Radical Alternating Copolymerization of Twisted 1,3-Butadienes with

Sep 29, 2014 - Styrene (St, Wako Pure Chemicals Co., Ltd., Osaka, Japan) and isoprene (IP, Tokyo Chemical Industry Co., Ltd., Tokyo) were distilled be...
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Radical Alternating Copolymerization of Twisted 1,3-Butadienes with Maleic Anhydride as a New Approach for Degradable Thermosetting Resin Asuka Tsujii,† Mami Namba,‡ Haruyuki Okamura,† and Akikazu Matsumoto*,† †

Department of Applied Chemistry, Graduate School of Engineering, Osaka Prefecture University, 1-1 Gakuen-cho, Naka-ku, Sakai-shi, Osaka 599-8531, Japan ‡ Department of Applied Chemistry and Bioengineering, Graduate School of Engineering, Osaka City University, 3-3-138 Sugimoto, Sumiyoshi-ku, Osaka 558-8585, Japan S Supporting Information *

ABSTRACT: We report a novel approach for the synthesis of a readily curable and degradable resin by the radical alternating copolymerization of 1,3-diene monomers with maleic anhydride. 2,4-Dimethyl-1,3-pentadiene predominantly produced an alternating copolymer rather than a Diels−Alder adduct during the reaction with maleic anhydride. We revealed the most stable conformation as a twisted diene structure of 2,4-dimethyl-1,3-pentadiene and the other methylsubstituted dienes by DFT calculations, while a completely planar structure was preferred for the s-cis and s-trans conformers of the nonsubstituted butadiene. The highly alternating repeating structure of the produced copolymers was revealed based on the NMR analysis and copolymerization reactivity ratios. The alternating copolymers including an anhydride moiety and a carbon-to-carbon double bond in each repeating unit was conveniently used for the thermosetting, subsequent degradation, and polymer−surface modification by postpolymerization reactions such as epoxy curing and oxidative ozonolysis.

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INTRODUCTION During the recent decade, living and sequence-controlled radical polymerizations have become a powerful tool for the design of smart polymer materials with the help of postpolymerization reactions using click chemistry.1 Alternating radical copolymerization has also been used for the structure control and materials design of polymers due to several advantages of the polymer synthetic method,2,3 e.g., the production of a high-molecularweight copolymer in high yield, a well-defined alternating sequence structure independent of the comonomer ratio in the feed, and the use of a variety of monomers. For example, the radical copolymerization of an electron-donating olefin, such as styrenes and vinyl ethers combined with maleic anhydride (MAn) as the typical electron-accepting monomer provides an alternating copolymer as the functional polymer precursor, of which the reactive anhydride moiety is further used for the synthesis of functional polymers,4 such as polymer drugs5 and polymer-surface functionalization.6 On the other hand, 1,3dienes, such as butadiene (BD), isoprene, cyclopentadiene, and furan, readily react with MAn to yield Diels−Alder adducts,7 but not alternating copolymers. In fact, we found very few reports about the radical copolymerization of 1,3-diene monomers with MAn in the literature.8 Thus, the directivity of MAn to Diels− Alder reactions is an obstacle for the copolymerization with 1,3dienes, while Diels−Alder polymer reaction is one of the most convenient and reliable methods for the synthesis of self-healing © XXXX American Chemical Society

materials, bioconjugated functional polymers, well-controlled polymer architectures, and controlled polymer networks.9 Interestingly, 3-methylenecyclopentene as the cyclic 1,3-diene monomer with a reactive exomethylene moiety produces an alternating copolymer during radical copolymerization with an electron-accepting N-substituted maleimide in high yield, because the Diels−Alder reaction of the cyclic monomer can be totally suppressed due to the fixed s-trans conformation.10 In this study, we propose the simple molecular design of noncyclic 1,3-diene monomers for the synthesis of alternating copolymers with MAn by the introduction of alkyl substituents in the diene moiety. We revealed that 2,4-dimethyl-1,3-pentadiene (DMPD) and several other methyl-substituted dienes with a twisted molecular conformation produced the alternating copolymers with MAn. The predominant copolymer formation is discussed based on the reactivity of the diene monomers with a twisted diene structure. An anhydride moiety and a carbon-to-carbon double bond in each repeating unit of the copolymers were available for the postpolymerization reactions, such as crosslinking using an epoxy compound11 and oxidative ozonolysis12 (Scheme 1). Reworkable UV- and thermally cured polymer materials are actually useful in various application fields, such as Received: July 29, 2014 Revised: September 18, 2014

