Stereospecific Radical Polymerization of Fluoroalkyl Acrylates

Transcription

1 Stereospeifi Radial Polymerization of Fluoroalkyl Arylates WEIHONG LIU, 1 TAMAKI NAKANO, 2 YOSHIO OKAMOTO 3 1 Joint Researh Center for Preision Polymerization (JRCPP)-Japan Chemial Innovation Institute (JCII), Graduate Shool of Engineering, Nagoya University, Chikusa-ku, Nagoya , Japan 2 Graduate Shool of Materials Siene, Nara Institute of Siene and Tehnology, Ikoma , Nara , Japan 3 Department of Applied Chemistry, Graduate Shool of Engineering, Nagoya University, Chikusa-ku, Nagoya , Japan Reeived 27 Deember 1999; aepted 7 January 2000 ABSTRACT: The free-radial polymerization of 2,2,2-trifluoroethyl arylate (TFEA), 1,1,1,3,3,3-hexafluoro-2-propyl arylate (HFiPA) and perfluoro-tert-butyl arylate (PFtBA) was arried out under various onditions and the stereostruture of the obtained polymers was investigated. Most polymerizations of the three monomers afforded polymers rih in diad syndiotatiity (r) in bulk or in solution; the r-speifiity was higher in the HFiPA and PFtBA polymerization than in the TFEA polymerization. Although the tatiity was nearly independent of reation temperature during the polymerization of TFEA, the r-speifiity inreased by lowering the reation temperature during the polymerization of the other two monomers. The polymerization stereohemistry was also affeted by the reation solvents inluding toluene, tetrahydrofuran, and fluoroalohols. It was noted that the stereohemistry of the polymerization of HFiPA and PFtBA also depended on the monomer onentration, and a lower monomer onentration led to a higher r-speifiity. By optimizing the aforementioned reation onditions, the poly(hfipa) having r 81% (polymerization in tetrahydrofuran at 98 C at [M] o 0.2M) and the poly(pftba) having r 77% (polymerization in toluene at 78 C at [M] o 0.2M) were obtained John Wiley & Sons, In. J Polym Si A: Polym Chem 38: , 2000 Keywords: radial polymerization; tatiity; stereohemistry; onentration effet; fluoroalkyl; arylate; solvent effet INTRODUCTION Correspondene to: Y. Okamoto ( okamoto@aphem. nagoya-u.a.jp) Journal of Polymer Siene: Part A: Polymer Chemistry, Vol. 38, (2000) 2000 John Wiley & Sons, In. The stereoregularity of vinyl polymers often has signifiant effets on the properties of polymer materials. Therefore, it is important to devise stereoregulation methods for the polymerization reations. The ontrol of the stereohemistry (tatiity) during free-radial polymerization is espeially intriguing and hallenging beause there are only limited suessful examples in spite of the onveniene and industrial importane of radial polymerization. It has been reported that monomer design, reation onditions (solvent, temperature, and monomer onentration), and additives an alter the stereohemistry of the radial polymerization of aryli monomers. 1 However, the effets of these fators have been shown not to be as signifiant on arylate polymerization as on metharylate polymerization. Methyl and 1024

2 POLYMERIZATION OF FLUOROALKYL ACRYLATES 1025 ethyl arylates almost always produe atati polymers regardless of the reation onditions, 2,3 and only arylates bearing a bulky group suh as isopropyl and trimethylsilyl an afford a syndiotati-rih polymer at low temperature during radial polymerization. 