Abstract
The copolymers of 2-methylene-1,3-dioxepane (MDO) and N-phenyl maleimide (NPM) prepared by radical polymerization with high thermal stability, glass transition temperature and optical transparency are presented. The polymers made under specific reaction conditions, i.e., 120°C and high amounts of MDO, had degradable ester units, which were formed via radical ring-opening polymerization of MDO. The formation of charge-transfer complex between MDO and NPM also led to the formation of high-molar-mass copolymers by simple mixing and heating of monomers without the use of any initiator. Structural characterization of the copolymers including mechanistic studies was carried out using nuclear magnetic resonance spectroscopy, and their thermal properties were studied using differential scanning calorimetry and thermogravimetric analysis.
1 Introduction
N-Substituted maleimides are interesting monomers for making transparent thermally stable polymeric materials (1–7). Under radical polymerization condition, N-substituted maleimides make either AB-alternating or sequence-controlled 2:1 AAB polymers, with vinyl monomers having an electron-rich double bond such as styrene, vinyl ether, α-substituted propylene, etc. (3). The alternating polymers are formed via a charge-transfer complex between electron-rich and electron-poor monomers (8–11). The maleimide ring structure in every repeat unit provides very high thermal stability and glass transition temperature to the resulting polymers (1–3).
Cyclic ketene acetals (CKAs) are interesting monomers with an electron-rich double bond due to the presence of two electron-donating oxygen atoms. They undergo radical and cationic ring-opening polymerization and provide polyesters, polyacetals or a combination of the two, depending on their structure and on the reaction conditions (12). 2-Methylene-1,3-dioxepane (MDO) is one of the very well studied CKAs in terms of homo- and copolymerization, with a large number of vinyl monomers such as methyl methacrylate (MMA) (13), styrene (14, 15), vinyl acetate (16, 17), propargyl acrylate (18), glycidyl methacrylate (GMA) (19), etc. Homopolymerization of MDO leads to an aliphatic biodegradable polyester with a polycaprolactone-like structure (20), whereas cationic polymerization of MDO provides mainly ring-retained acetal units in the polymer backbone (21).
Interestingly, in our previous work (22), MDO provided hydrolytically degradable polymers with high glass transition temperature (Tg) by simple mixing and heating with electron-deficient α-methylene-γ-butyrolactone, which is a bio-based monomer derived from white tulips. Enzymatically degradable MDO copolymers with poly(ethylene glycol) methyl ether methacrylate and 7-(2-methacryloyloxyethoxy)-4-methylcoumarin methacrylate showed self-assembly into micelles in water and were studied for the release of an anticancer drug, doxorubicin (23). After this report, similar biodegradable amphiphilic copolymers were used as a template for micelle formation and drug delivery (24, 25). In addition, controlled radical ring-opening polymerization of MDO provides an opportunity to produce degradable polyesters with low-molar-mass dispersity, tunable polymer chain length and functional end groups, and also provides the possibility of producing polyesters with complex architectures (26). For instance, Hedir et al. recently reported well-defined linear and hyperbranched functional polyesters by copolymerization of MDO and vinyl monomers under reversible addition-fragmentation chain transfer polymerization condition (17). The hyperbranched functional polyesters were used for the formation of nanoparticles. Delplace et al. (27) reported branched copolymers of different CKAs with oligo(ethylene glycol) methacrylate (OEGMA) and acrylonitrile (AN) by nitroxide-mediated radical ring-opening polymerization. The resulting poly(OEGMA-co-AN-co-CKA) showed a well-defined structure, good hydrolytic degradability and low cytotoxicity.
With an aim to provide a highly thermally stable (bio)degradable transparent cost-effective polymer, we studied the copolymerization behavior of MDO and N-phenyl maleimide (NPM). After radical ring-opening copolymerization, MDO was expected to provide degradable ester units on the polymer backbone and NPM was expected to introduce high thermal stability and glass transition temperature. We studied the copolymerization behavior of NPM and MDO using different reaction conditions such as monomer feeds and reaction temperatures, with and without radical initiators. The mechanistic aspects, structural and thermal properties of the resulting copolymers were studied using nuclear magnetic resonance spectroscopy (NMR), differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA). Hydrolytically degradable copolymers of MDO and NPM with high thermal stability, glass transition temperatures and optical transparency could be obtained under specific reaction conditions.
2 Experimental section
2.1 Materials
The monomer (MDO) was made according to published literature procedures (14). 2,2′-Azobis(isobutyronitrile) (AIBN, Aldrich, Germany) and NPM (Aldrich, Germany) were used after recrystallization from methanol. Di-tert-butyl peroxide (DTBP, Aldrich, Germany) was used as received. All solvents were distilled before use.