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dx.doi.org/10.1021/ma501555n | Macromolecules XXXX, XXX, XXX−XXX

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Scheme 1. Synthesis of Thermally Curable and Ozone-Degradable Polymers by Radical Alternating Copolymerization of DMPD as the Twisted 1,3-Diene Monomer with MAn

(Earth Walker Trading Co., Ltd., Hiroshima). The DFT calculations were carried out using Spartan’10 (Wave Function, Inc.) at the B3LYP/ 6-311G* level. Copolymerization Procedures. The copolymerization of DMPD with MAn or CAn was carried out in chloroform-d or 1,2-dichloroethane in the presence of AMVN and a small amount of mesitylene as the internal standard. The monomer conversions and yields of the Diels− Alder products were determined by 1H NMR spectroscopy, then the reaction mixture was poured into a large amount of diethyl ether/nhexane mixture (4/1 volume ratio) to precipitate the copolymers. The isolated copolymers were dried in vacuo at 110 °C. The yield of the copolymers was gravimetrically determined. The copolymers were purified by a repeated precipitation procedure using acetone and a diethyl ether/n-hexane mixture. The copolymerization of St with MAn was carried out in the presence of AIBN in chloroform at 40 °C for 5 h according to a procedure similar to that for the copolymerization of DMPD with MAn. The copolymer with St was precipitated using diethyl ether. Epoxy Curing. A mixture of the copolymer (10 wt %), BADGE ([anhydride]/[epoxy] = 1/1 molar ratio) and NDMBA (3 wt % to BADGE) in cyclohexanone was spin-coated on a Si plate at 1000 rpm for 30 s, then prebaked at 60 °C for 5 min, followed by curing at a determined temperature for 1 h in air. The insoluble fraction was determined based on a change in the film thickness after dipping in acetone for 5 min. Ozonolysis. The ozonolysis of the epoxy-cured film (thickness 0.8 μm) was carried out in acetone by bubbling ozone-containing air at room temperature. The ozone concentration was determined to be approximately 0.1 mmol/L based on the UV absorption spectrum of the solution at the gas−liquid equilibrium at 0 °C. After ozone bubbling took place for a determined time, nitrogen gas was bubbled, followed by the reduction of the produced ozonides with 5 wt % triphenylphosphine in acetone at room temperature for 10 min, then the film thickness was determined again. For the ozonolysis of the linear polymers, ozonecontaining air was bubbled into a THF solution containing the copolymer (Mn = 0.5−2.1 × 104, 0.5 mg/mL, 10 mL) at 0 °C followed by 5 min of nitrogen bubbling, then triphenylphosphine (7 mg/mL in THF, 1 mL) was added with stirring at 0 °C for 10 min. After the solvent was removed under reduced pressure, the products were analyzed by SEC.

adhesives, printing, coatings, mounting of electronic parts, and various composite materials.13

