Synthesis of High Performance Cyclic Olefin Polymers (cops) with Ester Group via Ring-opening Metathesis Polymerization

Novel ester group functionalized cyclic olefin polymers (COPs) with high glass transition temperature, high transparency, good mechanical performance and excellent film forming ability have been achieved in this work via efficient ring-opening metathesis copolymerization of

For some applications of COCs and COPs, high Tg is a requirement for keeping good dimensional stability under higher temperature [15,26,27].The Tg of COCs can be enhanced by increasing the cycloolefin content in the polymer chains [16,28].Another approach to improve the Tg of COCs is employing a bulkier cycloolefin monomer [15,27,29,30].Due to its steric effect, high Tg can be achieved at relatively low comonomer incorporation.For instances, we have prepared a novel COC with high Tg (207.0 °C) by copolymerizing a bulky monomer, exo-1,4,4a,9,9a,10-hexahydro-9,10(1′,2′)benzeno-l,4-methanoanthracene (HBM), with ethylene in which HBM content is only 30.4 mol% (1 in Scheme 2) [31].For COPs, which can be viewed conceptually as an alternating copolymer of ethene and a cyclic monomer, a sterically hindered cycloolefin seems the only way to improve their Tg [4].Recently, we have obtained a novel COP by ROMP of HBM and subsequent hydrogenation (2 in Scheme 2) with high Tg value of about 220 °C [32,33].The Tg can be adjusted at an attractive region of 160-220 °C via copolymerization of HBM with less bulky monomer (3 in Scheme 2).It was found that the Tg of the resulted COPs linearly increased with HBM content, which is easily controlled by changing feed ratios.
It is noteworthy that the most COCs and COPs are hydrophobic, which may limit some of their end uses [3,34].Effective improvement of the material performances, such as toughness, adhesion, paintability, and compatibility with other polymers, can be achieved by introducing some functional groups into polymer chains [35].For example, Arton ® (4 in Scheme 2), commercialized by JSR, is a cyclic olefin resin with a polar group in its molecular chain and is good in adhesion, adherence, and miscibility.Direct coordination (co)polymerization of monomers with polar functional groups is very difficult for COCs because of catalysts poisoning.Generally, polar monomer needs to be protected by pre-reaction with protective groups to avoid catalyst deactivation.Recently, Shiono and his co-workers introduced carbonyl or hydroxyl groups into copolymers via copolymerization of norbornene with ω-alkenylaluminum as a polar monomer precursor [35,36].Ru-mediated ROMP, nevertheless, has good tolerance towards various functional groups, which allows for the facile introduction of many different functional groups into COPs [37][38][39][40][41].In this work, we described the synthesis of high-performance functional COPs containing ester groups by copolymerization of HBM with three different comonomers using the Grubbs first generation catalyst, RuCl2(PCy3)2(CHPh) (Cy = cyclohexyl) (G1), and subsequent hydrogenation (5 in Scheme 2).

