Chinese Journal of Polymer Science Vol. 35, No. 1, (2017), 36−45 Chinese Journal of Polymer Science © Chinese Chemical Society Institute of Chemistry, CAS Springer-Verlag Berlin Heidelberg 2017

Ring-opening Metathesis Polymerization of Cis-5-norbornene-endo-2,3- dicarboxylic Anhydride Derivatives Using the Grubbs Third Generation Catalyst*

Ji-xing Yang, Li-xia Ren** and Yue-sheng Li School of Materials Science and Engineering and Tianjin Key Laboratory of Composite and Functional Materials, Tianjin University, Tianjin 300072, China

Abstract A series of cis-5-norbornene-endo-2,3-dicarboxylic anhydride (NDCA, M1) derivatives (M2−M4) with different types of nonpolar substituted groups were synthesized and characterized by 1H/13C-NMR and mass spectrometry (MS). Ring- opening metathesis polymerization (ROMP) of these monomers using the Grubbs third generation catalyst (G3) generated high molecular weight polymers with much improved solubility compared with the NDCA’s homopolymer. It was found that the solubility of these polymers increased with increased substituent’s steric hindrance. The living polymerization of NDCA derivative containing the bulkiest substituent (M4) catalyzed by G3 in tetrahydrofuran was confirmed by the kinetic studies with low polydispersity indices (PDI) (< 1.30). By using sequential ROMP, well-defined diblock copolymers containing anhydride groups were synthesized.

Keywords Living polymerization; Ring-opening metathesis polymerization; Amphiphilic block copolymer; Anhydride group

Electronic Supplementary Material Supplementary material is available in the online version of this article at http://dx.doi.org/10.1007/s10118-017-1873-1.

INTRODUCTION Over the past two decades, ring-opening metathesis polymerization (ROMP) has become a powerful and broadly applicable method for synthesizing macromolecular materials[1]. Especially, ROMP has proven to be a versatile method for creating well-defined block copolymers (BCPs) with low polydispersity indices (PDIs) due to its living nature[2, 3]. The advent of ruthenium (Ru) based catalysts, such as the commercial Grubbs catalysts, which exhibit a remarkable functional group tolerance, makes ROMP particularly valuable for synthesizing highly functionalized polymer materials[4]. Thus far, norbornene derivatives substituted with many kinds of polar groups including amide[5], ester[4, 6], and hydroxyl groups[7] have been successfully polymerized. Through sophisticated molecular design, norbornene derivatives containing more polar groups, such as carboxys[8], amines[9], and even sulfonate groups[10−12], can be also effectively polymerized. For instance, Sanda and Masuda et al. have achieved direct polymerization of norbornene monomers carrying amino groups or carboxyl groups without protection utilizing the Grubbs second generation catalyst (G2) in good yields by incorporating amino acid spacers between the polar groups and norbornene skeleton, which maybe prevent amino groups or carboxyl groups from interacting with the ruthenium center[13].

* This work was financially supported by the National Natural Science Foundation of China (Nos. 21234006 and 21574098). ** Corresponding author: Li-xia Ren (任丽霞), E-mail: [email protected] Received August 12, 2016; Revised September 9, 2016; Accepted September 12, 2016 doi: 10.1007/s10118-017-1873-1 ROMP of Cis-5-norbornene-endo-2,3-dicarboxylic Anhydride Derivatives 37