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EXPERIMENTAL SECTION

Materials. Commercially available maleic anhydride (MAn, Kishida Chemicals Co., Ltd., Osaka, Japan) was used after sublimation under reduced pressure (1.5 mmHg). 2,4-Dimethyl-1,3-pentadiene (DMPD), 1,3-pentadiene (PD, (3E)/(3Z) = 75/25), (3E)-1,3-hexadiene (1,3HD), and 2,4-hexadiene (2,4-HD, (2E,4E)/(2E,4Z)/(2Z,4Z) = 57/39/ 4) were purchased from Tokyo Chemical Industry Co., Ltd., Tokyo, and used as received. Styrene (St, Wako Pure Chemicals Co., Ltd., Osaka, Japan) and isoprene (IP, Tokyo Chemical Industry Co., Ltd., Tokyo) were distilled before use. Cyclohexanone (Nacalai Tesque, INC., Kyoto, Japan) was distilled under reduced pressure (1.5 mmHg). 2,2′Azobis(4-methoxy-2,4-dimethylvaleronitrile) (AMVN) and 2,2′-azobis(isobutyronitrile) (AIBN) were purchased from Wako Pure Chemicals Co., Ltd., Osaka, and recrystallized from methanol. Citraconic anhydride (CAn, Tokyo Chemical Industry Co., Ltd., Tokyo), mesitylene (Tokyo Chemical Industry Co., Ltd., Tokyo), N,N′-dimethylbenzylamine (NDMBA, Wako Pure Chemicals Co., Ltd., Osaka, Japan), and bisphenol A diglycidyl ether (BADGE, Tokyo Chemical Industry Co., Ltd., Tokyo) were used as received. General Procedures. The NMR spectra were recorded using JEOL ECS-400 and ECX-400 FT NMR spectrometers. Size exclusion chromatography (SEC) was carried out using Chromatoscience CS300C, JASCO PU-2080PLUS, JASCO DG-2080−53, JASCO RI-2031PLUS, TOSOH TSK-gel columns, GMHHR-N and GMHHR-H, and tetrahydrofuran (THF) as the eluent. The number- and weight-average molecular weights (Mn and Mw, respectively) and polydispersity (Mw/ Mn) values were determined by calibration with standard polystyrenes. The thermogravimetric analysis (TG) was carried out using Shimadzu TGA-50 in nitrogen at the flow rate of 10 mL/min and the heating rate of 10 °C/min to determine the onset temperature of decomposition (Td5). The differential scanning calorimetry (DSC) was carried out using Shimadzu DSC-60 in nitrogen at the heating rate of 10 °C/min to determine the glass transition temperature (Tg). After the sample was heated to a temperature over the Tg, it was cooled to 30 °C at the rate of 50 °C/min and then used for the measurement. Spin-coated films were prepared using a Mikasa 1H-D7 spin coater. Film thickness was determined by interferometric measurement using a Nanometrics Nanospec AFT M-3000 series. Water contact angles were determined using a DMs-400 contact angle meter (Kyowa Interface Science Co., Ltd., Saitama). Ozone gas was generated using Ozone Mart O3 Clear B

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Table 1. Radical Copolymerization of 1,3-Diene Monomers and MAn or CAna diene (mol/L)

anhydride (mol/L)

[AMVN] (mmol/L)

temp (°C)

time (h)

anhydride convn (%)

adduct yield (%)

copolymer yield (%)

DMPD (0.50) DMPD (0.50) DMPD (0.50) DMPD (0.50) DMPD (1.0) DMPD (1.5) DMPD (2.0) IP (0.50)b PD (0.50)b PD (0.50)b 1,3-HD (0.50) 2,4-HD (0.50)b DMPD (0.50) PD (0.50)

MAn (0.50) MAn (0.50) MAn (0.50) MAn (0.50) MAn (1.0) MAn (1.5) MAn (2.0) MAn (0.50) MAn (0.50) MAn (0.50) MAn (0.50) MAn (0.50) CAn (0.50) CAn (0.50)

0 10 10 50 100 150 200 10 10 200 10 10 50 50

40 30 40 40 40 40 40 30 30 30 30 30 40 40

5 5 5 5 5 5 5 1 24 3 5 24 5 5

26 54 66 77 86 88 91 89 81 d 82 73 d 24

25 12 10 11 10 11 9 65 39 26 46 56 ∼0 14

0 31.4 51.6 61.3 77.0 77.0 80.2 17.1 31.2 53.0 30.9 13.0 0 13.1

Mn/104

Mw/Mn

1.3 1.0 0.3 0.5 0.7 0.8 c 2.6 0.9 2.0 1.7

1.5 1.5 1.4 1.6 1.6 1.9 c 1.8 1.6 2.4 1.8

80% yield). The MAn conversion reached 90% while the yield of the Diels−Alder adduct remained at only 10% under the conditions shown in Figure 1. The alternating copolymeriza-

Figure 2. Comparison of MAn conversion and the copolymer and Diels−Alder adduct yields for the reaction of DMPD and 1,3-HD with MAn: [diene] = [MAn] = 0.50 mol/L, [AMVN] = 0.01 mol/L in CDCl3 at 30 °C for 5 h.

shows the comparison of the product yields during the reactions of DMPD and 1,3-HD with MAn. It indicated that the Diels− Alder adduct yield was significantly suppressed during the reaction of DMPD due to its sterically hindered 2,4,4-trimethylsubstituted diene structure, while the copolymer was produced in a similar yield. This supported the fact that the selective copolymer formation during the reaction of DMPD with MAn was due to the suppression of the Diels−Alder reaction by the sterically hindered substituents of the diene moiety. Furthermore, we also revealed that the reaction of citraconic anhydride (CAn) with DMPD gave trace amounts of the copolymer and Diels−Alder product (Table 1). It was previously reported that the introduction of a methyl group to the double bond of MAn significantly reduced the rate of the Diels−Alder reaction due to an enhanced steric hindrance.14 In the present study, the αmethyl substituent of CAn suppressed both the copolymerization and the Diels−Alder addition by the synergistic effects of multiple methyl substitution on the olefin and diene moieties.