Characterization
The NMR data of the monomer and polymer samples were obtained on a Bruker400 MHz spectrometer (400 MHz for 1 H, 100 MHz for 13 C, Bruker, Karlsruhe, Germany) at ambient temperature, with CDCl3 or CD2Cl2 as a solvent.Chemical shifts (δ) are reported in parts per million relative to the internal standard tetramethylsilane (TMS, δ = 0.00 ppm).Infrared spectra were acquired on a Jasco FT/IR-410 spectrometer (Jasco, Tokyo, Japan); wave numbers in cm −1 were reported for characteristic peaks.Differential scanning calorimetry (DSC) measurements were carried out under nitrogen atmosphere at a heating rate of 10 °C/min with a DSC1 Stare System (Mettler Toledo Instruments, Zurich, Swiss) calorimeter up to a temperature of 250 °C using a sample mass in the range of 5-10 mg.The glass transition temperature (Tg), was taken from the second heating scan and was recorded as the midpoint of the step change in heat capacity.The molecular weights and the molecular weight distributions of the polymer samples were determined at 35 °C by a waters 1525 type gel permeation chromatography (GPC).Polymer solutions for GPC tests were prepared with a concentration of ca.0.1% w/v in HPLC grade THF, and the calibration was made by polystyrene standard Easi-Cal PS-1 (PL Ltd., Agilen, Santa Clara, California, USA).Thermo gravimetric analyses (TGA) were performed on a Perkin-Elmer Pyris 1 (PerkinElmer Inc., Waltham, Massachusetts, USA) at a heating rate of 10 °C/min in nitrogen or air.The weights of the samples were of about 8 mg.Tensile tests were performed on an 8.9 kN, screw driven universal testing machine (Instron 1211, Canton, Boston, MA, USA) equipped with a 10 kN electronic load cell and mechanical grips.The tests were conducted at room temperature using a cross-head rate of 5 mm/min according to the ASTM standard.The data reported were the mean and standard deviation from at least ten determinations.The static water contact angles (WCA) on the surfaces of COPs films was measured at room temperature and 60% relative humidity using a contact angle goniometer (DSA, KRUSS GmbH, Bremen, Germany) by the sessile drop method with a 3 μL water droplet.The WCA values were recorded after 3 s from droplet deposition.For each value reported, at least five measurements taken from different surface locations were averaged.The angle measure was accurate to ±3.The transparency of copolymer film was recorded on a Shimadzu UV-3600 spectrophotometer (Shimadzu, Kyoto, Japan).X-ray photoelectron spectroscopy (XPS) was used to analyze the surface oxygen atomic state of copolymers with Mono X-Rource (Thermal ESCALAB 250, Thermo Fisher Scientific, Fair Lawn, NJ, USA).

Ring-Opening Metathesis (Co)polymerization and Hydrogenation Procedure
The mixtures of HBM and comonomer (total weight: 1.5 g) were dissolved in 25 mL dichloromethane (DCM) in a 60 mL glass reactor equipped with a mechanical stirrer at room temperature.Under nitrogen atmosphere, an appropriate amount of G1 ([monomers]/[G1]: 300/1), dissolved in 5 mL dichloromethane, was transferred into the reactor.The mixture was stirred for two hours, until the color changed from purple to red-orange.Excess ethyl vinyl ether was added and the reaction was allowed to stir for 30 min at room temperature.The residue was then precipitated by drop-wise addition of the reaction mixture into vigorously stirred methanol.The polymers were washed three times with methanol and dried in a vacuum oven overnight at 40 °C.
In a typical procedure, unsaturated polymer (1.0 eqv.), 4-methylbenzenesulfonhydrazide (5.0 eqv.), tripropylamine (5.5 eqv.) and a small amount of butylated hydroxytoluene were transferred into a 250 mL round-bottom flask.The mixture was thoroughly degassed and stirred under a nitrogen atmosphere.Then, 30 mL toluene was injected into round-bottom flask and the mixture was refluxed at 110 °C with vigorous stirring.After approximately 16 h, the solution was poured into 150 mL of stirring methanol to yield a white solid.The solid was filtered and washed with distilled water, and then the polymer was redissolved in 20 mL of hot toluene and poured into 150 mL of methanol.This procedure was repeated twice again in order to remove impurities and inorganic substances.The obtained white polymer was dried under vacuum at 50 °C for 24 h.The yields of all the saturated polymers were higher than 85%.The 1 H NMR spectra for the homopolymer have been reported [32], and copolymers were provided in Figures 1A and A1 (in Appendix).

Preparation of Film
The resultant polymer (adjusted to fit a total concentration of 8-10 wt%) was suspended in tetrachloroethane in a 50 mL round-bottom flask, and the mixture was heated at 60 °C for at least 10 h.After the complete dissolution, the thick solution was filtered using a filter.The obtained solution was spread on a glass plate in an oven with a nitrogen atmosphere.The solvent in the spread film was carefully evaporated at 55 °C for 12 h.Then the film was further dried at 60 °C for about 12 h.After the evaporation, the thickness of obtained film was measured about 80-140 μm.

