Periodic Vinyl Copolymers Containing γ-Butyrolactone via ADMET

May 25, 2012 - Two symmetric diene monomers, M6 and M8, were designed. Both monomers contain two γ-butyrolactone units, but they are different in the...
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Periodic Vinyl Copolymers Containing γ-Butyrolactone via ADMET Polymerization of Designed Diene Monomers with Built-in Sequence Zi-Long Li, Lei Li, Xin-Xing Deng, Li-Jing Zhang, Bo-Tao Dong, Fu-Sheng Du, and Zi-Chen Li* Beijing National Laboratory for Molecular Sciences (BNLMS), Key Laboratory of Polymer Chemistry & Physics of Ministry of Education, Department of Polymer Science & Engineering, College of Chemistry and Molecular Engineering, Peking University, Beijing 100871, China S Supporting Information *

ABSTRACT: The synthesis of a new family of periodic copolymers containing γ-butyrolactones by acyclic diene metathesis polymerization (ADMET) and their thermal properties are presented. Two symmetric diene monomers, M6 and M8, were designed. Both monomers contain two γ-butyrolactone units, but they are different in the length of methylene spacers between cyclic structures and terminal alkenes. The monomers have been prepared by a two-step approach; first, the atom transfer radical addition (ATRA) of diethyl meso-2,5-diiodohexanedioate with either 1,5-hexadiene or 1,7-octadiene was conducted to yield intermediates containing two γ-iodo ester sequences; subsequently, the specific sequence was transformed into γ-butyrolactone unit via intramolecular cyclization upon heating. The two monomers were polymerized using two Grubbs catalysts (Grubbs I and Grubbs II) to produce four polymers with moderate to high molecular weights (P6-1, P6-2, P8-1, and P8-2) and hydrogenation of which gave the final saturated polymers. The expected periodic copolymers have been obtained and were characterized with a variety of methods, indicating that the γ-butyrolactone units could endure the polymerization and hydrogenation. Polymers catalyzed by Grubbs II catalyst suffer from chain heterogeneity due to severe olefin isomerization. Thermal properties of the polymers were investigated via thermal gravimetric analysis (TGA) and differential scanning calorimetry (DSC). TGA measurements show that these polymers are stable up to 350 °C. DSC results demonstrate that the glass transition and melting behaviors of the polymers are not only affected by the rigidity of γ-butyrolactone units in polyethylene chains but also dependent on the methylene spacer length and chain homogeneity. Copolymerization of M6 or M8 with 1,9-decadiene resulted in random copolymers with lower γ-butyrolactone content and less regular chain structure. These copolymers exhibit lower Tg or Tm compared with the periodic copolymers.

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INTRODUCTION Copolymers of vinyl monomers are widely used as one family of important synthetic materials. Both the composition and sequence of the repeating units along the polymer main chain affect their properties. Copolymerization is a general way to produce these polymers. With the development of a variety of controlled/ “living” polymerization methods, control over molecular weight, composition, topology, and functionality of copolymers has been achieved.1,2 However, for most copolymerizations, due to the large difference in reactivities of monomers, precise sequence control in the formed polymers is challengeable even for copolymerization of two monomers. Gradient copolymers obtained by controlled polymerization can only be considered as roughly sequence-regulated copolymers, but not well-defined structures.3−6 Alternating copolymers from © 2012 American Chemical Society

certain monomer pairs represent some special cases with the simplest monomer sequence.7−11 Synthesis of sequence-regulated copolymers from three or more monomers by direct copolymerization is generally impossible. Periodic copolymer represents another kind of multisegmented copolymer with a precise arrangement of sequenced units in each segment. They are usually synthesized by polymerization of tailor-made monomers with built-in sequence by taking advantage of wide accessibility of organic reactions.12−14 Pioneered by Yokota,12 various unprecedented alternating and periodic vinyl copolymers have been synthesized. These materials exhibit quite different thermal Received: February 20, 2012 Revised: May 15, 2012 Published: May 25, 2012 4590

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Scheme 1. ADMET Polymerization of Structurally Symmetric Diene Monomers Containing Two γ-Butyrolactone Units