Figure 1. Time−conversion and time-yield curves for the reaction of DMPD with MAn: [DMPD] = [MAn] = 0.50 mol/L, [AMVN] = 0.05 mol/L in CDCl3 at 40 °C.

tion also proceeded during the reactions using isoprene (IP), 1,3pentadiene (PD), 1,3-, and 2,4-hexadienes (1,3-HD and 2,4-HD, respectively), but the copolymer yields were relatively low (13− 53%) due to a considerable amount of the Diels−Alder products in 26−65% yields. For the reactions of IP and 2,4-HD with MAn, the major product was the corresponding Diels−Alder adduct, but not the copolymer. Both reactions competitively occurred during the reactions of PD and 1,3-HD with MAn. Figure 2 C

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Conformation of Diene Monomers. We investigated the molecular conformation of various 1,3-diene monomers, of which the chemical structures are shown in Figure 3, based on the

Figure 3. Chemical structures of 1,3-diene monomers used for the DFT calculations.

Figure 4. Preferred conformation and dihedral angel values of the diene moiety for (a) DMPD and (b) BD determined by the DFT calculations at the B3LYP/6-311* level. The most stable s-trans and s-cis conformers are shown for each compound.

DFT calculations. The results are summarized in Table 2. A completely planar structure was preferred for the all s-trans conformers of the examined dienes, and also, the s-cis conformers of BD, (2E,4E)-2,4-HD, and (3E)-2MPD as the diene compounds involving no steric hindrance. For the s-cis conformers of the other substituted dienes, a twisted conformation was observed due to the increased steric hindrance of the alkyl substituents;15 i.e., the dihedral angles including the carbon-to-carbon double bonds [C(α)C(β)−C(γ)C(δ)] were 46−55° for (3Z)-2MPD, DMPD, and (2Z,4Z)-2,4-HD, and 32−39° for the other dienes. A methyl group on the butadiene skeleton with the Z-configuration exhibited greater steric repulsion which led to a more twisted molecular conformation. At the same time, the carbon-to-carbon bond length for the C(β)−C(γ) bonds increased for the dienyl moieties containing a twisted structure. Figure 4 shows the stable conformations for each s-cis and s-trans conformer of BD and DMPD as the typical results and the schematic illustration of steric repulsion between the methyl substituents on the diene moiety of DMPD leading to the formation of a twisted s-cis conformation. The induced magnitudes of the steric hindrance and the conformation change were amplified by the multiple methyl substitutions. It was noted that a twisted s-cis conformation for DMPD and (3Z)-2MPD was more energetically preferred than the planar s-trans one; ΔE = −4.45 and

−0.74 k/mol for DMPD and (3Z)-2MPD, respectively. This is in contrast to the positive ΔE values (12.8−16.8 k/mol) for the other dienes. The results obtained for the conformation analysis agreed well with the copolymerization results in Table 1 although the copolymerization data for each isomer shown in Figure 3 were not always available. Namely, the twisted s-cis conformations of the diene monomers preferred the alternating copolymerization rather than Diels−Alder reaction with MAn. In addition, the highest Mulliken negative atomic charges for the α-carbon of DMPD supported the fact that the propagation of an electrondeficient MAn radical favorably attacked the α-carbon with a high density (Table 2). The 13C NMR chemical shift values listed in Table S1 (See Supporting Information) resulted in a similar conclusion. The linear relationship between the 13C NMR chemical shift values and the e values of various vinyl monomers was previously reported in order to discuss the reactivity during the anionic, cationic, and radical polymerizations.16 The monomers with greater electron-donating characteristics are generally appropriate for the alternating copolymerization with electron-deficient monomers and provide a high molecular weight copolymer in high yield. In this study, DMPD as the most electron-donating monomer actually exhibited a high selectivity during the reaction with MAn to provide the alternating copolymer rather than the Diels−Alder adduct.