HBM/Comonomer Copolymerization
To obtain COPs with improved performances, such as adjustable Tg and hydrophilicity, we have designed and synthesized three norbornene derivatives with ester group as comonomers, 5-norbornene-2-yl methylacetate (NMA), 5-norbornene-2-yl methyl 2-ethylhexanoate (NME) and 5-norbornene-2-yl methyldodecanoate (NMD).Calculated from 1 H NMR spectra, it was found that all the comonomers were a mixture of endo/exo-isomer with mole ratio of 2:1.A series of ROMP at different HBM/comonomer molar ratios were then carried out using the Grubbs first generation catalyst Ru(CHPh)(Cl)2(PCy3)2 (Cy = cyclohexyl, G1), which exhibited very high activity to produce well-defined polymers.All the copolymerizations took place efficiently, affording copolymers in quantitative yields.Thus, copolymer compositions can be adjusted precisely and easily by varying the molar ratio of HBM to comonomers (NMA, NME and NMD) in the copolymerization reaction.The typical data including the molecular weights (MWs) and molecular weight distributions (MWDs) of the resultant COPs are summarized in Table 1.All these copolymers showed high MW, ranging from 40 kg/mol to 110 kg/mol, and relatively narrow MWDs with polydispersity index (PDI) values from 1.24 to 1.60.The content of HBM in the copolymers can be calculated by 1 H NMR spectra, and the experiment values were consistent with the theoretical ones.A typical 1 H NMR spectrum for HBM/NMA copolymer is shown in Figure 1A (entry 2, Table 1, CD2Cl2 at 25 °C).The resonances at δ 3.8-4.3ppm were assigned to the two protons of benzal and methylene, which directly connected norbornene skeleton, and protons of benzene ring in HBM were detected at 6.5-7.2 ppm.The HBM incorporations of the copolymers were calculated from the relative intensities of these protons using the equation: where I3.8-4.3 ppm is the benzal and methylene signals from HBM and comonomer units, respectively; and I6.5-7.2 ppm is from benzene ring in HBM unit.The ratio of the signal intensity was 90.8%, close to the theoretical value (90:10), which also supported the point that the feed cyclic olefins can be quantitatively converted into polymers under these conditions.These results clearly indicated that synthesis of various grades COPs is possible by this approach.The reactivity ratios of HBM and comonomers were estimated by the classical Fineman-Ross method, which were calculated from values of the monomer feed ratio and the analyzed copolymer compositions (see Figures A2-A4 and Tables A1-A3 in Appendix) [44].For the copolymerization of HBM and NMA, the monomer reactivity ratios (r) were determined to be rHBM = 1.65 and rNMA = 0.59 (Figure A2 and Table A2 in Appendix).This result indicates that HBM is more reactive than NMA, and the obtained copolymers are a kind of gradient copolymers [45].The high reactivity of HBM was possibly attributed to high ring strain.Furthermore, the ester group of NMA, especially the endo-isomer, may coordinate the active center, which decreased the reactivity of comonomer [46][47][48][49].The same method proved that copolymers of HBM with NME or NMD are also gradient ones (the value rHBM = 1.58, rNME = 0.52, and rHBM = 1.37, rNMD =0.7) (see Figures A3,A4 and Tables A2,A3 in Appendix).
Hydrogenation of the double bonds in main chain of ROMP products was helpful for improving the materials' tolerance towards heat and light.The effective hydrogenation of these copolymers was performed with excess toluenesulfonyl hydrazide.Complete hydrogenation can be confirmed via the disappearance of the signals assigned to the olefinic protons in main chain at 5.0-6.0 ppm in 1 H NMR spectrum (Figure 1A,b).The FT-IR spectra of selected copolymers before and after hydrogenation are shown in Figure 1B; the quite clear absorption peak at 968 cm −1 is assigned to the out of plane C-H ring bending vibration (Figure 1B,a).However, no absorptions are observed after hydrogenation, which further confirm the complete hydrogenation (Figure 1B,b).