properties and crystallinity behaviors from the corresponding statistical counterparts, indicating that monomer sequence plays a significant role in elucidating structure−property relationship of synthetic polymers. Later on, Cho summarized the synthesis of vinyl copolymers having controlled microstructures by ringopening polymerizations.13 Nevertheless, synthesis of these special cyclic monomers is rather time-consuming. Research on sequence-regulated polymers has recently become a popular field in polymer science, mainly due to the development in polymerization and characterization methods, as well as the elucidation of unprecedented property of sequenceregulated single polymer chain.15−18 A large number of recent contributions have already shed light on this exciting but not yet fully exploited research area.19−29 Among them, synthesis of carbon chain polymers with sequenced microstructures is the most challenging, and synthetic wisdom has already been reflected in some of the elegantly designed chain and stepwise processes.19−29 However, rather limited monomers can be selected for perfect sequence control via chain-growth polymerization. Acyclic diene metathesis polymerization (ADMET) has been proved to be a powerful and convenient method for synthesizing polyolefins via step-growth mechanism.30 The polymerization can tolerate polar groups using ruthenium-based catalysts (best known as Grubbs catalysts); therefore, many functionalized polyethylenes with well-defined microstructures and advanced functions have been synthesized through rational design of diene monomers.31−34 Among them, copolymers of ethylene with other vinyl monomers represent a series of precision polyethylenes with functional groups separated by aliphatic carbon chains of defined length.31,32,35−41 However, in most cases, only a single functional entity is introduced into the middle of the diene monomer structure, while more complex sequence has rarely been reported. We hypothesized that ADMET of diene monomers containing rigid cyclic structure would yield new types of periodic copolymer with good thermal stability and high crystallinity. In order to obtain these periodic polymers and study their structure−property relationship, we designed and synthesized two new diene monomers (M6 and M8) and studied their ADMET polymerization using two types of Grubbs catalysts. The obtained periodic copolymers contain two γ-butyrolactone moieties in each repeating unit, but they are different in methylene spacer length connecting the two cyclic moieties. Furthermore, copolymerization of those two monomers with 1,9-decadiene resulted in random copolymers with lower γ-butyrolactone content and less regular chain structure compared with the periodic copolymers. We demonstrated the effects of the cyclic structure, methylene

spacer length, and polymer chain homogeneity on the thermal properties of this family of polymers (Scheme 1).

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EXPERIMENTAL SECTION

Materials. 4-Methylbenzenesulfonhydrazide (TsNHNH2, 97%) was purchased from Acros. Triethylborane solution (BEt3, 1.0 M in hexanes) and Grubbs catalysts (first and second generations) were obtained from Aldrich. Tripropylamine (98%), ethyl vinyl ether (stabilized with 0.1% N,N-diethylaniline), and diethyl meso-2,5dibromohexanedioate (98%) were used as received from Alfa Aesar. Sodium iodide (NaI, 99.0%) was purchased from Tianjin Xuan’ang Sci. Ind. & Trade Co. Ltd. 1,5-Hexadiene (97%, Aldrich) and 1,7octadiene (98%, Aldrich) were dried over sodium under vigorous stirring and then vacuum-distilled before use. Other chemicals were all purchased from Beijing Chem. Reagent Co. and used as received unless otherwise noted. Dimethylformamide (DMF) and dichloromethane (CH2Cl2) were distilled over calcium hydride before use. Tetrahydrofuran (THF) was refluxed with sodium for 8 h and then redistilled. Acetone was refluxed with potassium permanganate for 8 h, redistilled, and stored over anhydrous sodium sulfate. Measurements. The number-average molecular weights (Mn) and polydispersity indices (PDI) of polymers were measured using a Polymer Laboratories gel permeation chromatograph (PL GPC50) equipped with three PLgel mixed-C columns. Chloroform was used as the mobile phase (40 °C, 1.0 mL/min), and the GPC was calibrated with polystyrene standard. 1H NMR (400 or 300 MHz) and 13C NMR (100.5 or 75 MHz) spectra were recorded on a Bruker ARX-400 spectrometer or a Varian Gemini 300 MHz spectrometer in CDCl3 with tetramethylsilane (TMS) as the internal reference for chemical shifts. Infrared spectra were recorded on a Bruker Vector-22 Fourier transform infrared spectrometer with potassium bromide (KBr) as dispersing medium, and OPUS/IR software was applied to manipulate the IR spectra. Thermal gravimetric analysis (TGA) was carried out using a Q600-SDT thermogravimetric analyzer (TA Co. Ltd.) with nitrogen purging rate set at 50 mL/min. Measurements were conducted from 50 to 600 °C at a heating rate of 10 °C/min. Calorimetric measurement was performed using a Q100 differential scanning calorimeter (TA Co. Ltd.) with nitrogen purging rate set at 50 mL/min. The program was set to finish two cycles in a temperature range of −50 to 200 °C at a heating/cooling rate of 10 °C/min. Data of endothermic curve were acquired from the second scan for each polymer sample. TA Universal Analysis software was applied for data acquisition and processing in TGA and DSC measurements. Electrospray ionization mass spectroscopy (ESI-MS) characterizations of monomers were performed using a Bruker APEX-IV Fourier transform mass spectrometer (positive ion mode). Matrix-assisted laser desorption/ionization time-offight mass spectroscopy (MALDI-TOF-MS) characterizations of polymers were conducted on a Bruker BIFLEX-III MALDI-TOF mass spectrometer (linear mode). Synthesis of Diethyl meso-2,5-Diiodohexanedioate.50,51 NaI (10.0 g, 66.7 mmol) was weighed and transferred into a 100 mL round-bottom flask, and 30 mL of acetone was added. The 4591