3,4 In this work, we examined the stereohemistry of the free-radial polymerization of 2,2,2-trifluoroethyl arylate (TFEA), 1,1,1,3,3,3- hexafluoroisopropyl arylate (HFiPA), and perfluoro- tert-butyl arylate (PFtBA) in searh of the monomer strutures, whih an lead to stereoregular polyarylates via a radial mehanism. EXPERIMENTAL Materials TFEA (TCI) and HFiPA (TCI) were ommerially available and distilled immediately before use. PFtBA was prepared with perfluoro-tert-butyl alohol and aryloyl hloride aording to a literature method 5 and purified by distillation. CF 3 CH 2 OH (Aldrih; purity 99%), (CF 3 ) 2 CHOH (Wako; purity 99%), (CF 3 ) 3 C(CH 3 )OH (P.C.R.; purity 97%), and (CF 3 ) 3 COH (Aldrih; purity 99%) were used as reeived. The other solvents were purified by distillation immediately before use. 2, 2 -Azobisisobutyronitrile (AIBN) and benzoyl peroxide (BPO) were purified by rerystallization from methanol. Benzoin (TCI) was used as reeived. Tri-n-butylborane (nbu 3 B) was obtained as a tetrahydrofuran (THF) solution (1.0M) (Aldrih) and used after removal of the solvent. Polymerization Polymerization was arried out in a glass ampoule under a nitrogen atmosphere. Polymerization using nbu 3 B was onduted as follows: nbu 3 B was added with a hypodermi syringe to a monomer solution kept at reation temperature. The polymerization reation was initiated by introduing a small amount of air with a syringe. The reation was terminated by adding a small amount of methanol solution of 2,6-di-tert-butylp-resol. The polymerizations with AIBN, BPO, and benzoin were arried out using the usual methods. The obtained poly(pftba) was preipitated into methanol, and the poly(tfea) and poly(hfipa) into a methanol water mixture (4/1, v/v). The polymers were olleted by entrifuging and dried under vauum. Measurements The 1 H NMR spetrum was measured with a Varian Gemini 2000 spetrometer at 400 MHz. Size exlusion hromatographi (SEC) analysis was aomplished on a Shodex System-21 SEC system equipped with a UV (254 nm) and an RI detetor using Shodex KF-803 and KF-806L olumns onneted in series (eluent, THF). The moleular weight of the polymers was alibrated with standard polystyrenes. The degree of polymerization (DP) of poly(pftba) was measured as poly(methyl arylate) derived from the original polymers by hydrolysis in H 2 SO 4 /MeOH (1/1, v/v) at 90 C, followed by methylation with diazomethane. The DSC measurement was performed on a Seiko SSC-5200 alorimeter at a heating rate of 10 C/ min under a nitrogen atmosphere. RESULTS AND DISCUSSION Tatiity Determination by 1 HNMR The diad tatiity of polyarylates has been determined by 1 H NMR analysis. 2,6 Figure 1 shows the 1 H NMR spetra of poly(tfea), poly(hfipa), and poly(pftba) obtained in this work. Beause the poly(hfipa) and poly(pftba) were insoluble in hloroform, a mixture of hloroform-d and 1,1,2- trifluoro-1,2,2-trihloroethane was employed as the solvent for the NMR measurements. Aording to the previous assignments made for other polyarylates, 2,6 the peaks at 2.5 [poly(tfea)] or 2.7 [poly(hfipa) and poly(pftba)] and ppm were assigned to the main-hain methine and methylene, respetively. The methylene proton peaks are split into three parts beause of the mainhain stereohemistry, whih is assigned to two meso and one raemo signal as shown in the figure. The diad meso ontent was alulated from the peak area ratio of the downfield-meso signal to the

3 1026 LIU, NAKANO, AND OKAMOTO temperatures. The polymers rih in syndiotati diad (r) were obtained; the r-speifiity inreased with the bulkiness of the monomers. The stereohemistry was also affeted by the reation temperature for HFiPA and PFtBA, but no lear temperature effet was observed for TFEA. In the polymerization of the two former monomers, the r-speifiity slightly inreased by lowering the reation temperature. Figure MHz 1 H NMR spetra of polyarylates: poly(tfea) of run 4 in Table I. (CDCl 3, 60 C) (A); poly(hfipa) of run 8 in Table I, [CF 2 ClCFCl 2 -CDCl 3 (1/1, v/v), 50 C] (B); and poly(pftba) of run 6 in Table V, CF 2 ClCFCl 2 -CDCl 3 (6/1, v/v), 50 C] (C). methine signal or aording to the following