2.2 Instrumentation
1H (300 MHz) and 13C (75 MHz) NMR spectra were recorded on a Bruker Ultrashied-300 spectrometer (Bruker, Germany) in CDCl3, using tetramethylsilane (TMS) as an internal standard. Deconvolution technology was used for NMR spectral analysis. UV-Vis measurement was carried out on a Hitachi U-3000 spectrometer. The molecular weights (Mn, GPC and Mw, GPC) and molar mass dispersities (Ð) of the polymers were determined by gel permeation chromatography (GPC). The columns and detector used were two PSS-SDV gel columns (PSS, Mainz, Germany) and a differential refractive index detector, respectively. DMF (HPLC grade, Aldrich, Germany) with LiBr (5 g/l in DMF) was used as an eluting solvent, with a flow rate of 0.5 ml/min. Poly(methyl methacrylate) (PMMA, PSS, Germany) calibration was employed for molar mass analysis. A Mettler thermal analyzer (Mettler, Gießen, Germany) with 851 TG and 821 DSC modules was used for determining the thermal stability and glass transition temperatures, respectively, in nitrogen. A heating rate of 10 K/min was used. The amount of sample used for the measurements was 10±1 mg for both DSC and TGA.
2.3 General procedure for the copolymerization of MDO and NPM
The reactions were carried out under argon in a Schlenk tube. A mixture of MDO and NPM at different mole ratios (MDO/NPM=1:9, 3:7, 5:5, 7:3 and 9:1) and a radical initiator (AIBN or DTBP) (1 wt.% of monomer) were dissolved in anisole and degassed by three freeze-vacuum-thaw cycles. The reaction was started by putting the tube in a preheated oil bath at different temperatures (60°C, using AIBN as the initiator, or 120°C, using DTBP as the initiator) for 2 h while stirring. After this, the reaction mixture was precipitated into methanol, filtered and dried at 40°C for 48 h.
2.4 Methanolysis of the MDO-NPM copolymer
The copolymer prepared at 120°C with an MDO/NPM ratio of 5:5 in the feed was used for hydrolysis experiment. A total of 0.30 g of copolymer was dissolved in a mixture of dioxane (15 ml) and methanolic KOH (25 ml, 5 wt.%) and heated to reflux for 24 h. After this time, the solution was neutralized with 3 ml of concentrated hydrochloric acid and then extracted with chloroform followed by washing with water. The solvent was evaporated under reduced pressure to get a yellow solid, which was dried under vacuum at 50°C for 2 days.
3 Results and discussion
3.1 Copolymerization of MDO and NPM at 60°C
Initially, various reactions for the copolymerization of MDO with NPM were carried out by changing the molar ratio of the two monomers in the feed (MDO/NPM=1:9, 3:7, 5:5, 7:3 and 9:1) at 60°C using AIBN as the initiator. The GPC traces for the reaction product were unimodal (Figure 1), with molar mass ranging from 6.38×104 to 2.08×105 and molar mass dispersity (Ð) from 1.7 to 2.0. During copolymerization, MDO could undergo either ring-opening polymerization, which produces ester units, or vinyl polymerization, which produces acetal rings, or a combination of the two (Scheme 1) (12). In previous studies, MDO showed the formation of quantitative ester units during copolymerization with vinyl monomers using radical initiators at a reaction temperature of between 60°C and 120°C (12, 23, 28, 29), whereas cationic polymerization of MDO provided mainly ring-retained acetal units (21). Therefore, the resulting copolymer structure was characterized by 1H NMR and 13C NMR spectroscopy (Figure 2). In the 13C NMR spectra, a very strong peak at δ=100 ppm was observed. This showed the presence of ring-retained acetal rings formed by vinyl polymerization of MDO as the major structure (Figure 2B). The characteristic proton signals of both ring-retained MDO and NPM units were also present and marked in the 1H NMR spectrum (Figure 2A). The peak integration at δ=6.5–7.5 ppm (aromatic protons of NPM) and δ=2.5 ppm [methylene group (a) of the MDO unit] was used for determining the copolymer composition, as listed in Table 1. The comonomer ratio of MDO and NPM in the resulting copolymers was almost 1:1 for different feed ratios. The copolymer with a very large amount of NPM in the feed (MDO/NPM=1:9) showed a deviation from the copolymer composition of 1:1 (MDO/NPM), and the ratio of comonomers in the copolymer was calculated as 1:3 (MDO/NPM). On using very large amounts of MDO in the feed (MDO/NPM=9:1), a small peak appeared at δ=4.0 ppm in the 1H NMR spectrum originating from the -C(O)OCH2- protons of ring-opened ester MDO units (Figure S1 in Supporting Information). This showed the formation of a mixed structure with both ester (formed by ring-opening) and ring-acetal units (formed by ring-retained vinyl polymerization) in copolymers when using a large amount of MDO monomer in the feed (path 3 in Scheme 1). The ratio of ring-opened and ring-retained units of MDO was determined by deconvolution of the overlapping 1H NMR peaks between δ=4.1–3.9 ppm. Only about 4 mol% of the total MDO units was ring opened and present as ester units in the MDO-NPM copolymer prepared with a feed monomer ratio of 9:1 (MDO/NPM). The results confirmed that the MDO units showed a high potential to undergo simple vinyl polymerization with ring-retained units at 60°C (path 1 in Scheme 1). The MDO units with ring-opened ester structure only existed in the MDO-NPM copolymer prepared with a monomer ratio of 9:1 (MDO/NPM) in the feed.