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heterogeneous mixture was heated to reflux for 5 min and then cooled down to room temperature. Diethyl meso-2, 5-dibromohexanedioate (5.0 g, 13.8 mmol) was added, and the mixture was heated to reflux for 24 h. The reaction was stopped upon cooling to room temperature and exposure to air. After the inorganic salts were filtered, the dark red organic layer was rotary evaporated. The obtained dark red solid was purified by silica chromatography separation to remove the iodine (petroleum ether/ethyl acetate = 10/1). The amorphous white solid was recrystallized from petroleum ether to obtain a white crystal. Yield: 75%. 1H NMR (400 MHz, CDCl3), δ (TMS, ppm): 4.26 (m, 6H), 2.00 (m, 4H), 1.29 (t, 6H). 13C NMR (100 MHz, CDCl3): δ 170.34, 61.61, 35.21, 18.66, 13.56. Anal. Calcd for C10H16O4I2: C, 26.45; H, 3.55. Found: C, 26.54; H, 3.55. Synthesis of preM6 and preM8. Diethyl meso-2,5-diiodohexanedioate (3.632 g, 8 mmol) was weighed and transferred into a 50 mL round-bottom flask. Then, 1,5-hexadiene (6.56 g, 80 mmol) and CH2Cl2 (4 mL) were sequentially added to the flask. The flask was sealed with a glass stopper, and the mixture was vigorously stirred at room temperature until a homogeneous solution was formed. BEt3 solution (1.6 mL, 1 M in hexane) was added dropwise with syringe. The mixture was stirred for 48 h, during which the stopper was removed and 10 mL of air was purged with a syringe every 4 h. The reaction was stopped and rotary evaporated to remove CH2Cl2 and excess 1,5-hexadiene. The viscous opaque oil was purified through silica chromatography separation (petroleum ether/ethyl acetate = 10/1) to obtain a viscous colorless oil. Yield: 75%. 1H NMR (400 MHz, CDCl3), δ (TMS, ppm): 5.76 (m, 2H), 5.04 (m, 4H), 4.16 (q, 4H), 3.95 (m, 2H), 2.72 (m, 2H), 1.37−2.34 (m, 16H), 1.27 (t, 6H). 13C NMR (100 MHz, CDCl3): δ 174.35, 136.34, 115.60, 60.27, 45.23, 44.68, 42.60, 41.93, 39.93, 38.62, 35.88, 34.11, 33.23, 29.62, 28.89, 28.24, 27.79, 14.13. Anal. Calcd for C22H36O4I2: C, 42.73; H, 5.87. Found: C, 42.86; H, 5.89. Synthesis of preM8 was conducted in a similar approach with 1,7octadiene as the starting material. Yield: 75%. 1H NMR (400 MHz, CDCl3), δ (TMS, ppm): 5.80 (m, 2H), 4.98 (m, 4H), 4.16 (q, 4H), 3.95 (m, 2H), 2.74 (m, 2H), 1.31−2.34 (m, 24H), 1.27 (t, 6H). 13C NMR (100 MHz, CDCl3): δ 174.00, 137.92, 114.26, 59.96, 44.77, 44.43, 42.05, 40.70, 39.31, 36.46, 34.67, 33.06, 29.44, 28.20, 27.79, 27.54, 13.98. Anal. Calcd for C26H44O4I2: C, 46.30; H, 6.58. Found: C, 46.44; H, 6.60. Synthesis of M6 and M8. In a magnetic bar and an adaptor equipped 100 mL round-bottom flask, compound preM6 (3.0 g) was added. The flask was placed in an oil bath set at 130 °C and stirred vigorously overnight. The adaptor was connected to a vacuum circulating water pump during the reaction. The reaction was stopped upon cooling to room temperature. A dark brown solid was obtained. The crude product was purified by silica chromatography separation (petroleum ether/ethyl acetate = 10/4 to 10/6). Subsequently, the amorphous white solid was recrystallized from ethyl ether. Yield: 65%. Tm1 = 42.7 °C, Tm2 = 84.3 °C. 1H NMR (400 MHz, CDCl3), δ (TMS, ppm): 5.80 (m, 2H), 5.02 (m, 4H), 4.44 (m, 2H), 1.37−2.83 (m, 18H). 13 C NMR (100 MHz, CDCl3): δ 178.50, 178.05, 136.81, 115.46, 77.95, 40.84, 40.41, 39.00, 38.51, 34.95, 34.52, 33.02, 29.30, 28.67, 28.22, 27.97, 27.44. Anal. Calcd for C18H26O4: C, 70.56; H, 8.55. Found: C, 70.28; H, 8.52. ESI-MS for M6: [M + H+] = C18H27O4, calcd: 307.190 39; found: 307.190 44. Monomer M8 was synthesized in a similar approach from preM8. Yield: 65%. Tm1 = 53.2 °C, Tm2 = 79.6 °C. 1H NMR (400 MHz, CDCl3), δ (TMS, ppm): 5.79 (m, 2H), 4.98 (m, 4H), 4.40 (m, 2H), 1.29−2.86 (m, 26H). 13C NMR (100 MHz, CDCl3): δ 178.70, 178.26, 138.35, 114.59, 78.76, 40.98, 40.52, 39.16, 38.63, 35.21, 34.82, 33.21, 28.81, 28.39, 28.04, 27.50, 24.64. Anal. Calcd for C22H34O4: C, 72.89; H, 9.45. Found: C, 72.60; H, 9.42. ESI-MS for M8: [M + H+] = C22H35O4, calcd: 363.252 99; found: 363.252 98. Polymerization. Take the synthesis of P6-1 as an example. Monomer M6 (153.0 mg, 0.5 mmol) and Grubbs I catalyst (4.1 mg, 5 μmol) were weighed and transferred sequentially into a 25 mL Schlenk flask equipped with a Teflon valve. Then, CH2Cl2 (1.0 mL, degassed by sonication prior to use) was added to dissolve the mixture to obtain a purple solution. The flask was connected to a reflux