equation: m% 2A 1 / A 1 A (1) where A 1 is the area of the downfield-meso signal and A 2 is the ombined area of the raemo and upfield-meso signals. Bulk Polymerization Table I shows the results of the bulk polymerization of TFEA, HFiPA, and PFtBA at different Polymerization in Various Solvents The solvent effet on the stereohemistry of the polymerizations of the three monomers was studied in toluene, tetrahydrofuran, 2-propanol, and fluoroalohols. Table II shows the results of the polymerization of TFEA in the various solvents. A lear solvent effet was found. Almost atati polymers with m/r 50/50 were obtained in THF, whereas all polymers produed in the other solvents were slightly rih in syndiotatiity. However, the r-ontents of the polymers obtained in the solvents other than (CF 3 ) 3 COH were similar to those of the polymers prepared in bulk. The r-speifiity was most enhaned by using (CF 3 ) 3 COH as the solvent. Bulky fluoroalohols inluding (CF 3 ) 3 COH have been reported to remarkably affet the stereohemistry of polymerizations of metharylates 7 and vinyl esters. 8 However, in this work, other bulky fluoroalohols, Table I. Free-Radial Bulk Polymerization of Fluoroalkyl Arylates by nbu 3 B-air a Temp. Yield b Monomer M n 10 4 ( C) (%) M w /M n m/r 1 e TFEA /55 2 TFEA /55 3 TFEA /55 4 TFEA /55 5 e HFiPA /60 6 HFiPA /62 7 HFiPA /65 8 HFiPA /66 9 e PFtBA DP /65 10 PFtBA 0 98 DP /67 11 PFtBA DP /69 12 PFtBA DP /69 a [nbu 3 B] 0 0.1M; [AIBN] 0 0.1M; time 5 h (runs 1 3, 5 7 and 9 11), 24 h (runs 4, 8 and 12). b MeOH-H 2 O (4/1, v/v)-insoluble part (runs 1 8); MeOH-insoluble part (runs 9 12). Determined by SEC with polystyrene standard. d Determined by 1 H NMR. e Initiator AIBN.

4 POLYMERIZATION OF FLUOROALKYL ACRYLATES 1027 Table II. Free-Radial Polymerization of TFEA by nbu 3 B-Air in Various Solvents a Temp. Yield b Solvent M n 10 4 ( C) (%) M w /M n m/r 1 THF /50 2 THF /51 3 THF /51 4 THF /51 5 e toluene /54 6 toluene /56 7 toluene /55 8 toluene /54 9 f (CH 3 ) 2 CHOH /53 10 (CH 3 ) 2 CHOH /52 11 CF 3 CH 2 OH /55 12 CF 3 CH 2 OH /53 13 (CF 3 ) 2 CHOH /56 14 (CF 3 ) 2 CH 3 COH /57 15 (CF 3 ) 3 COH /62 a [TFEA] 0 1.0M; [nbu 3 B] M; [AIBN] o 0.02M; time 24 h. b MeOH-H 2 O (4/1, v/v)-insoluble part. Determined by SEC with polystyrene standard. d Determined by 1 H NMR (400 MHz, CDCl 3, 60 C). e Initiator AIBN. f Initiation UV-AIBN. (CF 3 ) 2 CHOH and (CF 3 ) 3 CCH 3 OH, had a negligible r-enhaning effet ompared with (CF 3 ) 3 COH. The effet of (CF 3 ) 3 COH may be due to the hydrogen-bonding interation with the ester groups of the monomer and growing hain, whih makes these speies apparently bulkier. 7,8 A solvent effet was also observed during the polymerization of HFiPA (Table III). The polymerization in THF produed polymers with a lower moleular weight, and only oligomers were obtained at a high reation temperature possibly beause of the hain transfer to THF. The obtained polymers had poor solubility in toluene and hexane. Therefore, in these ases, the produts preipitated out from the polymerization systems. The polymerization in all the solvents produed a polymer with a higher syndiotatiity than that in bulk under the same temperatures. The polymerization in THF led to a higher r- speifiity than that in most of the other solvents. (CF 3 ) 3 COH also gave a polymer with an analogous r-ontent to that in THF and pyridine. In all the solvents tested, the r-speifiity was enhaned by lowering the reation temperature. Table IV shows the results of the polymerization of PFtBA in various solvents. The polymers were preipitated out during the polymerization in toluene, THF, and 2,2,2-trifluoroethanol, but the polymerization systems were homogeneous in (CF 3 ) 2 CHOH, (CF 3 ) 2 CCH 3 OH, and (CF 3 ) 3 COH. The polymerization in the solvents other than (CF 3 ) 3 COH afforded polymers with a higher r-ontent than that obtained in bulk. In (CF 3 ) 3 COH, unlike in the polymerization of less bulky TFEA, the r-speifiity dereased. A similar r-dereasing effet of (CF 3 ) 3 COH has also been observed during the polymerizations of bulky monomers suh as t-butyl metharylate 7 and vinyl pivalate. 