GPC curves of copolymers made by radical polymerization of MDO and NPM with different monomer ratios in the feed at 60°C.
Different possibilities during MDO-NPM copolymerization. Path 1: MDO copolymerized with ring-retained structure; path 2: MDO copolymerized with ring-opened structure; path 3: MDO copolymerized with ring-opened and ring-retained structure.
1H NMR (A) and 13C NMR (B) spectra of the MDO-NPM copolymer (MDO/NPM=5:5 in the feed) prepared at 60°C.
Copolymerization of MDO and NPM at 60°C.
| Entry | Feed ratio (molar ratio) | Yield, % | Mna | Ða | Copolymer composition (molar ratio)b | MDO with ring-opened structure, mol%b | ||
|---|---|---|---|---|---|---|---|---|
| MDO | NPM | MDOc | NPM | |||||
| 1d | 90 | 10 | 26.3 | 1.81×105 | 2.0 | 55 | 45 | 4 |
| 2d | 70 | 30 | 61.3 | 2.08×105 | 1.8 | 53 | 47 | –e |
| 3d | 50 | 50 | 52.4 | 1.81×105 | 1.8 | 49 | 51 | –e |
| 4d | 30 | 70 | 51.0 | 1.28×105 | 1.7 | 45 | 55 | –e |
| 5d | 10 | 90 | 37.9 | 6.38×104 | 1.9 | 25 | 75 | –e |
aCharacterized by GPC with an RI detector, calibrated with the PMMA standard (Figure 1).
bCharacterized by 1H NMR spectra of the MDO-NPM copolymers with a deconvolution method (Figure 2A).
cRing-opened and ring-retained MDO.
dInitiator: AIBN, 0.5 mol% of the monomer; reaction temperature, 60°C; reaction time, 2 h; solvent, anisole.
eThe content of MDO units with ring-opened structure was very low (lower than 1 mol% of all MDO units in the copolymer) and could not be calculated owing to the strong overlapping.
3.2 Copolymerization of MDO and NPM at 120°C
In an effort to produce MDO-NPM copolymers with higher contents of MDO units with a ring-opened structure, a series of MDO-NPM copolymers were prepared at 120°C with various monomer ratios in the feed. The GPC traces for the copolymers (MDO/NPM=9:1, 7:3, 5:5 and 3:7 in the feed) were unimodal (Figure 3). The copolymer made with a very high amount of NPM in the feed (MDO/NPM=1:9) showed bimodality (Figure 3). The microscopic structure of the MDO-NPM copolymers was characterized by 1H NMR (Figure 4A) and 13C NMR (Figure 4B) with full assignments. All copolymers in the 13C NMR spectra showed the presence of ketal carbon at δ=100 ppm due to the ring-retained acetal units from MDO. Compared with the 13C NMR spectra of copolymers prepared at 60°C (Figure 4B, black curve), an extra peak at δ=170 ppm in the 13C NMR spectrum of MDO-NPM prepared at 120°C (Figure 4B, red curve) was obvious, which corresponded to the carbonyl-carbon (k) of ring-opened MDO units (ester units, structure 3). Compared with the 1H NMR spectrum of the resulting copolymers prepared at 60°C (Figure 4A, black curve), the peak at δ=4.0 ppm in the 1H NMR spectrum due to the -OCH2- protons of ester units formed by ring-opening of MDO during copolymerization at 120°C was obvious (Figure 4A, red curve). Other characteristic peaks observed in the 1H NMR spectrum were δ=6.5–7.5 ppm (aromatic protons of NPM) and δ=2.4–3.1 ppm [methylene protons (a) of MDO with ring-retained structure 1 and -CH2C(O)O- protons (f) of MDO with ring-opened ester units (structure 3)]. With the use of deconvolution of overlapping peaks between δ=2.4–3.1 (Figure S2 in Supporting Information), the ratio of ester units in comparison to the ring-retained acetal units was determined (Table 2). The deconvolution was carried out using an NMR processing program (MestReNova). The copolymers prepared at 120°C showed enhanced tendency to undergo ring-opening polymerization with the formation of ester units in comparison to the polymers prepared at 60°C. An increase in the amount of ester units in the copolymers was obtained with an increase in MDO in the feed. The copolymer, which was prepared at 120°C with an MDO/NPM ratio of 9:1 in the feed, had 54 mol% of the total MDO units as ring-opened esters, whereas the copolymer with an MDO/NPM ratio of 5:5 in the feed showed 39 mol% of ring-opened structures. On reducing the amount of MDO to below 50 mol% in the initial feed, the amount of ring-opened MDO was strongly decreased and could not be determined with accuracy.