condenser which was equipped with an anhydrous CaCl2 loaded drying tube. Then the flask was placed in an oil bath set at 40 °C. Nitrogen purging was applied through the Teflon valve during the polymerization. As the polymerization proceeded, CH2Cl2 was partially evaporated and the reaction mixture became solidified after 12 h as the magnetic bar was unable to stir. Polymerization was quenched upon stopping nitrogen purging, and then 5 mL of chloroform and excess of ethyl vinyl ether were added. The solidified polymeric materials were dissolved, and the magnetic bar was able to stir again as the temperature was raised to the boiling point of chloroform. The concentrated brownish polymer solution was poured into l00 mL of cold methanol to afford precipitated white solid. After centrifugation and vacuum dryness, P6-1 was recovered in 90% yield. P6-2, P8-1, and P8-2 were synthesized in a similar way by ADMET of the corresponding monomers. Hydrogenation. Take the synthesis of hyP6-1 as an example. Polymer P6-1 (120.0 mg), TsNHNH2 (613.8 mg, 3.3 mmol), and tripropylamine (572.0 mg, 4.0 mmol) were weighed and transferred into a 10 mL round-bottom flask. Then, DMF (2 mL) was added to obtain a heterogeneous mixture. The flask was connected to a reflux condenser which was equipped with an anhydrous CaCl2 loaded drying tube. The flask was placed in an oil bath set at 130 °C under vigorous stirring. After 12 h, the reaction was stopped upon cooling to room temperature to obtain a dark red solution with white solid. The mixture was vacuum evaporated to remove DMF, and then 1 mL of chloroform was added to dissolve the crude hydrogenated product. The dark red solution was poured into 100 mL of cold methanol to obtain a white solid. After filtration, the polymer was redissolved in 1 mL of chloroform and poured into 100 mL of acidic cold methanol. After filtration and vacuum dryness, hyP6-1 was obtained in quantitative yield. Other polymer samples, hyP6-2, hyP8-1 and hyP8-2, were synthesized in a similar way from the corresponding polymer precursors.

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RESULTS AND DISCUSSION Monomer Synthesis. Our original idea was to synthesize a diene monomer with a defined ethyl acrylate and vinyl bromide sequence by performing atom transfer radical addition (ATRA) reaction between diethyl meso-2,5-dibromohexanedioate and excess 1,5-hexadiene.42 Subsequent ADMET polymerization of this monomer followed by hydrogenation will incorporate the vinyl monomer sequence into polyethylene chain. The addition reaction was successful to give a crude product as yellowish oil in about 80% yield. Interestingly, when trying to purify the ATRA crude product by heating under reduced pressure, we obtained a white solid in about 20% yield. NMR and IR spectra revealed that this compound was a new diene monomer containing two γ-butyrolactone moieties (data not shown). We then surveyed literature and realized that γ-halo ester has a high tendency to form the corresponding γ-butyrolactone in an intramolecular nucleophilic substitution fashion.43−46 Literature results also indicated that γ-iodo ester could form γ-butyrolactone under much ambient conditions with higher yield.45−49 Motivated by using this new monomer to obtain a family of polyethylene derivatives containing cyclic γ-butyrolactone units, we used diethyl meso-2,5-diiodohexanedioate to synthesize this kind of monomer. This compound can be obtained in good yield (75%) by iodination of diethyl meso-2,5-dibromohexanedioate in acetone at reflux temperature for 24 h (Figures S1 and S2).50,51 Subsequently, we synthesized preM6 and preM8 via BEt3-O2catalyzed iodine transfer addition (ITA) of diethyl meso-2,5diiodohexanedioate with 1,5-hexadiene and 1,7-octadiene, respectively (Scheme 2).47−49 The reaction was performed in THF at room temperature for 48 h, during which air was intermittently purged in with a syringe. Although side reactions of radicals 4592

