ISSN 0965-545X, Polymer Science, Ser. A, 2008, Vol. 50, No. 4, pp. 388–393. © Pleiades Publishing, Ltd., 2008. Original Russian Text © S.Yu. Kochev, Yu.A. Kabachii, 2008, published in Vysokomolekulyarnye Soedineniya, Ser. A, 2008, Vol. 50, No. 4, pp. 607–613.

CHEMICAL TRANSFORMATIONS

Addition of Thiol-Ended Polystyrene to Acrylates S. Yu. Kochev and Yu. A. Kabachii Nesmeyanov Institute of Organoelement Compounds, Russian Academy of Sciences, ul. Vavilova 28, Moscow, 119991 Russia e-mail: [email protected] Received February 7, 2007; Revised Manuscript Received July 3, 2007

Abstract—Polystyrene containing end thiol groups has been prepared via the aminolysis or alcoholysis of its trithioester under conditions established previously for the model reaction of bis(1-phenylethyl) trithiocarbonate. The aminolysis of polystyrene with Mn = 8.2 × 103 by n-hexylamine at 100°C in toluene led to the formation of a polymer with Mn = 4 × 103 and a polydispersity coefficient of 1.13. The unimodal molecular-mass distribution of the polymer and its narrow polydispersity that is nearly equal to the initial polydispersity suggest that all trithiocarbonate groups of the starting polystyrene are involved in hydrolysis. The addition of 1-phenylethylthiol and thiol-ended polystyrene to methyl acrylate and methyl methacrylate via the Michael hydrolysis has been studied, and it has been demonstrated that the addition proceeds quantitatively at room temperature. The addition may be directly implemented with the use of dithioesters in the presence of the catalytic amount of a base; therefore, the stage of thiol synthesis can be eliminated. For the quantitative addition to acrylates, the preliminary hydrolysis of the polymer is however preferable. DOI: 10.1134/S0965545X08040056

INTRODUCTION The free-radical polymerization with the use of dithioesters as regulators of chain growth has gained wide acceptance for the synthesis of polymers of desired molecular masses [1–3]. The as-synthesized polymeric dithioesters, in turn, may serve as a source of polymers carrying end thiol groups [4, 5]. This paper is continuation of previous studies [6] devoted to the preparation and use of polystyrene carrying end thiol groups. The synthesis of this polymer via aminolysis or alcoholysis of a macromolecular trithioester was investigated, and its ability to add to α,β-unsaturated conjugated carbonyl compounds was examined. Particular attention was given to the quantitative aspects of chemical transformations. EXPERIMENTAL Analytical Techniques NMR spectra were measured on a Bruker spectrometer operating at 400.13 MHz (1H) and 100.1 MHz (13C). CDCl3 was used as a solvent. The yield of methyl-3-(1-phenylethylthio) propionate and methyl3-(1-phenylethylthio) isobutyrate was determined by gas chromatography using diphenyl as an internal standard. Measurements were performed on a Tsvet 800 chromatograph equipped with a flame-ionization detector, helium as a carrier gas (30 ml//min), and a 2-m stainless steel column packed with 5% SE-30 on Inerton N-Super.

Molecular mass and molecular-mass distribution were estimated by GPC on a chromatograph equipped with a Knauer HPLC 64 pump, a Milton Roy spectrophotometric detector, and three PL-GEL columns with measurement limits up to 102, 103, and 105. THF was used as a solvent; the elution rate was 1 ml/min. The chromatographic system was calibrated relative to reference polystyrene samples. The relative error in molecular-mass measurements was 10%. Materials 1,4-Dioxane and toluene were distilled over CaH2, and THF was distilled over LiAlH4. 1-Hexylamine (98%, Aldrich) and pentaerythritol tetraacrylate (Aldrich) were used as received. Methyl acrylate (MA, 99%, Aldrich), methyl methacrylate (MMA, 99%, Aldrich), and styrene (99%, Aldrich) was vacuum distilled over ë‡ç2 and stored at –15°ë under argon. Acetic acid (99.0%, Aldrich) was degassed via three freeze– pump–thaw cycles and stored in a Schlenk vessel under argon. To prepare a solution of tetrabutylammonium hydroxide ([Bu4N]OH, 1.025 M), equimolar amounts of tetrabutylammonium bromide and NaOH in methanol were mixed and the reaction solution was filtered. The filtrate was degassed via three freeze–pump–thaw cycles and stored in a Schlenk vessel under argon. Bis(1-phenylethyl) trithiocarbonate (PTC), phenylethyldithio benzoate (PTB), (S)-1-phenylethyl-(O)-ethyl xanthate (PX), and 1-phenylethylthiol (PET) were prepared in accordance with [6].