Table 2. DFT Calculation Results for the Determination of the Most Stable Conformers of Various 1,3-Diene Monomers s-trans

a

s-cis

diene

dihedral anglea (deg)

bond length for Cβ−Cγ (Å)

Mulliken atomic chargeb

dihedral anglea (deg)

bond length for Cβ−Cγ (Å)

Mulliken atomic chargeb

ΔE (kJ/mol)

BD IP (3E)-PD (3Z)-PD (3E)-1,3-HD (3Z)-1,3-HD (2E,4E)-2,4-HD (2E,4Z)-2,4-HD (2Z,4Z)-2,4-HD (3E)-2MPD (3Z)-2MPD 4MPD DMPD

180.00 179.82 180.00 179.78 179.69 179.44 180.00 180.00 180.00 180.00 179.44 179.72 179.50

1.456 1.467 1.454 1.455 1.435 1.456 1.454 1.455 1.455 1.465 1.477 1.454 1.470

−0.431 −0.456 −0.437 −0.432 −0.437 −0.431 −0.174 −0.168 −0.178 −0.465 −0.463 −0.436 −0.469

0.00 38.54 32.84 37.37 32.91 32.82 0.00 38.99 54.81 0.02 46.23 38.76 47.19

1.470 1.478 1.468 1.467 1.468 1.466 1.468 1.467 1.472 1.478 1.470 1.466 1.477

−0.412 −0.448 −0.422 −0.429 −0.421 −0.428 −0.150 −0.163 −0.155 −0.455 −0.458 −0.432 −0.461

16.71 12.79 14.92 14.85 14.63 14.88 16.76 15.15 13.33 15.28 −0.74 15.25 −4.45

For C(α)C(β)−C(γ)C(δ). bOn the α-carbon of the diene moiety. D

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Figure 5. 2D NMR spectra of poly(2,4-HD-alt-MAn) in acetone-d6 at room temperature. Asterisks indicate the peaks due to the solvents. Copolymerization conditions: [2,4-HD] = [MAn] = 0.5 mol/L, [AVMN] = 10 mmol/L in 1,2-dichloroethane at 30 °C for 24 h.

Structure of the Copolymers. The NMR spectroscopic and size exclusion chromatography (SEC) analyses revealed the alternating repeating structure of the copolymer of the dienes and MAn with a predominantly 1,4-repeating structure and an Mn value of 3−26 × 103. On the basis of the results of the 2D NMR analyses, the ratios of the 1,4- and 1,2 (or 3,4)-repeating structures were determined to be 5/1, 10/1, and 40/1 for poly(PD-alt-MAn), poly(IP-alt-MAn), and poly(2,4-HD-altMAn), respectively (Figure 5).17 These results suggested that the sterically hindered and internal diene structure of 2,4-HD favored the 1,4-propagation rather than the 1,2-propagation. No selectivity in the formation of the 1,4-trans and 1,4-cis repeating structures was observed for these copoylmers. The NMR spectra of poly(DMPD-alt-MAn) was too complicated to accurately determine the fine structure of the repeating units and their fractions. The composition of the resulting copolymers was always constant as 1:1, independent of the comonomer composition in the feed, as shown in the comonomer-copolymer composition curves produced for the copolymerizations of PD and 2,4-HD with MAn in 1,2-dichloroethane at 30 °C (Figure 6). The monomer reactivity ratios (r1 and r2) were determined to be 0.017 and 0.012 for the copolymerization of PD (M1) with MAn (M2) and 0.021 and ∼0 for the copolymerization of 2,4-HD (M1) with MAn (M2) based on the nonlinear least-squares curve fitting method. The determined monomer reactivity ratios supported the highly alternating copolymerization characteristic of these systems. Epoxy-Curing and Ozonolysis of the Copolymers. The copolymers of the diene monomers with MAn were soluble in acetone, 1,2-dichloroethane, and THF, and insoluble in methanol, toluene, and n-hexane. A transparent film was obtained by casting the solutions of the copolymers. The thermal properties of the alternating copolymers obtained from the dienes and styrene with MAn are shown in Table 3. The onset decomposition temperature (Td5) of poly(DMPD-alt-MAn) was determined to be 280 °C by a thermogravimetric (TG) analysis, being lower than those for the other copolymers (Td5 > 310 °C). This was because depolymerization was accelerated by the increased steric repulsion between the substituents in poly-

Figure 6. Comonomer−copolymer composition curves produced for the copolymerizations of PD and 2,4-HD (M1) with MAn (M2) in 1,2dichloroethane at 30 °C.

