyst decreased when more amounts of 4-methoxyphenol added (esp. at the mass ratio of DMCC/inhibitor from 0.5 to 0.25, entries 7–9, Table 3). The competitive coordination of PO and 4-methoxyphenol to the zinc centre²² might account for the decrease of productivity of the DMCC catalysts with increasing the amount of 4-methoxyphenol.

Table 3 The effect of the mass ratio of catalyst/4-methoxyphenol on the GMA–CO₂ copolymerization^a

Entry	DMCC/inhibitor	FCO2 (%)	Wcc (wt%)	Mn (kg mol^−1)	Mw/Mn	Productivity (g polymer per g Zn)
a Reaction condition: Zn–Co(III) DMCC 20 mg, GMA 3 ml temperature 60 °C, pressure 4.0 MPa, time 13 h. The molar fraction of carbonate units (FCO2) and the weight percentage of cyclic carbonate byproduct in total product (Wcc, wt%) were calculated by integrating the ^1H NMR peak area: FCO2% = (A5.2–4.9 − A4.6)/(A5.2–4.9 − A4.6 + A3.8–3.5/3), Wcc (wt%) = 186 × A4.6/(186 × A5.2–4.9 + 142 × A3.8–3.5/3). Mn and Mw/Mn of the crude copolymers were determined by GPC at 40 °C, using polystyrene standards for calibration.
1	∞	—	—	—	—	660.7
2	2.86	55.5	19.9	9.4	2.5	687.0
3	2.00	54.8	20.1	7.9	2.3	778.5
4	1.50	53.9	20.3	5.2	2.4	644.5
5	1.00	56.0	19.7	4.6	2.5	620.2
6	0.67	55.2	18.8	4.0	2.6	575.3
7	0.5	53.6	20.4	3.2	2.5	545.8
8	0.33	54.3	19.2	2.9	2.8	480.2
9	0.25	54.4	18.9	2.2	3.3	316.1

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The end groups of the resultant GMA–CO₂ copolymer were analyzed by the electrospray ionization-tandem mass spectrometry (ESI-MS) in a positive mode. The tested copolymer sample had a F_CO2 of 55%, M_n of ca. 720 and a M_w/M_n of 3.2, which was prepared at 60 °C and 4.0 MPa CO₂ pressure for 1.5 h under a mass ratio of catalyst/inhibitor of 0.2 (Zn–Co(III) DMCC/4-methoxyphenol = 20 mg/100 mg). Fig. 3 shows the ESI-MS spectrum of this GMA–CO₂ copolymer with a m/z range from 300 to 1450. Five species with various ether linkage contents were observed and ascribed as follows: HO–(GMA–CO₂)_n–(GMA)₁–H + Na⁺ (n = 1, 2, 3, 4, 5, 6), HO–(GMA–CO₂)_n–(GMA)₂–H + Na⁺ (n = 0, 1, 2, 3, 4, 5,6), HO–(GMA–CO₂)_n–(GMA)₃–H + Na⁺ (n = 1, 2, 3, 4, 5), HO–(GMA–CO₂)_n–(GMA)₄–H + Na⁺ (n = 0, 1, 2, 3, 4), HO–(GMA–CO₂)_n–(GMA)₅–H + Na⁺ (n = 1, 2, 3) (Table 4). The five species were found to own two terminal hydroxyl groups, indicating that the initiation reaction was triggered by Zn–OH group of Zn–Co(III) DMCC catalyst, which was responsible for one terminal hydroxyl group. The other hydroxyl group was produced by the chain transfer reaction to 4-methoxyphenol, trace water, or polymers in the reaction system. With increasing the amounts of 4-methoxyphenol, the rate of the chain transfer rate increased, resulting in low MWs of the resultant copolymers. Since no copolymers with 4-methoxyphenol group were observed, we proposed that the new site of Zn–methoxyphenol from chain transfer reaction of propagating chain to 4-methoxyphenol was hard to initiate the GMA–CO₂ copolymerization. This also made the activity of the catalyst decreased with increasing the amounts of end 4-methoxyphenol. Such copolymer has low MW and two end –OH groups, thus it could be potentially applied to make polyurethanes.