Thermal Properties of COPs
The glass transition temperatures (Tg) of these obtained polymers were examined by DSC analyses.The typical DSC curves of homopolymers of comonomers are shown in Figure A5 (in Appendix).The Tg decreased substantially after hydrogenation.After hydrogenation, the double bond was replaced by single bond and the flexibility of the polymer chain increased, resulting in lower Tg of the polymer.For example, the Tg value of poly(NMA) (22.4 °C) is 9 °C higher than that of the corresponding hydrogenated polymer H-poly(NMA) (13.7 °C).Additionally, poly(NMA) exhibit substantially higher Tg values as compared with poly(NME), both before and after hydrogenation.For the hydrogenated homo-polymers, the Tg value of H-poly(NMA) (13.7 °C) is higher by 48 °C than that of poly(NME) (−34.4 °C).This result seems to arise mainly from the reduced flexible side chain of poly(NMA).It is noteworthy that a melting temperature (−18.2 °C) was observed in the DSC curve of poly(NMD) because the long alkyl side chain led to side chain crystallization.As expected, all the hydrogenated copolymers were amorphous, which only exhibited apparent Tg without any melting temperatures (see Figures A6-A8 in Appendix).With increase of the comonomer incorporation, the Tg decreased linearly, as shown in Figure 2A.Interestingly, the H-poly(HBM-co-NME) copolymer exhibited the similar Tg values (NME 19.5 mol%, Tg 145.5 °C, entry 10, Table 2) to H-poly(HBM-co-NMD) copolymer (NMD 22.0 mol%, Tg 134.1 °C, entry 14, Table 2), whereas H-poly(HBM-co-NMA) copolymer exhibited significantly higher Tg (NMA 18.2 mol %, Tg 188.3 °C, entry 3, Table 2) at the same level of comonomer incorporation.The differences became more significant with the increase of the comonomer incorporation.When the content of NMA in copolymer was 40 mol%, the Tg still remained above 150 °C.The results indicated that a wide range of flexible comonomers (0-40 mol%) incorporation lead to different Tg values in a broad range (220-150 °C).Moreover, all the lines showed good correlation so that various grades of heat-resistant COPs can be obtained predictably by deliberate modulation of the composition of copolymers.
The thermal stability of the obtained copolymers was investigated by TGA.The typical measurement curves are shown in Figure 2B.All copolymers are thermally stable below 350 °C in nitrogen and exhibit two similar thermal decomposition stages above this temperature.The first stage degradation (350-400 °C) was attributed to decomposition of retro Diels-Alder reaction forming anthracene.The second degradation stage (400-500 °C) corresponded to degradation of copolymer structure in main chain.The thermal decomposition stages of H-poly(HBM) were similar to those of the copolymers, which indicated that comonomer incorporation showed little effect on thermal stability of the copolymers.Figure 2C showed the TGA curves of H-poly(HBM-co-NMA) copolymers at different levels of NMA.With increase in the comonomer content, the weight loss of the first decomposition stage decreased, which was consistent with the weight occupancy of benzene ring.Meanwhile, the copolymers also showed very similar thermal decomposition behavior in air and in nitrogen (see Figure A9 in Appendix).

Transparency of Cyclic Olefin Copolymer
Since the molecular weights values of the copolymers seemed to be high enough to form self-standing films, we tried to prepare the copolymer films by casting the tetrachloroethane solutions of the copolymers.To our delight, the self-standing transparent films with a thickness of 80-140 μm were successfully obtained for all these copolymers.The UV-vis measurement indicates that the COPs were always transparency irrelevant to the content and type of comonomers and the transmittance of the COPs was higher than 85% (Figure 3A).