Table 3. Characterization of Alternating Copolymers Produced during Alternating Radical Copolymerization of Diene and Styrene Monomers with MAn copolymer

yield (%)

Mn/104

Mw/Mn

Tg (°C)

Td5 (°C)

poly(DMPD-alt-MAn) poly(PD-alt-MAn) poly(2,4-HD-alt-MAn) poly(St-alt-MAn)

72.6 49.3 28.5 87.3

1.2 1.5 1.7 6.8

1.6 2.0 1.8 2.0

95 80 88 208

280 336 327 312

a

Copolymerization conditions: [DMPD] = [MAn] = 1.0 mol/L, [AMVN] = 20 mmol/L in chloroform at 30 °C for 10 h; [PD] = [2,4HD] = [MAn] = 0.5 mol/L, [AMVN] = 10 mmol/L in 1,2dichloroethane at 30 °C for 24 h; [St] = [MAn] = 1.0 mol/L, [AIBN] = 10 mmol/L in chloroform at 60 °C for 4 h.

(DMPD-alt-MAn) during the thermal degradation process. The glass transition temperature (Tg) was found to be 80−95 °C based on the differential scanning calorimetry (DSC) traces. The E

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Tg values increased with an increase in the number of the methyl substituents in the copolymers. The poly(St-alt-MAn) obtained from the alternating radical copolymerization of St with MAn exhibited a higher Tg value due to the less-flexible chain structure including a rigid benzene ring. The alternating copolymers included an anhydride moiety in the repeating units and, therefore, thermal curing immediately proceeded during the reaction with an epoxy compound, bisphenol A diglycidyl ether (BADGE), in the presence of a base catalyst, N,N-dimethylbenzylamine (NDMBA) (Figure 7a,

significantly depended on the reaction temperature, as shown in Figure 8b. The lower the reaction temperature, the more often chain scission occurred. This is because the ozone concentration in THF increased at a lower temperature, being determined by the gas−liquid distribution coefficient of ozone and the constant partial pressure and concentration in a gas phase. The change in the molecular weights and the number of scissions of the polymer chains (N) for the degradation of the copolymers under various conditions are summarized in Table 4. The chain-scission efficiency of the double bond included in the main chain of poly(DMPD-alt-MAn) reached 12.6% during the ozone degradation at 0 °C for 15 min. As mentioned in the previous section, we could not determine the microstructure (the ratio of 1,4- and 1,2-repeating structures) of poly(DMPD-alt-MAn). Therefore, the degradation efficiency is not discussed from the viewpoint of the polymer repeating fashion, although the 1,2repeating structure has no contribution to a decrease in the molecular weight of the copolymers during the ozone degradation. Thermolysis data shown in Table 4 indicated the excellent thermal stability of poly(DMPD-alt-MAn) in air at a high temperature under the dark conditions in contrast to the rapid ozone degradation. The alternating repeating structure is not essential for occurrence of the degradation, but it is important for the effective cross-linking and the subsequent ozone degradation of poly(DMPD-alt-MAn) including the maximum contents of the both MAn and diene repeating units. The ozone degradation of poly(PD-alt-MAn) similarly occurred, but no chain scission was observed for poly(St-alt-MAn) including no CC moiety in the polymer chain. These results suggested the application of the quick degradation of poly(DMPD-alt-MAn) for the resolubilization of the cured resins by ozonolysis. Actually, the cured and insoluble film was readily decomposed by ozonolysis, resulting in the resolubilization of the cross-linked polymers, as shown in Figure 7b. The thickness of the epoxy-cured film of poly(DMPDalt-MAn) rapidly decreased during the ozonolysis for several minutes. In contrast, no degradation occurred during the reaction of poly(St-alt-MAn). The oxidative ozonolysis was also valid for modification of the polymer surfaces by the introduction of polar functional groups such as an aldehyde and carboxylic acid. Figure 7c and Table 5 show the results for a comparison of the changes in the water contact angles during the ozonolysis of the epoxy-cured poly(DMPD-alt-MAn) and poly(St-alt-MAn). The ozonolysis was carried out in air containing ozone at room temperature. The water contact angle value decreased from 77.9 to 69.6° during the 30 min ozonolysis of the cured poly(DMPD-alt-MAn) film, while no change was observed for poly(St-alt-MAn) under similar conditions. The change in the film thickness was slight during the surface-modification of the cured films using air containing ozone. It was also confirmed that PPh3 as the reducing agent only reacted with the produced ozonides, judging from the water contact angle values with and without the reaction with PPh3 before ozonization. The MAn repeating units included in the copolymers can also be reacted with any other multifunctional compounds, such as diols and diamines, to similarly give cross-linked polymers and gels. It can be also transformed into other high-performance transparent polymers with an excellent thermoresistance, such as the copolymers of dialkyl fumarates and N-substituted maleimides using monofunctional alcohols and amines, respectively.18 We are now continuing the studies on the