Polymers containing anhydride group are functional polymers due to the reactivity of anhydride groups and show wide applications in processing, separation and so on[14, 15]. Maleic anhydride modified polypropylene (MA-PP) is the most important commercially functionalized PP polymer for its low cost, high activity, and good processing ability[16]. The anhydride group can be transformed into carboxylic acid or carboxylate, which could be used in separation techniques[17]. The anhydride group can also react with other functional groups such as amino group, which could further broaden its application[18]. However, the preparation of polymers containing anhydride group including post-modification, copolymerization with other monomers is limited. ROMP has been utilized in preparation of polymers containing anhydride group[18]. Buchmeiser and coworkers reported the synthesis of cross-linked polymer containing anhydride group by ROMP of cis-endo-5-norbornene-2,3- dicarboxylic anhydride (NDCA, M1 in Scheme 1) and monomer containing bi-norbornene followed by hydrolysis. The resultant polymer material showed applications in solid-phase extraction (SPE) as well as for air and water clean-up[19]. Grubbs et al. reported on the homo-polymerization of NDCA monomers, however the product was insoluble in the solvent[20]. To date, synthesis of homo- and block copolymers containing anhydride group by controlled/living polymerization is very challenging due to the high polarity of anhydride group. In this study, non-polar substituent was introduced into NDCA to improve the solubility of the corresponding homopolymer (Scheme 1). Different substituents were selected in the preparation of NDCA derivatives to study the size/steric hindrance effect of the substituent on the solubility of polymer containing the anhydride as well as the polymerization behavior. Furthermore, block copolymers containing anhydride groups with well-defined structure were prepared.

Scheme 1 Monomers synthesized via D-A reaction and its homopolymerization

EXPERIMENTAL Materials All materials were obtained from Sigma-Aldrich or TCI and used without further purification except otherwise specified. Anhydrous solvents used in this work were purified by Solvent Purification System (SPS) purchased from Mbraun. Grubbs third generation catalyst RuCl2(3-bromopyridine)2(H2IMes)(CHPh) (G3, H2IMes = N,N- dimesityl-4,5-dihydroimidazol-2-ylidene), synthesized by the method reported previously[21−23], were stored and weighted in the glovebox. Fresh cyclopentadiene (CPD) was cracked slowly from dicyclopentadiene at 168 °C and collected at 0 °C. Phenylmaleic anhydride (PMA)[24] and 9,10-dihydro-9,10-ethenoanthracene-11,12- dicarboxylic anhydride (DHEADCA)[25, 26] were prepared according to literatures with modification (see Supporting Information, SI). 38 J.X. Yang et al.

Characterizations All experimental procedures involving the preparation of air- and/or moisture-sensitive compounds were conducted in Mbraun glovebox or under a nitrogen (N2) atmosphere using standard Schlenk technique unless otherwise noted. Gel permeation chromatography (GPC) was performed on a Waters 1525 instrument with tetrahydrofuran (THF) as eluent at a flow rate of 1.0 mL/min at 35 °C. Linear polystyrene (PS) standards (Easi- Cal PS-1, PL Ltd.) were used as calibration. 1H and 13C-NMR spectra were recorded on a Bruker-400 MHz spectrometer (399.65 MHz for 1H and 100.40 MHz for 13C, respectively) at 25 °C. Mass spectra were recorded on Quattro Premier XE spectrometer with Ionization Mode (ESI). Differential scanning calorimetry (DSC) measurements were performed on a Netzsch 204F1 in nitrogen atmosphere. The samples were heated up and cooled down at a rate of 10 K/min. Glass transition temperature (Tg) was taken from the second heating run and was read as the midpoint of change in heat capacity. Thermal gravimetric analysis (TGA) was performed using a SDTQ 600 TGA Instrument under nitrogen flow at a heating rate of 10 K/min from 40 °C to 800 °C. Synthesis of M2 Monomer M2 was synthesized according to previous reports with modification[27, 28]. Methyl maleic anhydride (MMA) (10 g, 89 mmol) and benzene (40 mL) were added into a round bottom flask (50 mL) equipped with a magnetic stirrer. Freshly cracked CPD (15 mL, 4 eq. of MMA) was added into the flask. The mixture was stirred at room temperature (R.T.) for 20 h followed by reacting at 50 °C for additional 5 h. The solvent was evaporated under vacuum, and a white solid was obtained. The pure product was obtained through recrystallization from 1 ethyl ether/hexane for twice (14.5 g, 92%). H-NMR (400 MHz, CDCl3, δ): 6.35 (dd, 1H), 6.26 (dd, 1H), 3.42−3.43 (br m, 1H), 3.10 (d, 1H), 3.00 (br m, 1H), 1.76−1.82 (m, 2H), 1.59 (s, 3H); 13C-NMR (100.4 MHz,