388

ADDITION OF THIOL-ENDED POLYSTYRENE TO ACRYLATES

The synthesis of all other compounds is described below. Synthesis of polystyrene. PS was synthesized as described in a patent [7] with the use of thermal initiation at styrene : PTC = 100 : 1 (mol/mol). After 50 h at 110°ë, the conversion of styrene and the yield of polystyrene were ~81%. These values correspond to Mn = 8.6 × 103. As evidenced by GPC, Mn = 8.2 × 103 and Mw /Mn = 1.12. Synthesis of methyl-3-(1-phenylethylthio) propionate. In a Schlenk vessel equipped with a magnetic stirrer, THF (10 ml), 1-phenylethylthiol (2.35 g, 17.0 mmol), and MA (1.54 g, 17.9 mmol) were loaded with a syringe under argon. [Bu4N]OH (1.025 M, 3.3 ml, 3.4 mmol) was added under cooling (ice/water) and intense stirring. The mixture was allowed to stand at room temperature for 48 h and heated at 40°ë for 1 h. THF was evaporated under reduced pressure, and the residue was dissolved in methylene chloride (70 ml). The solution was washed first with water acidified with hydrochloric acid to a weakly acidic reaction and then two times with water and dried over Na2SO4. Methylene chloride was evaporated under reduced pressure, and the residue was distilled under vacuum. Methyl-3(1-phenylethylthio) propionate (2.0 g, 51%) was isolated as a colorless liquid with a boiling point of 113– 115°ë/1 mmHg and the content of the basic product greater than 99% (gas chromatography). 1H NMR (CDCl ): δ 1.57 (d, 3H, CH ), 2.47 (t, 2H, 3 ç 3 CH2), 2.56 (t, 2H, CH2), 3.66 (s, 3H, CH3O), 3.99 (q, 1H, CH), 7.23–7.36 (m, 5H, C6H5) ppm. NMR (CDCl3): δë 22.30 (CH2), 25.96 (CH3), 34.13 (CH2), 44.02 (CH3O), 51.49 (CH), 126.9, 127.0, 128.3, 143.5 (C6H5), 172.1 (C=O) ppm. Synthesis of methyl-3-(1-phenylethylthio) isobutyrate. In a Schlenk vessel equipped with a magnetic stirrer, THF (10 ml), 1-phenylethylthiol (2.35 g, 17.0 mmol), and MMA (1.79 g, 17.9 mmol) were loaded with a syringe under argon, and [Bu4N]OH (1.025 M, 0.83 ml, 0.85 mmol) was added under cooling (ice/water) and intense stirring. The synthesis was performed in a manner similar to that described above. Methyl-3-(1-phenylethylthio) isobutyrate (2.84 g, 70%) was isolated as a colorless liquid with a boiling point of 104–110°ë/1 mmHg and the content of the basic product was greater than 99% (gas chromatography). 1H NMR (CDCl ): δ 1.13–1.18 (m, 3H, CH ), 1.56 3 ç 3 (d, 3H, CH3), 2.33–2.39 (m, 1H, CHCH2), 2.50–2.90 (m, 2H, CH2CH), 3.66 (d, 3H, CH3O), 3.92–4.00 (m, 1H, CH), 7.24–7.33 (m, 5H, C6H5) ppm. 13C

NMR (CDCl3): δë 16.47, 16.61 (CH3), 22.21, 22.30 (CH3), 34.09, 34.20 (CH2), 39.53, 39.58 (CH), 44.01, 44.30 (CH3O), 51.41 (CH), 126.83, 126.94, 126.99, 128.16, 128.20, 143.40, 143.47 (C6H5) ppm. 13C