Figure 7. (a) Effect of curing temperature on the insoluble fraction of poly(DMPD-alt-MAn) and poly(St-alt-MAn) for 1 h: [anhydride]/ [epoxy] = 1/1 in mol/mol, NDMBA 3 wt %. Film thickness of 0.8−1.1 μm. (b) Effect of ozonolysis time on the remaining thickness of epoxycured poly(DMPD-alt-MAn) and poly(St-alt-MAn) films in acetone at 0 °C. Film thickness of 0.8 μm. (c) Change in water contact angles during the ozonolysis of epoxy-cured poly(DMPD-alt-MAn) and poly(St-alt-MAn) films in air at room temperature.

see also Scheme 1).11 The insoluble fraction of the epoxy-cured poly(DMPD-alt-MAn) significantly increased in a temperature range above the Tg value. The Td5 value of poly(DMPD-altMAn) increased to 301 °C and the Tg was not observed after the epoxy curing. Poly(St-alt-MAn) was similarly cured during a heating process in a higher temperature range in the presence of the epoxy compound. The unsaturated carbon-to-carbon bond included in the main chain of the copolymers was tolerant to the addition of carbon and oxygen radicals and other chemical reagents due to its internal olefin structure. However, the scission of the polymer chains during oxidative ozonolysis readily occurred12 and it was confirmed by monitoring the change in the molecular weight distribution of the linear copolymer, as shown by a change in the SEC chromatograph (Figure 8a). The Mn value of poly(DMPDalt-MAn) decreased from 1.2 × 104 to 1.1 × 103 during the ozonolysis for 15 min in THF at 0 °C. The rate of chain scission

Figure 8. (a) Change in the molecular weight distribution of linear poly(DMPD-alt-MAn) during ozonolysis in THF at 0 °C for (a) 0, (b) 1, (c) 5, and (d) 15 min. (b) Temperature dependence of the ozonolysis of poly(DMPD-alt-MAn). F

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Table 4. Change in Molecular Weights and Chain Scission Numbers of the Alternating Copolymers Prepared in This Study under Various Degradation Conditionsa copolymer

degradation method

poly(DMPD-alt-MAn)

ozonolysis

temp (°C) 0 0 0 0 0 −73 −40 30

poly(DMPD-alt-MAn)

thermolysis 150 150 150

poly(PD-alt-MAn)

ozonolysis 0 0

poly(St-alt-MAn)

ozonolysis 0 0 0

time (min)

Mn/104

0 1 2 3 5 15 1 1 1 0 60 120 300 0 1 2 0 5 10 15

1.17 0.53 0.40 0.35 0.25 0.11 0.15 0.33 0.88 1.17 1.12 0.94 0.77 0.49 0.22 0.15 2.94 2.87 2.60 2.75

Mw/104 Mw/Mn number of chain scissions, Nb 1.88 1.00 0.74 0.63 0.45 0.16 0.25 0.66 1.40 1.88 1.87 1.66 1.52 0.74 0.34 0.23 7.45 7.69 7.63 7.70

1.61 1.88 1.84 1.78 1.82 1.48 1.65 1.99 1.59 1.61 1.68 1.77 1.98 1.52 1.59 1.52 2.53 2.68 2.93 2.80

0 1.2 1.9 2.3 3.7 9.6 6.8 2.5 0.3 0