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Scheme 2. Monomer Synthesisa

Reagents and conditions: (a) NaI, acetone, 60 °C, 24 h; (b) 10 equiv, 1,5-hexadiene or 1,7-octadiene, BEt3, air, THF, rt, 48 h; (c) bulk, reduced pressure, 130 °C, 12 h. a

Figure 1. 1H and 13C NMR spectra of M6 (A) and M8 (B) in CDCl3.

Scheme 3. ADMET Polymerization and Hydrogenationa

a

Reagents and conditions: (a) 1.0 mol % Grubbs catalyst, CH2Cl2, N2 purge, 40 °C, 12 h; (b) TsNHNH2, NPr3, DMF, 130 °C, 12 h.

were confirmed by elemental analysis, ESI-MS and 1H, 13C NMR spectra (Figure 1). ADMET Polymerization and Hydrogenation. ADMET polymerization of M6 and M8 were performed using either 1.0 mol % Grubbs I or Grubbs II catalyst in CH2Cl2 at 40 °C under continuous nitrogen purge (a technique applied to remove the generated ethylene).34 The corresponding polymeric products

such as coupling, disproportionation, hydrogen abstraction, and cyclization may occur during the ITA reaction, pure product was obtained after chromatography separation as viscous colorless oil in good yield (75%). The structures were confirmed by NMR spectra (Figures S3−S6). Finally, lactonization was performed in bulk at 130 °C under reduced pressure to give two ADMET monomers M6 and M8 in good yield (65%). Their structures 4593

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Table 1. Molecular Weight and Thermal Data for Unsaturated and Saturated Polymers polymer

Mna

P6-1 P6-2 P8-1 P8-2 hyP6-1 hyP6-2 hyP8-1 hyP8-2

4200 6000 9200 7900 5000 8400 12900 10900

Mnb

Mwa

Mw/Mna

Tgc (°C)

Tmc (°C)

ΔHmc (J/g)

Tdd (°C)

3900 5800 9200 8400

7300 11800 21200 22600 7800 12300 25300 24000

1.75 1.97 2.31 2.87 1.55 1.46 1.97 2.21

29.3 43.9 n.d. 24.7 n.d. 37.9 n.d. n.d.

n.d. n.d. 106.8 n.d. 131.6 n.d. 146.3 99.8

n.d. n.d. 34.2 n.d. 41.2 n.d. 44.4 29.3

307 302 312 309 352 356 353 358

a

Determined via GPC (1 mL/min in CHCl3) using polystyrene calibration. bDetermined via 1H NMR (400 MHz in CDCl3) spectrum according to end-group analysis. DP of polymer is calculated using peak area integration ratio of signals assigned to γ-methine on the γ-butyrolactone unit and terminal methyl groups, respectively. Mn (by NMR) = DP × repeating unit mass. cDetermined via DSC, 10 °C/min scan rate, values determined from second scan data. dOnset decomposition, determined via TGA, 10 °C/min scan rate.

The data also suggests that polymers derived from M8 possess higher molecular weight than those from M6 though suffer from higher polydispersities (up to 2.87 for P8-2). In general, higher molecular weight polymers are anticipated via ADMET polymerization using Grubbs II catalyst as in the case of P6-2 compared with P6-1. However, P8-2 possesses lower Mn but higher Mw and PDI than P8-1, indicating that more fractions of low molecular weight components exist in the polymer sample. In order to obtain high molecular weight polymers, we carried out ADMET polymerization of M6 using Grubbs I catalyst in high boiling point solvents such as toluene and trichlorobenzene (TCB). In both solvents, precipitation of polymers was observed after 2 h. GPC and NMR measurements confirmed that only oligomers of M6 were obtained (Figures S8−S10). We attributed this to the low solubility of the oligomers in these two solvents, thus limiting the formation of high molecular weight polymers. We also measured the FT-IR spectrum of each sample, and one typical spectrum of P6-1 is shown in Figure 2B while other spectra are shown in Figure S11. The characteristic peak of γ-butyrolactone unit (1763 cm−1) is clearly resolved in the polymer, though it shifts 8 cm−1 to long wavenumber as compared to that in the monomer (1755 cm−1). In addition, two peaks at 910 and 3078 cm−1 in the monomer assigned to C−H planar vibration and CC stretching of α-olefin moved to 970 cm−1 assigned to the C−H out-of-plane bending of 1,2disubstituted olefin of the unsaturated polymers (asterisk labeled peak). This result confirms that γ-butyrolactone can endure the polymerization condition to form polymers with γ-butyrolactone units in the polymer main chain. The polymers were further characterized by NMR. As an example, Figure 3 shows the 1H and 13C NMR spectra of P6-1 and P6-2. Compared with M6, the polymer samples exhibit signals of internal alkenes (5.4 ppm). The cis−trans ratio of the internal alkenes was calculated. The peak at chemical shift higher than 5.44 ppm is assigned to E-type olefins, while that lower than 5.44 ppm is assigned to Z-type olefins; thus, the cis− trans ratio is calculated via comparing these two peak integration ratios. The results are summarized in the notes of Figure 3 and Figure S12, respectively. Signals of terminal alkenes substantially decrease for P6-1 and completely disappear for P6-2. Theoretically, terminal olefin groups (5.0 and 5.7 ppm) should be maintained at any time during ADMET polymerization; therefore, Grubbs II catalyst may exhibit different catalytic behavior. Since olefin isomerization and subsequent interchain exchange intrinsically occur while performing ADMET with Grubbs II catalyst,53−56 this result is