CDCl3, δ): 174.90, 170.90, 137.20, 135.20, 53.70, 53.40, 52.00, 50.60, 46.60, 21.30; MS (ESI), m/z theoretical + Calc. for [C10H10O3 + Na] 201.06; Found 201.10. Synthesis of M3 Monomer M3 was synthesized from PMA and CPD following the similar procedures as M2[29, 30]. The yield was l 71%. H-NMR (400 MHz, CDCl3, δ): 7.60−7.30 (m, 5H), 6.46 and 6.39 (m, 2H), 3.88 (d, 1H), 3.61 (m, 1H), 13 3.55 (m, 1H), 1.80−1.68 (m, 2H); C-NMR (100.4 MHz, CDCl3, δ): 172.60, 170.70, 137.28, 136.94, 136.35, + 129.19, 128.42, 126.93, 62.62, 53.46, 52.62, 50.96, 46.92; MS (ESI), m/z theoretical Calc. for [C15H12O3 + Na] 263.08; Found 263.2. Synthesis of M4 Monomer M4 was synthesized as follows. 9,10-Dihydro-9,10-ethenoanthracene-11,12-dicarboxylic anhydride (DHEADCA) (8 g, 29.2 mmol), dissolved in DCM (50 mL), were added into a 100 mL round bottom flask equipped with a magnetic stirrer. CPD (30 mL, 364 mmol) was added and stirred at R.T. for 12 h. During the reaction, white solids were precipitated from the solution. Another 14 mL of CPD (170 mmol) was added and stirred overnight. The off-white solid was filtered and washed with abundant hexane. The product was purified by recrystallizing from hexane/DCM mixture as off-white microcrystalline. After dried under vacuum for 10 h, l 6.5 g product was obtained with yield of 63%. H-NMR (400 MHz, CDCl3, δ): 7.11−7.35 (m, 8H), 6.41 (m, 2H), 13 4.64 (s, 2H), 3.01 (m, 1H), 1.19 (d, 1H), 0.18 (d, 2H); C-NMR (100.4 MHz, CDCl3, δ): 173.00, 141.40, 141.20, 140.00, 127.60, 127.40, 125.40, 125.30, 67.40, 48.60, 47.90, 47.50; MS (ESI), m/z theoretical Calc. for + [C23H16O3 + Na] 363.11; Found 363.2. Synthesis of M5 Monomer M5 was synthesized according to literature[25] with excellent yield of 93%. lH-NMR (400 MHz,

CDCl3, δ): 7.45−8.03 (m, 10H), 6.43 (q, 1H), 5.94 (q, 1H), 4.51 (t, 1H), 3.35−3.97 (d, 1H), 3.15 (s, 1H), 1.88 (d, 13 1H), 1.48 (d, 1H); C-NMR (100.4 MHz, CDCl3, δ): 199.80, 198.50, 136.10, 135.90, 135.60, 133.60, 132.10, 131.90, 127.60, 127.70, 127.50, 127.30, 49.50, 47.70, 46.90, 46.30; MS (ESI), m/z theoretical Calc. for + [C21H18O2 + Na] 325.13; Found 325.20.

ROMP of Cis-5-norbornene-endo-2,3-dicarboxylic Anhydride Derivatives 39

Typical Procedure of Homopolymerization

All polymerizations were conducted at R.T. in a glovebox under N2 atmosphere. In a 10 mL vial, a certain amount of G3 was added and dissolved in 1 mL of solvent (DCM or THF). Calculated amount of the monomer was dissolved in another vial with certain amount of solvent. The monomer solution was poured into the catalyst solution under vigorous stirring. At the end of the polymerization, a few drops of ethyl vinyl ether (EVE) were added to terminate the polymerization. The polymers were precipitated from hexane, collected by filtration and followed by vacuum drying at 40 °C. Typical Procedure of Block Copolymerization