Hydrophilicity of Cyclic Olefin Copolymer
The lack of polar groups in COPs has limited their application properties.Because of the ester groups in the comonomers, the resultant copolymers in this work should exhibit some hydrophilic properties.The water contact angles (WCA) of the copolymer films were measured to assess the hydrophilic properties of the COPs and the typical results were shown in Figure 3B.The contact angle varied in inverse proportion to the ester groups' content in the copolymer (103°-84° in 10-60 mol% of NMA in the copolymers).It was speculated that the contact angle of H-poly(HBM-co-NMA) copolymer is smaller than the other copolymers, because short alkyl side chain (methyl) of NMA may make the oxygen more easily exposed on the surface of polymer, resulting in the increase of the polarity of the copolymer.However, it turned out that the result was not consistent with this speculation.As we can see from Figure 3B, the H-poly(HBM-co-NMA) copolymers showed higher contact angles than the H-poly(HBM-co-NME) and H-poly(HBM-co-NMD) ones; namely, the polarity of the H-poly(HBM-co-NMA) copolymers was smaller than the others under the same comonomer incorporation.It was possible that hydrophobic long alkyl side chain in H-poly(HBM-co-NME) and H-poly(HBM-co-NMD) copolymers caused its rearrangement and extended toward the inside of polymer membrane, which exposed the ester group on the surface of the polymer membrane.However, for H-poly(HBM-co-NMA) copolymer, hydrophobic short alkyl side chain (methyl) randomly distributed on the surface of polymer membrane causing low oxygen concentration on the surface of polymer membrane.
In order to prove this assumption, X-ray photoelectron spectroscopy (XPS) studies were conducted to evaluate the oxygen concentration on the surface of copolymer films.The results showed that the oxygen concentration in H-poly(HBM-co-NMA) (20 mol%) copolymers surface was indeed lower than H-poly(HBM-co-NMD) (20 mol%) (see Figure A10 in Appendix), which was consistent with the higher WCA of poly(HBM-co-NMA)(Figure 3B).The oxygen concentration increased and the WCA decreased correspondingly with the increase of the content of NMA in the copolymer.

Mechanical Properties of Cyclic Olefin Copolymer
In this work, the COPs with higher flexible comonomer incorporation even display higher Tg, which may be promising for improving mechanical performance without sacrificing high Tg of the COPs.Therefore, the tensile test was further conducted to verify their mechanical performance.The stress-strain behaviors were measured under the similar conditions as a previous report [15].The typical results are provided in Table 2, for the HBM homopolymer with Tg of 216.3 °C, a strain at break of 1.9% was observed (entry 1, Table 2).With increase of the content of comonomer, the strain at break (εb) of the copolymer increased gradually and the tensile strength (σ) and tensile modulus (E) decreased gradually (entries 2-6, 7-10, 1-14, Table 2).The enhanced flexibility can be clearly explained by the increased chain entanglement as copolymers contain higher amount of flexible comonomers units.Although the σ and the E value decreased, εb gradually increased from 2.8% to 9.7% as the content of NMA increased.All copolymers were similar flexible at the same level of comonomer incorporation.Meanwhile, all the copolymers exhibited higher Tg, more than 150 °C, except that NMA incorporation was 60 mol%, which indicated the resulted copolymers possess both high Tg and improved mechanical performance.

A2. Synthesis of 5-Norbornene-2-yl Methyl 2-Ethylhexanoate (NME)
(a) A 0.25 mol of isocaprylic acid was placed in a dried, 500-mL, three-necked, round-bottomed flask equipped with a magnetic stirrer, a dropping funnel, a water condenser and a thermometer.On the top of the condenser, an exit tube with a gas-absorption trap was attached to remove the reaction of hydrogen chloride.When the solution was heated to 60 °C, thionyl chloride 0.5 mol was added dropwise under stirring.The reaction mixture was heated and refluxed on a steam bath, until no more hydrogen chloride evolved.Excess thionyl chloride was removed by distillation at atmospheric pressure.Then, isocaprylic acid chloride was distilled off under reduced pressure.The yield of the product: 95%.