were designated as P6-1, P6-2, P8-1, and P8-2, respectively. Then these polymers were hydrogenated to give the final polymers: hyP6-1, hyP6-2, hyP8-1, and hyP8-2 (Scheme 3). ADMET polymerization follows a step-growth mechanism, and high molecular weight polymers can only be obtained after a long polymerization time. Initially, we conducted the polymerization of these two monomers for 5 days, during which solid products were precipitated from the polymerization mixture due to low solubility of the polymers in dichloromethane. We then isolated the polymers by filtration and tried to redissolve them in common solvents like toluene, CHCl3, THF, and DMF; however, we found that these solvents could hardly dissolve the precipitated polymers even when heated to the boiling points of these solvents. Therefore, in this study, all the polymerizations were stopped after 12 h. In this case, the obtained polymers could dissolve in chloroform. Typical exhaustive hydrogenation conditions were applied to the aforementioned four polymer samples using tri-n-propylamine and 4-methylbenzenesulfonhydrazide (TsNHNH2) in DMF at 130 °C.52 Since hydrogenation should be performed at high temperature (TsNHNH2 decomposes above 110 °C), DMF was selected as the solvent. Although the reaction mixture was heterogeneous as polymers could not be completely dissolved, double-bond conversion exceeding 99% was accomplished using this method. All the data are summarized in Table 1. We first measured the molecular weights of these polymers by GPC with chloroform as the eluent. Figure 2A shows the

Figure 2. (A) GPC traces of P6-1 and hyP6-1. (B) FT-IR spectra of M6, P6-1, and hyP6-1.

typical GPC profile of P6-1, a monopeak was observed, and the Mn was 4200 with a PDI of 1.75 as against polystyrene standard. Other polymer samples give similar GPC traces (Figure S7). Data in Table 1 indicate that moderate to high molecular weight polymers have been obtained in spite of the relatively high PDI values due to step-growth mechanism. 4594

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consistent with our expectation. For examples involving metathesis reaction such as ring-closing metathesis (RCM), benzoquinone has been proved to be effective to prohibit olefin isomerization while the production yield is not severely affected.57−61 Therefore, in order to reduce the extent of olefin isomerization, we deliberately performed ADMET polymerization of M8 using Grubbs II catalyst with 2.0 mol % additive

such as benzoquinone, 4,5-dichloro-3,6-dioxocyclohexa-1,4diene-1,2-dicarbonitrile (DDQ), acetic acid, and maleic anhydride. As the polymerization proceeded, we observed that the bubbling rate of the reaction systems was much slower than the ones without additives. GPC analysis shown in Figure S13 clearly indicates that only very low molecular weight oligomers of M8 have been obtained. These results indicate that the four additives severely affect the catalytic reactivity of Grubbs II catalyst, thus prohibiting molecular weight increase. Accordingly, in this work, the polymerization was carried out without additives. In the 1 H NMR of P6-2, besides the normal internal alkenes at 5.4 ppm, alkene signals ranging from 5.5 to 5.7 ppm, were observed. These peaks can be assigned to the A-type olefin formed by isomerization (see Figure 3), counting 31% of the total olefins. The NMR spectra of P8-1 and P8-2 are shown in Figure S12. The spectra are in general the same as those of P6-1 and P6-2 and are consistent with the expected structures. However, in the case of P8-2, the ratio of A-type olefin sharply decreases to only 2% of the total olefins. We speculate that the inevitable olefin isomerization should occur within a limited length of carbon chains, so that double bond in P6-1 has more opportunities to form the A-type olefin. However, other possible chemical structure of olefins such as B-type olefin could not be identified in any of these polymer samples. Since γ-butyrolactone is a rigid five-membered ring structure holding thermodynamically high stability and specific configuration, formation of B-type olefin may be energetically unfavorable. The four polymers were hydrogenated under the abovementioned conditions to yield the final polymers. They show slightly higher Mn and lower PDI than the precursors (Figure 2, Figure S7, and Table 1). We attributed this to the additional precipitation when purifying the polymers.62 IR spectra (Figure S11)

Figure 3. (a) 1H NMR and (b) 13C NMR spectra of P6-1 and P6-2, respectively (solvent: CDCl3, peaks of CDCl3 are cut off in 13C NMR spectra). Cis/trans ratios of P6-1 and P6-2 are 36/64 and 33/67, respectively.