GPC traces of polymers made by radical polymerization of NPM and MDO with different monomer ratios in the feed at 120°C.
1H NMR and 13C NMR spectra of the MDO-NPM copolymer, using a monomer ratio of 7:3 (MDO/NPM) in the feed as an example. (A) 1H NMR and (B) 13C NMR; black curve: copolymerization at 60°C; red curve: copolymerization at 120°C.
Copolymerization of MDO and NPM at 120°C.
| Entry | Feed ratio (molar ratio) | Yield, % | Mna | Ða | Copolymer composition (molar ratio)b | MDO with ring-opened structure, mol%b | ||
|---|---|---|---|---|---|---|---|---|
| MDO | NPM | MDOc | NPM | |||||
| 6d | 90 | 10 | 22.6 | 1.03×105 | 2.1 | 59 | 41 | 54 |
| 7d | 70 | 30 | 55.3 | 1.13×105 | 2.1 | 57 | 43 | 45 |
| 8d | 50 | 50 | 85.1 | 6.03×104 | 2.1 | 54 | 46 | 39 |
| 9d | 30 | 70 | 82.4 | 5.46×104 | 2.7 | 36 | 64 | –e |
| 10d | 10 | 90 | 81.9 | 6.59×104 | Bimodal | 16 | 84 | –e |
aCharacterized by GPC with an RI detector, calibrated with the PMMA standard.
bCharacterized by 1H NMR spectra of the MDO-NPM copolymers with a deconvolution method.
cRing-opened and ring-retained MDO.
dInitiator: AIBN, 0.5 mol% of the monomer; reaction temperature, 60°C; reaction time, 2 h; solvent, anisole.
eThe content of MDO units with ring-opened structure was very low and could not be calculated owing to the strong overlapping.
3.3 Copolymerization mechanism
The comonomer-copolymer composition curves for copolymerization at 60°C and 120°C are shown in Figure 5. The monomer reactivity ratios at different temperatures could be determined on the basis of the comonomer-copolymer composition curves for the MDO and NPM copolymerization at 60°C and 120°C. The rMDO and rNPM values were 0.024 and 0.120, respectively, for the copolymerization at 60°C, which were much lower than the monomer reactivity ratios for the copolymerization at 120°C (rMDO=0.084 and rNPM=0.32). These results also suggested the alternating copolymerization tendency of MDO and NPM at 60°C.
Comonomer-copolymer composition curves for the MDO and NPM copolymerization at different temperatures (entries 1–5 in Table 1 and entries 6–10 in Table 2). The curve was drawn using a nonlinear least-squares curve-fitting method. Red curve: Fit curve of the copolymerization at 60°C; navy curve: fit curve of the copolymerization at 120°C. Fitting equation:
NPM has an electron-deficient double bond, whereas the double bond of MDO is electron rich because of the neighboring electron-donating oxygen atoms. The simple mixing of the two monomers could lead to the formation of a charge transfer complex and spontaneous polymerization without any initiator. In fact, heating a mixture of NPM and MDO (1:1 molar ratio) without any initiator at 60°C and 120°C led to high-molar-mass polymers in 22 h with unimodal GPC curves but with low yield (entry 11 and entry 14 in Table 3). The yield was around 10% at 60°C, whereas it was around 32% at 120°C. The copolymer composition (MDO/NPM) was almost 1:1 in both cases. With the polymerization at 120°C, 37% of the total MDO units were ring opened in the form of esters (entry 14 in Table 3), whereas the MDO was polymerized in the form of ring acetals at 60°C (entry 11 in Table 3).