8 As observed during the polymerizations of HFiPA, a higher r- speifiity was ahieved at a lower temperature for the polymerization of PFtBA in toluene. Effet of Monomer Conentration Although the monomer onentration generally does not affet the stereohemistry of radial polymerization, we reently found that the stereohemistry of the radial polymerization of triphenylmethyl metharylate was remarkably dependent on monomer onentration. 9 In this work, as shown in Table V, a marked monomer onentration effet on the stereohemistry was also found

5 1028 LIU, NAKANO, AND OKAMOTO Table III. Free-Radial Polymerization of HFiPA by nbu 3 B-Air in Various Solvents a Temp. Yield b Solvent M n 10 4 ( C) (%) M w /M n m/r 1 2 THF /70 2 THF /75 3 THF /76 4 THF /79 5 e toluene /64 6 toluene /67 7 toluene /68 8 toluene /70 9 hexane g /67 10 diethyl ether g /78 11 f pyridine /70 12 (CH 3 ) 2 CHOH g /66 13 (CH 3 ) 2 CHOH g /68 14 (CH 3 ) 2 CHOH g /69 15 CF 3 CH 2 OH /65 16 (CF 3 ) 2 CHOH /66 17 (CF 3 ) 2 CH 3 COH /65 18 (CF 3 ) 3 COH /71 a [HFiPA] 0 1.0M; [nbu 3 B] 0 0.1M; [AIBN] o 0.02M; [benzoin] M; time 24 h. b MeOH-H 2 O (4/1, v/v)-insoluble part. Determined by SEC with polystyrene standard. d Determined by 1 H NMR (400 MHz, 50 C) in CDCl 3 -CF 2 ClCFCl 2 (1/1, v/v). e Initiator AIBN. f Initiation UV-Benzoin. g Containing 10 vol % of THF. in the polymerization of HFiPA in THF. In the temperature range of 0 98 C, the r-speifiity was higher at a lower monomer onentration. This effet was more obvious at a lower temperature and an r-ontent of 81% was ahieved at [M] o 0.2M at 98 C. A similar onentration effet was also observed in the polymerization of PFtBA in toluene (runs 5 7 in Table IV). The r-ontent inreased from 70% to 77% with a derease in the monomer onentration from 4.0 to 0.2M at 78 C. The onentration effets may be explained by postulating the existene of onformers for the growing speies, whih have different stereospeifiities of monomer addition as in the ase of the triphenylmethyl metharylate polymerization. 9 The simplest assumption may be as follows. There are two interhangeable speies in the reation system: one has a higher r-speifiity and the other has a lower r-speifiity. The latter is initially formed upon the monomer addition but it hanges with a rate omparable to the monomer addition rate of the former as the former is thermodynamially more stable. Hene, at the lower monomer onentration where the interhange rate may be relatively higher than the monomer addition rate, a higher r-speifiity is attained. Beause the effet was obvious in the work only at a relatively low temperature, the energy barrier between the two speies may be rather low. Ativation Parameters for the Polymerization Using the results from the temperature dependene of tatiity so far desribed, the differenes in the ativation enthalpy ( H i H s ) and ativation entropy ( S i S s ) between the isotatispeifi and syndiotati-speifi propagations were estimated for the polymerizations in bulk, toluene, and THF. The parameters were determined by the plots of the stereohemistry data aording to the following equation: 10 ln P i /P s S i S s /R H i H s /RT (2) where P i and P s are mole frations of the isotati and syndiotati diads, respetively. R is the gas