Copolymerizations were also conducted at R.T. in a glovebox under N2 atmosphere. A certain amount of G3 was added into a 10 mL vial and dissolved in 1 mL of THF. M5 was dissolved in 1.5 mL of THF and added into the vial under vigorous stirring. After polymerized for 1 h, a certain amount of M4 dissolved in 3.5 mL of THF was poured into the vial, and the polymerization lasted for 5 h. A few drops of EVE were then added to terminate the polymerization. The polymer was precipitated from hexane and collected by filtration. Pure block copolymers (poly(M5)-b-poly(M4)) were obtained after vacuum drying at 40 °C for 10 h. Procedures of Hydrolysis Experiment Poly(M5)-b-poly(M4) (0.2 g, 0.31 mmol of anhydride group) block copolymer, dissolved in 15 mL of THF, was added into a flask filled with NaOH aqueous solution (5 mL, 8 eq.). The mixture was heated to 90 °C and refluxed for 36 h. When the temperature was cooled to R.T., the mixture was acidified with dilute HCl. Petrol ether was added slowly to precipitate the polymer from the solution. The product was collected and dried under vacuum at 40 °C for 2 days after filtration, followed by washing with excessive water till neutral. Block copolymer containing carboxylic acid was thus obtained (0.135 g, 67.5%).

RESULTS AND DISCUSSION It is well known that the Grubbs catalysts are the most efficient and widely used catalysts for ROMP, albeit different generations (G1, G2, and G3) display distinct reaction characteristics. Compared with G1 and G3, G2 exhibits an extremely small initiation/propagation ratio owing to its slow initiation and extremely fast propagation[31]. Accordingly, the products obtained using G2 catalyst are of relatively broad molecular weight distribution (MWD) and much higher molecular weight than the theoretical value. Hence, G2 is not suitable for controlled/living polymerization of ROMP. On the other hand, G3 catalyst shows ultrafast initiation, much higher activity and tolerance of different functional groups[23, 32]. Herein, G3 was applied as catalyst in order to obtain controlled/living polymerization of monomer containing anhydride group. Monomer Design and Synthesis To investigate the polymerization behavior of monomer containing anhydride group, improving polymer’s solubility is the primary requirement. In general, the solubility of polymers containing anhydride in common solvents such as DCM and THF could be improved by incorporating non-polar substitutent groups into the monomer. For this reason, different substituents had been introduced into NDCA as shown in Scheme 1. Three NDCA derivatives, M2, M3 and M4, with gradually enlarged non-polar substituents, were prepared with good to high yields via the D-A reaction between freshly cracked CPD and MMA, PMA, and DHEADCA, respectively. Figure 1 shows the 1H-NMR spectra of the corresponding pure monomers. Only one stereoisomer (endo-adduct) was found for both M2 and M3 because of the endo-rule. Since their molecular structures were asymmetric, the protons on the double bond were nonequivalence and two peaks were found. However, the D-A reaction of CPD with DHEADCA gave a mixture of endo- and exo-adduct as indicated in the 1H-NMR spectrum (Fig. S5 in SI). Pure endo-adduct (M4) (Fig. 1 top) could be obtained through recrystallization from hexane/DCM. As the molecular structure of M4 was symmetric exactly, only one peak of norbornene alkene protons was found. Furthermore, 13C-NMR and mass spectrometry (MS) were used to characterize these monomers. 40 J.X. Yang et al.