Comparison of MDO-NPM copolymerization with different reaction conditions in anisole [monomer ratio in the feed: 1:1 (MDO/NPM)].
| Entry | Inhibitor/initiator | Reaction temperature, °C | Yield, % | Mna | Ða | Copolymer composition (molar ratio)b | MDO with ring- opened structure, mol%b | |
|---|---|---|---|---|---|---|---|---|
| MDOc | NPM | |||||||
| 11d | – | 60 | 9.4 | 2.66×105 | 1.9 | 48 | 52 | –e |
| 12f | Hydroquinone | 60 | 6.6 | 2.57×104 | 1.6 | 47 | 53 | –e |
| 13g | AIBN | 60 | 52.4 | 1.81×105 | 1.8 | 49 | 51 | –e |
| 14d | – | 120 | 31.7 | 5.36×104 | 2.6 | 53 | 47 | 37 |
| 15f | Hydroquinone | 120 | 10.3 | 4.14×104 | 2.9 | 54 | 46 | 21 |
| 16g | DTBP | 120 | 85.1 | 6.03×104 | 2.1 | 54 | 46 | 39 |
aCharacterized by GPC with an RI detector, calibrated with the PS standard.
bCharacterized by 1H NMR spectra of the MDO-NPM copolymers with a deconvolution method.
cRing-opened and ring-retained MDO.
dSpontaneous copolymerization; reaction time: 22 h.
eThe content of MDO units with ring-opened structure was very low and could not be calculated owing to the strong overlapping.
fCopolymerization with the inhibitor; cinhibitor=1 wt.% of the monomer; reaction time: 22 h.
gCopolymerization with the initiator; cinitiator=0.5 wt.% of the monomer; reaction time: 2 h.
The alternating copolymerization of electron-rich vinyl monomers (styrene, acrylates, etc.) with electron-deficient vinyl monomers like 2,3,4,5,6-pentafluorostyrene, maleic anhydride, N-phenyl maleimide, etc., was widely researched by Saegusa (9) and Hall (10). Based on their work, the alternating copolymerization could be preceded by diradical or zwitterion mechanism (Scheme 2).
Mechanism of the MDO-NPM copolymerization.
To obtain an insight into the mechanism of polymerization, a blank reaction was carried out. Equimolar amounts, 2.12 mmol each of MDO and NPM, were dissolved in 1 ml of CDCl3 and stirred under argon for 1 h. The resulting complex was directly analyzed by 1H NMR spectroscopy. TMS was used as the internal standard. The 1H NMR spectra (Figure 6) showed that peak a corresponding to the protons from the CH=CH group of the NPM monomer was shifted from δ=6.78 to δ=6.56 (peak a′) and that peak b corresponding to the protons from the CH2=C group of the MDO monomer was shifted from δ=3.41 to δ=3.30 (peak b′). This result strongly suggested the interaction between the double bonds of MDO and NPM. Because of the formation of the MDO-NPM complex, the other proton signals of the MDO and NPM monomer also showed a small shift in the peaks in the 1H NMR spectrum.
Comparison of the 1H NMR spectra of the MDO-NPM complex with the MDO and NPM monomers.
To determine the mechanism of this reaction, MDO-NPM copolymerization was carried out in the presence of a radical inhibitor such as hydroquinone. Although polymer yields were less, the presence of hydroquinone did not completely inhibit the polymerization and the copolymer composition was 1:1 (MDO/NPM). MDO was predominantly present as an acetal ring at 60°C, whereas 21% of the total MDO units were ring opened to ester units at 120°C, which was much less in comparison to the product obtained either via simple mixing of two monomers or by radical polymerization (Table 3). The occurrence of polymerization even in the presence of hydroquinone indicated the mixed mechanism occurring during NMP-MDO copolymerization. A similar behavior was observed during the copolymerization of MDO with a tulipalin-A-based vinyl monomer in our previous study (22). The addition of hydroquinone led to ionic polymerization overweighing the radical one, thereby providing more ring-retained MDO units. MDO cationic polymerization provided ring-retained structures in the majority of cases, as shown previously in the literature (21).
3.4 Thermal, optical and hydrolytic degradability studies
The thermal stability of the MDO-NPM copolymers made by radical polymerization was performed by TGA (Figure S3 in Supporting Information). All polymers were highly thermally stable (decomposition temperature was higher than 300°C). Because of the presence of ester units formed by ring-opening of MDO during copolymerization, the MDO-NPM copolymers with monomer ratios of 9:1, 7:3 and 5:5 (MDO/NPM) in the feed at 120°C showed a two-step thermal decomposition (initial decompositions at 320°C and 410°C). Whereas on reducing the amount of MDO to below 50 mol% in the initial feed, the amount of ring-opened MDO in the MDO-NPM copolymer was strongly decreased and it showed a one-step decomposition (decomposed at 410°C) (Table 4).