Figure 4. (A) 1H NMR and (B) 13C NMR spectrum of hyP6-1 (solvent: CDCl3, peaks of CDCl3 are cut off in 13C NMR spectrum), respectively; MALDI-TOF-MS spectra of hyP6-1 (C) and hyP6-2 (D). 4595

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Figure 5. TGA curves of M6 (A), P6-1 (B), and hyP6-1 (C).

Figure 6. Endothermic DSC curves of eight polymer samples.

lengths of methylene spacers in the backbone as in the case of hyP8-2. Thermal Properties. Thermal properties of two monomers and eight polymers were systematically investigated with TGA and DSC (Figures 5 and 6, Figures S16 and S17), and the data are collected in Table 1. The TGA curve of M6 exhibits a twostage decomposition ascribed to pyrolysis of α-olefin containing carbon chain and γ-butyrolactone, respectively. However, for P6-1, the TGA curve shows only a slight weight loss below 350 °C, suggesting the terminal alkenes pyrolysis. Above 350 °C, the polymer suffered from catastrophic decomposition and it completely decomposed at about 465 °C. Other polymers behave similarly. After hydrogenation, the thermal stability of all the polymers further increased. The onset weight loss occurred above 352 °C, exceeding the onset Td value of high density polyethylene (about 300 °C). Importantly, the Td values of these polymers are even higher than those of the previously reported precision polyethylene with a single flexible functional group placed on a specific position of the repeating unit, suggesting that the rigid γ-butyrolactone unit derived from the specific sequence of vinyl iodide−ethyl acrylate plays a critical role in substantially enhancing the thermal stability of the synthesized polymers. Glass transition and melting behaviors of these polymers were measured by DSC, and the curves are shown in Figure 6. Since functional group incorporation and olefin isomerization prohibit regular packing of polyethylene chains, only hyP6-1, P8-1, hyP8-1, and hyP8-2 exhibit semicrystallinity with Tms ranging from 100 to 146 °C. P6-1, P6-2, hyP6-2, and P8-2 are amorphous polymers, exhibiting Tgs in a temperature range from 25 to 44 °C (Table 1). These Tg values are much higher than that of polyethylene (commonly considered not exceeding −20 °C).63,64 Again, rigid γ-butyrolactone unit in the polymer main chain may account for such an increase. Before hydrogenation, P8-1 is the only polymer exhibiting a melting peak, which is due to long and relatively regular methylene spacer.

show complete disappearance of the alkene groups, indicating quantitative hydrogenation of the main-chain double bonds. The 1H and 13C NMR spectra of hyP6-1 are shown in Figure 4 (spectra of other hydrogenated polymer samples are shown in Figure S14). Signals of the double bonds disappear, but signals of terminal methyl groups can be identified in both spectra. Besides that, the maintenance of signals of γ-butyrolactones and aliphatic carbon chains indicates that the exhaustive hydrogenation has been successfully conducted without affecting the polymer backbone. However, information on olefin isomerization disappears after hydrogenation. On the basis of these NMR spectra, we calculated the molecular weight of the hydrogenated polymers by end-group analysis (see Table 1), and the results are in good agreement with the GPC determined values. The fine structures of the polymers were characterized by MALDI-TOF-MS, and the spectra of hyP6-1 and hyP6-2 are shown in Figure 4. A family of peaks with a regular interval of 280 (theoretical repeating unit mass) indicates the integrity of the polymer chain structures. However, the spectra also exhibit minor series with peak mass shifted from the corresponding major series by the mass of 14 × n, implying that different length of methylene spacers exist in the polymer structures. This phenomenon further verifies olefin isomerization during ADMET polymerization, which is more pronounced in hyP6-2. For polymers hyP8-1 and hyP8-2, the MS spectra become even more distributed (Figure S15), especially for hyP8-2. The results are reasonable considering that longer carbon chain between two terminal alkenes in monomer M8 provides more possibilities to form different isomerized olefin structures.53−56 Thus, Grubbs II catalyst exhibits higher catalytic activity for the ADMET polymerization of the two monomers to yield higher molecular weight polymers, but it will cause isomerization during polymerization, yielding polymers with chain heterogeneity. Increasing the length of methylene spacer between terminal alkene and γ-butyrolactone unit of monomer will cause more isomerization, generating polymers with irregular 4596