1 Fig. 1 H-NMR spectra of NDCA derivatives in CDCl3 ROMP of NDCA Derivatives Although the synthesis of these NDCA derivatives (M2−M4) can be found elsewhere, their ROMP behavior has not been studied. Herein, the homopolymerizations of M1 and all these synthesized NDCA derivatives (M2−M4) were explored utilizing efficient G3 catalyst under inert atmosphere at R.T. in DCM and THF. The results are listed in Table 1. In the polymerization of M1, white solid appeared upon adding NDCA solution into the catalyst solution due to the very poor solubility of poly(M1), which was similar to a reported study[20]. Thus, GPC analysis of poly(M1) in THF was impossible. For the polymerization of M2, in which a methyl substitute was incorporated, in DCM, no precipitate was observed in the early stage. However, precipitation was observed after ca. 15 min and poly(M2) showed very high MW and multimodal distribution while analyzed by GPC in THF (entry 3, Table 1). The wide distribution might be attributed to a heterogeneous polymerization process. When the polymerization was performed in THF, no solid was observed during the whole process. Nevertheless, the MWD was somewhat broad (PDI = 3.13) (entry 4, Table 1). With a bulkier phenyl group in M3, poly(M3) showed much better solubility compared to poly(M1) and poly(M2). The polymerization solution remained clear during the entire process in either DCM or THF. However, the MWD was still a little broad (PDI ca. 2.5) (entries 5 and 6, Table 1). Surprisingly, with the bulkiest substituent in M4, poly(M4) had narrow MWD (PDI = 1.38, entry 7 in Table 1). In particular, when polymerized in THF, PDI of poly(M4) was as low as 1.21, indicating that the polymerization of M4 was most likely a living polymerization (entry 8, Table 1). It was believed that the bulky substitute played an important role in the observed narrow MWD. The steric hindrance of M4 may have greatly reduced the occurring of both intramolecular and intermolecular chain transfer reaction. To confirm the living polymerization of M4 catalyzed by G3 in THF, a series of homopolymerization with different

M4/G3 feed molar ratio were conducted under the same polymerization conditions. As shown in Fig. 2, Mn increased linearly with increase in M4/G3 feed molar ratio, and PDI was consistently narrow for all polymerizations (PDI < 1.30). This result indicated that ROMP of M4 catalyzed by G3 in THF was a living polymerization. At the end of the polymerization, monomer conversion was calculated from 1H-NMR analysis of the crude product by comparing the integration of the protons of alkene from the corresponding monomer and polymer. As shown in Fig. S6, the signal from the alkene group of the monomer was located at δ = 6.20−6.50, wherever, the peaks at δ = 5.00–6.00 were assigned to the alkene groups from the polymer. The monomer conversions of all these NDCA derivatives reached above 97%, which demonstrated the very high activity and functional group ROMP of Cis-5-norbornene-endo-2,3-dicarboxylic Anhydride Derivatives 41 tolerance of G3. It should be noted that the yields of poly(M2), poly(M3) and poly(M4) were less than 100% due to weight loss during the work up of these polymers[33]. The polymers were purified by precipitating for several times and followed by vacuum dry. Figure 3 shows the 1H-NMR spectra of poly(M2) and poly(M4). The 1H- NMR spectra of M3 and poly(M3) are shown in Fig. S7. As observed, there were no peaks at δ = 6.20−6.50, indicating the purity of the polymers. Moreover, no signal of carboxylic acid was observed at δ = 9.00−13.00 in DMSO, which meant that the anhydride group was very stable during the entire process (Fig. S7 in SI). Dissolving capacity in different solvents of the resultant anhydride group containing polymers was further studied. The results are summarized in Table 2. It was found that the solubility of the anhydride-containing polymers was significantly improved with increased steric hindrance of the substituent groups. The solubility of poly(M4) was greatly improved as it was dissolved in most of common organic solvents such as ethyl acetate (EA), toluene, and acetone.

Table 1 Results of homopolymerization of monomers mediated by catalyst G3 a b −3 c c Entry Monomer Solvent [G3] (mol/L) [M]/[G3] t (h) Conv. (%) Yield (%) Mn × 10 PDI 1 M1 DCM 0.625 100 3 98 55 − d − d 2 M1 THF 8.333 100 3 99 50 − d − d 3 M2 DCM 1.250 100 12 97 86 187 e − e 4 M2 THF 1.250 100 3 97 90 31.4 3.13 5 M3 DCM 1.250 100 12 98 80 47.8 2.52 6 M3 THF 1.250 100 3 98 85 49.6 2.57 7 M4 DCM 1.250 100 7 98 79 23.6 1.38 8 M4 THF 0.800 100 3 97 80 24.9 1.21 a Monomer conversion was determined by comparing the peak areas of the norbornene alkene protons and the alkene protons 1 b c of polymer from H-NMR spectra in CDCl3 for the crude mixtures; Petrol ether-insoluble part; Number-average molecular weight (Mn) and PDI of the resultant polymer determined by GPC at 35 °C in THF versus narrow polystyrene standards; d THF insoluble; e Multimodal distribution