Thermal characterization of MDO-NPM copolymers prepared at different temperatures.
| Entry | Copolymer composition (molar ratio)a | MDO with ring-opened structure, mol%a | Decreasing T, °Cb | Tg, °Cc | |
|---|---|---|---|---|---|
| MDOd | NPM | ||||
| 6e | 59 | 41 | 54 | 330, 416 | 106 |
| 7e | 57 | 43 | 45 | 322, 409 | 104, 142 |
| 8e | 54 | 46 | 39 | 326, 405 | 97, 142 |
| 9e | 36 | 64 | –f | 409 | 94, 222 |
| 10e | 16 | 84 | –f | 425 | 93, 272 |
aCharacterized by 1H NMR spectra of the MDO-NPM copolymers with a deconvolution method.
bCharacterized by TGA.
cCharacterized by DSC.
dRing-opened and ring-retained MDO.
eInitiator: DTBP, 0.5 mol% of the monomer; reaction temperature, 120°C; reaction time, 2 h; solvent, anisole; same as entries 6–10 in Table 2.
fThe content of MDO units with ring-opened structure was very low and could not be calculated due to the strong overlapping.
The glass transition temperatures for the copolymers prepared at 120°C were determined using a DSC technique. The copolymers showed two glass transition temperatures except for the sample with an MDO/NPM ratio of 9:1 in the feed. Homo-poly(NPM) showed a very high glass transition temperature (more than 300°C) owing to the rigid ring structure (1). The glass transition temperature of poly(MDO) is dependent on the mode of polymerization. The poly(MDO) with 100% ring-opened ester units showed a Tg of -60°C (30). Because of the ring structure, the poly(MDO) with 100% ring-retained units had a higher glass transition temperature than the poly(MDO) with 100% ring-opened ester units. A hypothetical Tg of the NPM-MDO copolymer, which had an assuming composition of NPM and 100% ring-opened MDO units at 1:1, could be calculated as 73°C using the Fox equation:
where W is the weight ratio of the monomers in the NPM-MDO copolymer.
The copolymers prepared at 120°C in the present work showed a mixed structure with a ring-retained acetal and ring-opened ester units from MDO besides NPM rings in the polymer backbone. Because of this immiscible random structure of the resulting copolymer (i.e. the block of ring-retained MDO-co-NPM and the block of ring-opened MDO-co-NPM), two glass transition temperatures between the Tg of homopoly(NPM) and that of poly(MDO) with a fully ring-opened structure were observed in the DSC curves (Figure 7). Because the amount of ring-opened MDO was strongly decreased, the MDO-NPM copolymer prepared at 120°C with decreasing amount of MDO below 50 mol% (i.e. the ring-retained structure predominating) in the initial feed had a similar Tg with the copolymer prepared at 60°C (ring-retained structure only). As an example, the copolymer of entry 9 in Table 2 had a glass transition temperature at 94°C and 222°C, which was similar to the Tg of the copolymer of entry 4 in Table 1 (Tg1=90°C, Tg2=225°C). With increasing amount of MDO units with a ring-opened structure, the difference between these two glass transition temperatures was reduced (Table 3).
![Figure 7: DSC trace of the MDO-NPM copolymer [monomer ratio in the feed: 5:5 (MDO/NPM), entry 8 in Table 2].](/document/doi/10.1515/epoly-2015-0096/asset/graphic/j_epoly-2015-0096_fig_007.jpg)
DSC trace of the MDO-NPM copolymer [monomer ratio in the feed: 5:5 (MDO/NPM), entry 8 in Table 2].
The films of MDO-NPM copolymers with higher ester content (entries 6–8 in Table 2) were prepared using spin coating. The optical transparency of the films was characterized by UV-Vis spectroscopy (Figure 8). All the MDO-NPM copolymer films showed a high transmittance (higher than 90%) in the range of 300–800 nm. Because of the phenyl group on the polymer chains, the transmittance was strongly decreased in the range of 200–300 nm.
Because of the hydrolytically unstable linkage of the ester group from the ring-opened MDO units on the NPM-MDO copolymer chains, the copolymers prepared with higher MDO monomer ratio in the feed at 120°C (entries 6–8 in Table 2) were degradable under basic condition. Alkaline hydrolysis was carried out to study the hydrolytic degradable tendency of the NPM-MDO copolymers (entry 8 in Table 2 was used as an example). After 24 h of degradation with KOH in methanol/ dioxane cosolvent, the NPM-MDO copolymer was completely degraded. The remaining solid was <5 wt.% of the original NPM-MDO copolymer and appeared as fragments of degraded polymer from the 1H NMR spectrum (Figure S4).
4 Conclusion
Copolymers of 2-methylene-1,3-dioxepane (MDO) and N-phenyl maleimide (NPM) with high thermal stability, glass transition temperatures and optical transparency were successfully made. A mixed copolymer structure with ester, ring-acetal and NPM ring units in the backbone was observed using NMR spectroscopy. The relative amounts of these three units in the polymer backbone were dependent on the reaction temperature and on the amount of MDO in the feed. Increased temperature (120°C) and higher amounts of MDO in the feed resulted in polymers with more ester units. The high-molar-mass polymers were generated by simple mixing of two monomers, MDO and NMP, at 60°C and 120°C due to charge transfer complex formation. The resulting polymers have potential applications as degradable transparent polymers with high glass transition temperature for use in the packaging industry and therefore require further characterization in the future for their mechanical properties.