A5. Reactivity Ratios of Copolymerization
Copolymerization of HBM and NMA using the Grubbs first generation catalyst Ru(CHPh)(Cl)2(PCy3)2 (Cy = cyclohexyl, G1) was performed in dichloromethane at room temperature with various monomer feed compositions (<10% conversion).The reactivity ratios can be obtained by using Equation (A1) (the Fineman-Ross method), for which the reactivity ratio of NMA (rNMA) is the negative value of the slope and reactivity ratio of HBM (rHBM) is the intercept.
where f refer to the ratio of NMA and HBM compositions in copolymers, respectively; F refer to the ratio of the feed compositions of NMA and HBM monomers.The values of various parameters for calculating reactivity ratios of NMA and HBM are summarized in Table A1. Figure A1 shows the plot of (f−1)/F versus f/F 2 .From data fitting by the least-squares regression, the obtained reactivity ratios of HBM (rHBM) and NMA (rNMA) are 1.65 and 0.59, respectively.The value rHBM × rNMA = 0.96, being less than 1, shows that the copolymers synthesized using G1 are random ones.Figure A2 shows the plot of (f−1)/F versus f/F 2 .From data fitting by the least-squares regression, the obtained reactivity ratios of HBM (rHBM) and NME (rNME) are 1.58 and 0.52, respectively.The value rHBM × rNME = 0.82, being less than 1, shows that the copolymers synthesized using G1 are random ones.Figure A3 shows the plot of (f−1)/F versus f/F 2 .From data fitting by the least-squares regression, the obtained reactivity ratios of HBM (rHBM) and NME (rNME) are 1.37 and 0.7, respectively.The value rHBM × rNMD = 0.96, being less than 1, shows that the copolymers synthesized using G1 are random ones.

Scheme 2 .
Scheme 2. Structures of COPs in previous reports and the synthetic routes in this work.

Figure 2 .
Figure 2. (A) plots of Tg of the copolymer vs. comonomers incorporation; (B) TGA curves of the copolymers of HBM with different comonomers in N2; (C) TGA curves of the copolymers of HBM with NMA in N2.

Figure A2 .
Figure A2.Plot of (f-1)/F versus f/F 2 with least-squares regression line for HBM and NMA copolymerization.

a
Copolymers obtained at low conversion (<10%) from different monomer feed compositions; b f, m NME /m HBM , and mNME and m HBM refer to NME and HBM compositions in copolymers, respectively; c F, M NME /M HBM , and M NME and M HBM refer to the feed compositions of NME and HBM monomers, respectively.

Figure A3 .
Figure A3.Plot of (f−1)/F versus f/F 2 with least-squares regression line for HBM and NME copolymerization.

a
Copolymers obtained at low conversion (<10%) from different monomer feed compositions; b f, m NMD /m HBM , and m NMD and m HBM refer to NMD and HBM compositions in copolymers, respectively; c F, M NMD /M HBM , and M NMD and M HBM refer to the feed compositions of NMD and HBM monomers, respectively.

Figure A4 .
Figure A4.Plot of (f−1)/F versus f/F 2 with least-squares regression line for HBM and NMD copolymerization.

Figure A6 .
Figure A6.DSC curves of copolymers of HBM and NMA.

Figure A7 .
Figure A7.DSC curves of copolymers of HBM and NME.

Figure A8 .
Figure A8.DSC curves of copolymers of HBM and NMD.

Figure A9 .
Figure A9.Thermogravimetric analyses of compolymers of HBM with different comonomers in air.

Table 2 .
Tensile Properties of the COPs films a .
a Samples were prepared via solution casting and were measured at cross-head rate of 5 mm/min; b Tensile strength; c Tensile modulus; and d Strain at break.

Table A1 .
Various a Copolymers obtained at low conversion (<10%) from different monomer feed compositions; b f, m NMA /m HBM , and m NMA and m HBM refer to NMA and HBM compositions in copolymers, respectively; c F, M NMA /M HBM , and M NMA and M HBM refer to the feed compositions of NMA and HBM monomers, respectively.