Fig. 2 Variation of Mn and PDI as functions of [M4]/[G3] feed molar ratio in ROMP of M4 in THF Block Copolymerization of M4 and M5 Block copolymers containing anhydride group is very important, since they can be transformed into carboxylic acid or carboxylate containing amphiphilic block copolymers, which are of great scientific interest because of possible applications in the areas of advanced materials and nano-pharmaceuticals[33−35]. In this work, 5-norbornene-endo, exo-2,3-diylbis(phenylmethanone) (M5), which was synthesized via the D-A reaction of freshly cracked cyclopentadiene with trans-dibenzoylethylen[25], was chosen as the hydrophobic monomer due to its nonhydrolysable character[33]. This monomer was also characterized by (1H, 13C) NMR spectroscopy (Fig. S8a in SI) and MS. The living manner of M5 homopolymerization promoted by G3 in THF was confirmed by an extremely low PDI value of poly(M5) (< 1.1) (entry 1, Table 3). 42 J.X. Yang et al.

Fig. 3 1H-NMR spectra of (A) M2 and poly(M2); (B) M4 and poly(M4)

Table 2 Dissolving capacity of anhydride group containing homopolymers a Polymer EA Toluene DCM Chloroform Acetone THF DMSO DMF Poly(M1) − − − − − − + + Poly(M2) − − − − + + + + Poly(M3) − − + + + + + + Poly(M4) + + + + + + + + a Test method: 10 mg of polymer dissolves in 1 mL of solvent at R.T.; (−) Insolubility or poor solubility; (+) Good solubility

Block copolymers were synthesized through sequential addition method as shown in Scheme 2. M5 was added and reacted for 1 h to make the first sequence, followed by addition of M4 and reacted for another 5 h to make sure that M4 was consumed completely. As shown in Table 3, three diblock copolymers were obtained (entries 2−4, Table 3). Due to the complicated 1H-NMR spectra of block copolymers (Fig. S9 in SI), the molar ratios between M4 and M5 in the block copolymers were difficult to be calculated. No monomer peaks were observed in the 1H-NMR spectra of the block copolymers (Fig. S9 in SI). Representative GPC traces for entry 4 (Table 3) are shown in Fig. 4. Compared with the first block, the elution time of the diblock copolymer was significantly shortened, which indicated the successful synthesis of diblock copolymer. Narrow MWDs were obtained for both initial poly(M5) block (PDI < 1.11) and final poly(M5)-b-poly(M4) diblock polymer (PDI < 1.29). The preliminary investigation on the hydrolysis of anhydride group was conducted to obtain amphiphilic diblock copolymer containing carboxyl groups. The hydrolysis experiment of poly(M5)-b-poly(M4) (entry 3,

Table 3) was conducted with excessive NaOH in THF/H2O mixture under refluxing. The anhydride group could be easily hydrolyzed to carboxyl group as proved by 1H-NMR (Fig. S10 in SI) and IR analysis (Fig. S11 in SI).