Acknowledgments
We would like to thank DFG for financial support.
Funding: Deutsche Forschungsgemeinschaft, (Grant/Award Number: AG 24/18-1).
References
1. Doi T, Akimoto A, Matsumoto A, Oki Y, Otsu T. Alternating copolymerization of N-(alkyl-substituted phenyl)maleimides with isobutene and thermal properties of the resulting copolymers. J Polym Sci, Part A: Polym Chem. 1996;34:2499–505.10.1002/(SICI)1099-0518(19960915)34:12<2499::AID-POLA23>3.0.CO;2-2Search in Google Scholar
2. Hisano M, Takeda K, Takashima T, Jin Z, Shiibashi A, Matsumoto A. Sequence-controlled radical polymerization of N-Substituted maleimides with 1-methylenebenzocycloalkanes and the characterization of the obtained copolymers with excellent thermal resistance and transparency. Macromolecules 2013;46:7733–44.10.1021/ma401499vSearch in Google Scholar
3. Suwier DR, Steeman PA, Teerenstra MN, Schellekens MA, Vanhaecht B, Monteiro MJ, Koning CE. Flexibilized Styrene-N-substituted maleimide copolymers with enhanced entanglement density. Macromolecules 2002;35:6210–6.10.1021/ma020084rSearch in Google Scholar
4. Matsumoto A, Kubota T, Otsu T. Radical polymerization of N-(alkyl-substituted phenyl)maleimides: synthesis of thermally stable polymers soluble in nonpolar solvents. Macromolecules 1990;23:4508–13.10.1021/ma00223a002Search in Google Scholar
5. Ahn KD, LeeYH, Koo D, II. Synthesis and polymerization of N-(tert-butyloxycarbonyl)maleimide and facile deprotection of polymer side-chain t-BOC groups. Polymer 1992;33:4851–6.10.1016/0032-3861(92)90702-XSearch in Google Scholar
6. Hill DJ, Shao LY, Pomery PJ, Whittaker AK. The radical homopolymerization of N-phenylmaleimide, N-n-hexylmaleimide and N-cyclohexylmaleimide in tetrahydrofuran. Polymer 2001;42:4791–802.10.1016/S0032-3861(00)00867-3Search in Google Scholar
7. Gacal B, Cianga L, Agag T, Takeichi T, Yagci Y. Synthesis and characterization of maleimide (Co)polymers with pendant benzoxazine groups by photoinduced radical polymerization and their thermal curing. J Polym Sci, Part A: Polym Chem. 2007;45:2774–86.10.1002/pola.22034Search in Google Scholar
8. Rzaev ZM, Milli H, Akovali G. Complex-radical alternating copolymerization of trans-stilbene with N-substituted maleimides. Polym Int. 1996;41:259–65.10.1002/(SICI)1097-0126(199611)41:3<259::AID-PI591>3.0.CO;2-9Search in Google Scholar
9. Saegusa T. Spontaneous alternating copolymerization via zwitterion intermediates. Angew Chem Int Ed Engl. 1977;16:826–35.10.1002/anie.197708261Search in Google Scholar
10. Hall HK. Bond-forming initiation in spontaneous addition and polymerization reactions of alkenes. Angew Chem Int Ed Engl. 1983;22:440–55.10.1002/anie.198304401Search in Google Scholar
11. Yamamoto SI, Sanda F, Endo T. Spontaneous alternating copolymerization of methoxyallene with N-Phenylmaleimide. Macromolecules 1999;32:5501–06.10.1021/ma990104ySearch in Google Scholar
12. Agarwal S. Chemistry, chances and limitations of the radical ring-opening polymerization of cyclic ketene acetals for the synthesis of degradable polyesters. Polym Chem. 2010;1:953–64.10.1039/c0py00040jSearch in Google Scholar
13. Roberts GE, Coote ML, Heuts JP, Morris LM, Davis TP. Radical ring-opening copolymerization of 2-methylene 1,3-dioxepane and methyl methacrylate: experiments originally designed to probe the origin of the penultimate unit effect. Macromolecules 1999;32:1332–40.10.1021/ma9813587Search in Google Scholar
14. Agarwal S. Microstructural characterisation and properties evaluation of poly (methyl methacrylate-co-ester)s. Polym J 2006;39:163–74.10.1295/polymj.PJ2006137Search in Google Scholar
15. Morris LM, Davis TP, Chaplin RP. An assessment of the copolymerization reaction between styrene and 2-methylene-1, 3-dioxepane. Polymer 2001;42:495–500.10.1016/S0032-3861(00)00336-0Search in Google Scholar
16. Agarwal S, Kumar R, Kissel T, Reul R. Synthesis of degradable materials based on caprolactone and vinyl acetate units using radical chemistry. Polym J 2009;41:650–60.10.1295/polymj.PJ2009091Search in Google Scholar