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polymerization of two new symmetric diene monomers. The unique sequence of vinyl iodide−ethyl acrylate resulted from the iodine transfer radical addition of diethyl meso-2,5diiodohexanedioate, and a diene was effectively transformed into the corresponding γ-butyrolactone structure. ADMET polymerization and subsequent exhaustive hydrogenation were successfully conducted to obtain polyethylenes containing main chain γ-butyrolactone structure. Grubbs II catalyst is more efficient for ADMET polymerization but causes severe olefin isomerization. In this respect, Grubbs I catalyst should be a better choice for sequence-engineered ADMET polymerization. Thermal properties of this family of carbon chain polymers are dependent on the methylene spacer length and chain homogeneity. In general, longer and more uniform methylene spacer in the polymer main chain is a benefit for crystalline regions formation. Incorporation of rigid cyclic γ-butyrolactone structure in a periodic order increases the stiffness of polymer main chain. As a consequence, these polymers exhibit higher Tg, Tm, and Td than those of polyethylene. Random copolymerization of M6 or M8 with 1,9-decadiene induces the loss of periodicity, thus restricting polymer chain regular packing. By taking advantage of ATRA reaction in synthesizing diene monomers, we are currently working on more complicated sequence-regulated carbon chain polymers via ADMET polymerization.

This indicates that methylene spacer length and regularity are both necessary for unsaturated samples to form crystalline regions. As discussed above, the length of methylene spacer is longer in M8 than in M6, and the polymerization with Grubbs I catalyst causes only slight isomerization. However, for the saturated polymer samples, hyP6-2 is the only one without obvious melting peak due to short and severely isomerized methylene spacer, suggesting that either extending the length of methylene spacer or diminishing the extent of olefin isomerization will benefit saturated samples forming crystalline regions. Moreover, since hydrogenation transforms the internal alkenes to saturated aliphatic carbon chain thus promotes crystallinity, the Tm value of hyP8-1 (146.3 °C) is much higher than that of P8-1 (106.8 °C), consistent with the results described in the literature.38 For the two saturated samples with minor isomerization, hyP6-1 and hyP8-1, the Tm values of the two polymers are comparable to those of the previously reported high-density polyethylenes, but the ΔHm values are much lower, implying that incorporation of γ-butyrolactone may severely hinder crystallinity of aliphatic carbon chains (and vice versa). Overall, saturated polymer samples all exhibit good thermal stability with the onset Td values exceeding 350 °C due to rigid γ-butyrolactone structure incorporation. For the same reason, all the amorphous polymer samples exhibit Tgs higher than polyethylene. On the other hand, using M8 and Grubbs I catalyst for ADMET polymerization will extend methylene spacer length and increase methylene spacer regularity, respectively, thus benefiting the formation of crystalline regions. To further understand the effects of periodic sequence and the content of rigid γ-butyrolactone units on the thermal properties of polyethylene derivatives, we carried out copolymerization of M6 or M8 with 1,9-decadiene using Grubbs I catalyst in three different molar ratios (25/75, 50/50, and 75/25). GPC measurements revealed formation of copolymers with Mn ranging from 5000 to 7500 (Figure S18 and Table S1). By comparing the peak area integration of the methine protons at 4.4 ppm to the internal alkene protons at 5.3 ppm in NMR spectra (Figure S19), the compositions of these copolymers were calculated. The molar ratios of the two monomers in the copolymers are very close to the feed ratios, implying random copolymerization. After hydrogenation, saturated copolymers were obtained. The thermal properties of these four series of random copolymers were also measured (Figures S20−S22 and Table S1). All the copolymers show high thermal stability up to 350 °C. Copolymers from monomer M6 and 1,9-decadiene are amorphous, and all the others are semicrystalline. In general, either the glass transition temperatures or the melting temperatures increase with increasing the incorporation ratio of M6 or M8, and the saturated copolymer shows higher melting temperature than the unsaturated precursor. In addition, the melting temperatures of all the copolymers are lower than those of homopolymers of either M6 or M8, hydrogenation further increase the melting temperature of each sample. These results further confirmed the importance of rigid γ-butyrolactone units and chain regularity on the polymer properties. Copolymerization has two effects on the polymer chain structure; it decreases the content of rigid γ-butyrolactone unit and destroys the periodic order, thus restricting polymer chain regular packing.

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ASSOCIATED CONTENT

S Supporting Information *

Supporting 1H NMR, 13C NMR spectra, and GPC traces of the polymers, TGA and DSC curves. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail [email protected]; Tel +86-10-6275-5543; Fax +86-106275-1708. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work is financially supported by National Natural Science Foundation of China (No. 21090351) and National Basic Research Program of China (No. 2011CB201402).

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REFERENCES

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CONCLUSIONS We demonstrated a simple strategy to synthesize periodic vinyl copolymers containing γ-butyrolactone units via ADMET 4597

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