Table 3 Results of diblock copolymerization a c b Conv. Yield First block Diblock copolymer Entry M5/M4/G3 Solvent t (h) −4 d d −4 d d (%) (%) Mn × 10 PDI Mn × 10 PDI 1 50/0/1 THF 1 99 60.6 1.10 1.06 − − 2 25/75/1 THF 1/5 99 85.0 0.46 1.09 1.98 1.26 3 50/50/1 THF 1/5 99 83.6 1.13 1.11 2.63 1.26 4 75/25/1 THF 1/5 99 80.3 2.10 1.10 3.15 1.29 a b c d Polymerization at R.T.; Molar ratio; Petrol ether-insoluble part; Mn and PDI were determined by GPC at 35 °C in THF versus narrow PS standards ROMP of Cis-5-norbornene-endo-2,3-dicarboxylic Anhydride Derivatives 43

Scheme 2 Synthesis of block copolymer from monomers by sequential addition method

Fig. 4 GPC elution profiles after each reaction step (entry 3, Table 3) Thermal Properties of Polymers Containing Anhydride Thermal properties of these newly obtained polymers, especially those containing anhydride groups, were examined by TGA and DSC. The results are summarized in Table 4. The thermal stability of the resultant polymers was first investigated by TGA. Note that when the samples were heated above 100 °C, a minor weight loss for all the anhydride containing polymers was observed. This might be attributed to the release of adsorbed water (H2O), which was very difficult to remove because of relatively strong hydrogen bonding between H2O and the anhydride group. Thus, the starting decomposition temperature here was recorded as the onset weight loss temperature after all H2O was removed. As shown in Fig. 5, the thermal stability of NDCA’s homopolymer was poor. The thermal stability was improved by introducing substituents into M1. Poly(M2) and poly(M3) with small substituent to moderate substituent showed higher thermal decomposition temperatures (Td,s, ca. 330 °C) than poly(M1). However, with the bulkiest substituent, poly(M4) showed a relatively low Td,s of about 290 °C, which might be attributed to the easier retro-D-A reaction to release anthracene[36, 37]. The poly(M5) owned the highest Td,s of 375 °C because of its stable structure. The typical block copolymer (entry 3, Table 3) was also studied by TGA. The Td,s of poly(M5)-b-poly(M4) was very close to poly(M4) with Td,s of 288 °C. The thermal transition temperatures of these polymers were investigated by DSC. Because of low Td,s of poly(M1), no transition temperature such as glass transition temperature (Tg) was observed. As shown in Fig. 6, poly(M2) had a Tg of 281 °C, while poly(M3) with bulkier phenyl substituent showed a relatively low Tg of 258 °C. The phenyl group may block the interaction between polar anhydride groups (both intermolecular and intramolecular) to some extent, and Tg of poly(M3) thus was decreased. Due to the very steric bulk of M4, Tg of poly(M4) may be higher than its Td,s. Therefore, glass transition was not displayed in DSC curve within the range of test temperature. Tg of poly(M5) was found to be 137 °C, which was close to the value previously reported (143 °C)[33]. The thermal transition of poly(M5)-b-poly(M4) block copolymer (entry 3, Table 3) was also measured by DSC; however, no obvious transition temperature was found. 44 J.X. Yang et al.

Table 4 Thermal properties of the resultant polymers a b c Polymer Tg Td,s Td,10 Th,max Poly(M1) − d 140 190 194 Poly(M2) 281 333 353 429 Poly(M3) 258 325 347 390 Poly(M4) − d 291 303 341 Poly(M5) 137 375 382 402 Poly(M5)-b-poly(M4) − d 288 297 316 a The start decomposition temperature; b The temperatures of 10% weight loss; c The highest decomposition temperature; d “−”Not detected

Fig. 5 TGA curves of anhydride-containing polymers Fig. 6 DSC curves of anhydride-containing polymers

CONCLUSIONS Norbornenyl monomers containing anhydride with different bulky substituent were synthesized. The polymers were prepared via ROMP of the monomers catalyzed by G3 at room temperature. By incorporation of bulky groups, the solubility of homopolymers containing anhydride in common solvent was greatly improved. Well- defined anhydride group containing diblock copolymers with narrow PDI were prepared by a sequential addition approach. Amphiphilic diblock copolymers with carboxylic acid as the hydrophilic groups were prepared by hydrolysis of the block copolymers containing anhydride. And, by introducing bulky substituents, the thermal stability of corresponding polymers was improved.

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