17. Hedir GG, Bell CA, Ieong NS, Chapman E, Collins IR, O’Reilly RK, Dove AP. Functional degradable polymers by xanthate-mediated polymerization. Macromolecules 2014;47:2847–52.10.1021/ma500428eSearch in Google Scholar
18. Maji S, Mitschang F, Chen L, Jin Q, Wang Y, Agarwal S. Functional poly(Dimethyl Aminoethyl Methacrylate) by combination of radical ring-opening polymerization and click chemistry for biomedical applications. Macromol Chem Phys. 2012;213:1643–54.10.1002/macp.201200220Search in Google Scholar
19. Undin J, Finne-Wistrand A, Albertsson AC. Adjustable degradation properties and biocompatibility of amorphous and functional Poly(ester-acrylate)-based materials. Biomacromolecules 2014;15:2800–7.10.1021/bm500689gSearch in Google Scholar PubMed
20. Bailey WJ, Ni Z, Wu SR. Synthesis of poly-ε-caprolactone via a free radical mechanism. Free radical ring-opening polymerization of 2-methylene-1,3-dioxepane. J Polym Sci, Polym Chem Ed. 1982;20:3021–30.10.1002/pol.1982.170201101Search in Google Scholar
21. Zhu PC, Wu Z, Pittman CU. Ring opening during the cationic polymerization of 2-methylene-1,3-dioxepane: cyclic ketene acetal initiation with sulfuric acid supported on carbon. J Polym Sci, Part A: Polym Chem. 1997;35:485–91.10.1002/(SICI)1099-0518(199702)35:3<485::AID-POLA11>3.0.CO;2-KSearch in Google Scholar
22. Agarwal S, Kumar R. Synthesis of high-molecular-weight tulipalin-a-based polymers by simple mixing and heating of comonomers. Macromol Chem Phys. 2011;212:603–12.10.1002/macp.201000673Search in Google Scholar
23. Jin Q, Maji S, Agarwal S. Novel amphiphilic, biodegradable, biocompatible, cross-linkable copolymers: synthesis, characterization and drug delivery applications. Polym Chem. 2012;3:2785–93.10.1039/c2py20364bSearch in Google Scholar
24. Cai T, Chen Y, Wang Y, Wang H, Liu X, Jin Q, Agarwal S, Ji J. One-step preparation of reduction-responsive biodegradable polymers as efficient intracellular drug delivery platforms. Macromol Chem Phys. 2014;215:1848–54.10.1002/macp.201400311Search in Google Scholar
25. Cai T, Chen Y, Wang Y, Wang H, Liu X, Jin Q, Agarwal S, Ji J. Functional 2-methylene-1,3-dioxepane terpolymer: a versatile platform to construct biodegradable polymeric prodrugs for intracellular drug delivery. Polym Chem. 2014;5:4061–8.10.1039/C4PY00259HSearch in Google Scholar
26. Semsarilar M, Perrier S. ‘Green’ reversible addition-fragmentation chain-transfer (RAFT) polymerization. Nat Chem. 2010;2:811–20.10.1038/nchem.853Search in Google Scholar
27. Delplace V, Tardy A, Harrisson S, Mura S, Gigmes D, Guillaneuf Y, Nicolas J. Degradable and comb-like PEG-based copolymers by nitroxide-mediated radical ring-opening polymerization. Biomacromolecules 2013;14:3769–79.10.1021/bm401157gSearch in Google Scholar
28. Agarwal S, Ren L. Polycaprolactone-based novel degradable ionomers by radical ring-opening polymerization of 2-methylene-1,3-dioxepane. Macromolecules 2009;42:1574–9.10.1021/ma802615fSearch in Google Scholar
29. Agarwal S, Naumann N, Xie X. Synthesis and microstructural characterization of ethylene carbonate-ε-caprolactone/l-lactide copolymers using one- and two-dimensional NMR spectroscopy. Macromolecules 2002;35:7713–7.10.1021/ma020584kSearch in Google Scholar
30. Agarwal S, Speyerer C. Degradable blends of semi-crystalline and amorphous branched poly(caprolactone): effect of microstructure on blend properties. Polymer 2010;51:1024–32.10.1016/j.polymer.2010.01.020Search in Google Scholar
Supplemental Material:
The online version of this article (DOI: 10.1515/epoly-2015-0096) offers supplementary material, available to authorized users.
©2015 by De Gruyter
This article is distributed under the terms of the Creative Commons Attribution Non-Commercial License, which permits unrestricted non-commercial use, distribution, and reproduction in any medium, provided the original work is properly cited.