6 POLYMERIZATION OF FLUOROALKYL ACRYLATES 1029 Table IV. Free-Radial Polymerization of PFtBA by nbu 3 B-Air under Various Conditions a [PFtBA] 0 Temp. Yield b Solvent DP (M) ( C) (%) M w /M n m/r 1 e toluene /65 2 f toluene /67 3 toluene /71 4 toluene /73 5 toluene /75 6 toluene /77 7 toluene /70 8 THF /71 9 CF 3 CH 2 OH /68 10 (CF 3 ) 2 CHOH /71 11 (CF 3 ) 2 CH 3 COH /71 12 (CF 3 ) 3 COH /56 a Conditions: [PFtBA] o /[nbu 3 B] o 10/1; [PFtBA] o /[AIBN or BPO] o 50/1; time 24 h. b MeOH-insoluble part. Determined as poly(methyl arylate) by SEC. d Determined by 1 H NMR (400 MHz) in CDCl 3 -CF 2 ClCFCl 2 (1/6, v/v). e Initiator BPO. f Initiator AIBN. onstant (1.987 al/mol K) and T is the absolute temperature. The data for the polymerizations in bulk, toluene, and THF are plotted aording to eq 2 as shown in Figures 2, 3, and 4, respetively. The ativation parameters are summarized in Table VI. Exept for the polymerization of TFEA, where no lear influene of temperature was onfirmed, negative S i S s and positive H i H s values were obtained. This indiates that the syndiotati-speifi propagation is favored by Table V. Effet of Monomer Conentration on the Stereohemistry of Polymerization of HFiPA in THF using nbu 3 B-Air as an Initiator a [M] o Temp. Yield b M n 10 4 (M) ( C) (%) M w /M n m/r / / / / / / / / / / /70 a [nbu 3 B] M (runs 1 4); 0.05M (run 5) and 0.1M (runs 6 13); time 24 h. b MeOH-H 2 O (4/1, v/v)-insoluble part. Determined by SEC with polystyrene standard. d Determined by 1 H NMR (400 MHz, 50 C) in CDCl 3 -CF 2 ClCFCl 2 (1/1, v/v).

7 1030 LIU, NAKANO, AND OKAMOTO Figure 2. Temperature effet on the stereohemistry of the bulk polymerizations of TFEA (F), HFiPA ( ), and PFtBA (Œ). Figure 4. Temperature effet on the stereohemistry of the HFiPA polymerization in THF (F: [M] o 1.0M; : [M] o 2.0M; and Œ [M] o 5.4M). both entropy and enthalpy. However, the enthalpy ontribution may be greater than that of entropy beause the absolute values of S i S s are relatively small. From the ativation parameters listed in Table VI at different onentrations, we also noted that a larger value of H i H s was obtained at a lower monomer onentration. This orresponds to the fat that the temperature effet was larger at a lower monomer onentration. The results obtained here may suggest that the stereospeifiity of the assumed speies with a higher r- speifiity, whose population is higher at a lower monomer onentration, is more sensitive to temperature than that of the other speies. Thermal Properties of the Polymers The thermal properties of the vinyl polymers are generally affeted by side group, main-hain tatiity, and moleular weight. Figure 5 show the Table VI. Ativation Parameters for the Radial Polymerization of Fluoroalkyl Arylates under Different Conditions S i S s H i H s Monomer Solvent [M] o (al deg 1 mol 1 ) (al mol 1 ) Figure 3. Temperature effet on the stereohemistry of the TFEA (F), HFiPA ( ), and PFtBA (Œ) polymerizations in toluene ([M] o 1.0 M). TFEA none HFiPA none PFtBA none TFEA toluene HFiPA toluene PFtBA toluene TFEA THF HFiPA THF HFiPA THF HFiPA THF

8 POLYMERIZATION OF FLUOROALKYL ACRYLATES 1031 Figure 5. DSC urves of poly(fluoroalkyl arylate)s: poly(tfea) of run 15 in Table II (A 1 ), and run 4 in Table II (A 2 ); poly(hfipa) of run 10 in Table V (B 1 ), and run 4 in Table III (B 2 ); and poly(pftba) of run 6 in Table IV (C 1 ), and run 12 in Table IV (C 2 ). (Heating rate: 10 C/min; N 2 flow: 35 ml/min.) DSC urves of the three kinds of polymers with different tatiities prepared in this study. The midpoint of the heat apaity transition was taken as the glass-transition temperature (T g ). 