Fig. 3 ESI-MS spectrum of the GMA–CO₂ copolymer with ms [300–1450].

Table 4 m/z species of the crude GMA–CO₂ copolymer (ESI full ms [300–1450])

Species	m/z
HO–(GMA–CO2)n–(GMA)1–H + Na^+ (17 + 186n + 142 + 1 + 23, n = 1, 2, 3, 4, 5, 6)	369, 555, 741, 927, 1113, 1299
HO–(GMA–CO2)n–(GMA)2–H + Na^+ (17 + 186n + 142 × 2 + 1 + 23, n = 0, 1, 2, 3, 4, 5, 6)	325, 511, 697, 883, 1069, 1255, 1441
HO–(GMA–CO2)n–(GMA)3–H + Na^+ (17 + 186n + 142 × 3 + 1 + 23, n = 1, 2, 3, 4, 5)	653, 839, 1025, 1211, 1397
HO–(GMA–CO2)n–(GMA)4–H + Na^+ (17 + 186n + 142 × 4 + 1 + 23, n = 0, 1, 2, 3, 4)	609, 795, 981, 1167, 1353
HO–(GMA–CO2)n–(GMA)5–H + Na^+ (17 + 186n + 142 × 5 + 1 + 23, n = 1, 2, 3)	937, 1123, 1309

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Such GMA–CO₂ copolymers from Zn–Co(III) DMCC catalysis only have low glass transition temperatures (T_gs) due to the massive ether linkages in the copolymer. T_g value of a sample with a F_CO2 of 68% and M_n of 8.5 kg mol⁻¹ was only 18 °C. However, it has massive pendant acrylate groups, which could be utilized to prepare the thermosets with degradable carbonate units. Herein, two kinds of the cross-linked polymers were synthesized by reacting with allyl alcohol and acrylic acid in the presence of 2,2′-azobisisobutyronitrile (AIBN), as seen in Scheme 2. Both thermosets also presented massive hydroxyl (or carboxylic) groups, which were proved by FT-IR spectra, as shown in Fig. 4, in which the stretching vibration peak of C=C bond (1640 cm⁻¹) of both copolymers disappeared and the IR peaks resulted from –OH groups (3500 cm⁻¹) were observed. After cross-linked reaction with allyl alcohol (or acrylic acid), the produced cross-linked polymers showed improved T_gs of 105 °C (or 120 °C, Fig. 5). The carbonate units in such thermosets may make them to be easily degraded by water and enzyme.²³

Scheme 2 The curing reaction of the GMA–CO₂ copolymer with allyl alcohol or acrylic acid via free radical polymerization.

Fig. 4 FT-IR spectra of cross-linked polymers from GMA–CO₂ copolymer and allyl alcohol (A) or acrylic acid (B).

Fig. 5 The T_gs of GMA–CO₂ copolymer (A) and cross-linked polymers from GMA–CO₂ copolymer and allyl alcohol (B) or acrylic acid (C).

Conclusions

The GMA–CO₂ copolymerization was successfully performed by using the nanolamellar Zn–Co(III) DMCC catalyst, leading to poly(carbonate-co-ether) with pendant acrylate groups. The resultant poly(carbonate-co-ether) had a F_CO2 up to 68%. The molecular weight of poly(carbonate-co-ether)s could be adjusted by varying the amount of 4-methoxyphenol. Moreover, we provided a new kind of thermoset with high T_g and massive pendant hydroxyl (or carboxylic) groups, which may be degraded by water and enzyme.²³

Acknowledgements

The authors are grateful for the financial support by the National Science Foundation of the People's Republic of China (no. 21274123 and 21074106), and the Science and Technology Plan of Zhejiang Province (no. 2010C31036).

Notes and references

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