11 Poly(TFEA) exhibited the lowest T g among the three polymers. As shown in Figure 5 (A 1 and A 2 ), the poly(tfea)s having the same moleular weight (M n ) and different r-ontents, 51% (run 4 in Table II) and 62% (run 15 in Table II), exhibited T g s of 7 and 6 C, respetively, indiating that tatiity has little effet on the T g of the poly(tfea) in the above range of tatiity. The poly(hfipa)s exhibited higher T g s than the poly(tfea)s. The poly(hfipa)s having idential moleular weights (M n ) exhibited obviously different T g s beause of their different tatiities. As shown in Figure 5 (B 1 and B 2 ), the polymers with an r-ontent of 64% (run 10 in Table V) and 79% (run 4 in Table III) exhibited T g of 15 and 8 C, respetively. Similarly, the poly- (HFiPA)s (run 9 in Table III and run 9 in Table V) having M n with r-ontents of 67% and 76% exhibited T g of 21 and 16 C, respetively. These results mean that a higher r-ontent results in a lower T g for poly(hfipa) in the tested range of tatiity. This is in ontrast to the T g -tatiity relation for poly(methyl metharylate) where T g inreases with an inrease in the r-ontent. 12 Tatiity strongly affeted the thermal property of poly(pftba). The poly(pftba) with an r-ontent of 77% (DP 112, run 6 in Table IV) exhibited a sharp endothermi peak at 50 C, whih was asribed to a melting transition, whereas poly(pftba) with an r-ontent of 56% (DP 42, run 12 in Table IV) exhibited a glass transition at 34 C. This implies that the morphology of the polymer hanged from amorphous to a semirystalline state beause of the tatiity hange from an atati range to a syndiotati range. A similar result has also been found for syndiotati poly(n-butyl arylate)s with melting endothermi peaks, whih inrease as the r-ontent inreases. 13 CONCLUSION The free-radial polymerization of TFEA, HFiPA, and PFtBA was arried out and the stereohemistry of the polymerization was studied. Monomer struture signifiantly affeted the stereohemistry; HFiPA and PFtBA produed syndiotati polymers, while TFEA led to a lower r-seletivity. The stereohemistry of the TFEA polymerization was almost independent of temperature in the bulk, THF, and toluene, whereas that of the HFiPA and PFtBA polymerizations was learly dependent on temperature. Solvents also affeted the stereohemistry of propagation. During the polymerization of HFiPA in THF, the effet of monomer onentration was observed. By optimizing the reation onditions, a polymer with an r-ontent of 81% was obtained from HFiPA. The thermal properties of the polymers were influened by the side group and the main-hain tatiity. The poly(pftba) with a relatively high r-ontent (77%) exhibited a melting transition. This work was supported in part by the New Energy and Industrial Tehnology Development Organization (NEDO) for the projet on Tehnology for Novel High- Funtional Materials in Industrial Siene and Tehnology Frontier Program, AIST. REFERENCES AND NOTES 1. For a review, see: Nakano, T.; Okamoto, Y. ACS Symposium Series 685, 1998, pp Matsuzaki, K.; Uryu, T.; Ishida, A. J Polym Si Part A , 5, 2167.

9 1032 LIU, NAKANO, AND OKAMOTO 3. Uryu, T.; Shiroki, H.; Okada, M.; Hosonuma, K.; Matsuzaki, K. J Polym Si Part A , 9, Matsuzaki, K.; Okada, M.; Hosonuma, K. J Polym Si Part A , 10, Roitman, J. N.; Pittman, A. G. J Polym Si Polym Lett 1972, 10, Liu, W.; Nakano, T.; Okamoto, Y. Polym J 1999, 31, Isobe,Y.; Yamada, K.; Nakano, T.; Okamoto, Y. Maromoleules 1999, 32, Yamada, K.; Nakano, T.; Okamoto, Y. Maromoleules 1998, 31, Nakano, T.; Matsuda, A.; Okamoto, Y. Polym J 1996, 28, Fordham, J. W. L. J Polym Si 1959, 39, Sanhis, A.; Masegosa, R. M.; Rubio, R. G.; Prolongo, M. G. Eur Polym J 1994, 30, Kitayama, T.; Masuda, E.; Yamaguhi, M.; Nishiura, T.; Hatada, K. Polym J 1992, 24, Kitayama, T.; Kawauhi, T.; Tabuhi, M.; Hatada, K. Polym Prepr Jpn 1999, 48, 1289.