POLYMER SCIENCE

Series A

Vol. 50

No. 4

2008

389

IR: ν 2990, 1750, 1470, 1225, 1180, 720–780 cm–1. Addition of 1-phenylethylthiol to MA and MMA. Before these experiments, a gas chromatograph was calibrated relative to methyl-3-(1-phenylethylthio) propionate and methyl-3-(1-phenylethylthio) isobutyrate. Diphenyl was used as an internal standard. All operations were performed at room temperature under argon. In a typical experiment, a Schlenk vessel equipped with a magnetic stirrer was loaded in the flow of argon with diphenyl (20 mg), THF (3 ml), MMA (30 mg, 0.300 mmol) or MA (25.8 g, 0.300 mmol), 1-phenylethylthiol (20.7 mg, 0.150 mmol) or PTB (38.7 mg, 0.150 mmol) or PX (33.9 mg, 0.150 mmol), respectively. Then, MeOH (50 µl) was added with the aid of a syringe purged with argon and degassed under vacuum via three freeze–pump–thaw cycles. After a solution of [Bu4N]OH (1.025 M, 15 µl, 0.015 mmol) in methanol was added, the reaction solution was sampled (0.4 µl) and the content of methyl-3-(1-phenylethylthio) propionate or methyl-3-(1-phenylethylthio) isobutyrate was estimated by gas chromatography. Aminolysis and alcoholysis of polystyrene. All operations were performed under argon. In a typical experiment, a Schlenk vessel equipped with a magnetic stirrer was purged with argon and loaded with polystyrene (0.5 g, 6.1 × 10–2 mmol) and a solvent (0.3 ml of toluene for aminolysis or 0.5 ml of dioxane for alcoholysis. The vessel was closed and degassed by repeating freeze–pump–thaw cycle three times. 1-Hexylamine (2.3 equiv) or a NaOH solution in methanol (2.408 M, 4 equiv) was added with a syringe purged with argon, and the vessel was placed in an oil or water bath at 100 or 60°ë. The reaction mixture was allowed to stand at these temperatures for 5 h or 50 min, respectively. The polymer solution was cooled to room temperature, acidified with 1.5-fold excess of degassed acetic acid, dissolved in methylene chloride, and precipitated into methanol. The polymer was filtered, washed with methanol, and vacuum dried at 40–50°ë. From polystyrene with Mn = 8.2 × 103 and Mw /Mn = 1.12, polystyrene with end thiol groups (PST) with Mn = 4.0 × 103 (the calculated Mn = 4.1 × 103) and Mw /Mn = 1.13 was prepared via aminolysis. The alcoholysis of the above polystyrene gave the polymer with Mn = 4.18 × 103 and Mw /Mn = 1.19. Addition of thiol-ended PS to MA and MMA. All operations were performed under argon. In a typical experiment, a Schlenk vessel equipped with a magnetic stirrer was purged with argon and loaded with polystyrene (0.5 g) and dioxane (0.5 ml) or THF (0.5 ml). The vessel was closed and degassed by repeating freeze–pump–thaw cycle three times. MA or MMA (2.0 equiv) and a methanol solution of [Bu4N]OH (1.025 M, 0.1 equiv) was added with a syringe purged with argon. The reaction mixture was allowed to stand at room temperature for 3 h. The polymer solution was acidified with the excess amount of degassed acetic

390

KOCHEV, KABACHII (‡)

8

6

4

2

4 δH, ppm

2

0

(b)

8

6

0

Fig. 1. 1H NMR spectrum of adduct of thiol-ended PS with (a) MA and (b) in CDCl3.

acid, dissolved in methylene chloride, and precipitated into methanol. The polymer was filtered, washed with methanol, and dried under vacuum at 40–50°ë. The NMR spectra of MA- and MMA-based adducts are shown in Fig. 1.

Addition of thiol-ended PS to pentaerythritol tetraacrylate. In the reaction with pentaerythritol tetraacrylate, a solution of pentaerythritol tetraacrylate (0.4 equiv), polystyrene (1.0 g), and dioxane (1.0 ml) were degassed via three freeze–pump–thaw cycles. POLYMER SCIENCE

Series A

Vol. 50

No. 4

2008

ADDITION OF THIOL-ENDED POLYSTYRENE TO ACRYLATES

Then, the degassed solution of [Bu4N]OH (1.025 M, 0.1 equiv) in methanol was added with the aid of a syringe purged with argon, and the mixture was allowed to stay at room temperature overnight. RESULTS AND DISCUSSION Synthesis of Polystyrene with End Thiol Groups As was shown in [6], the use of PTC for the controlled free-radical polymerization of styrene is preferable for the subsequent quantitative hydrolysis of thioester groups and synthesis of the thiol-ended polymer. Polystyrene with Mn = 8.2 × 103 and Mw /Mn = 1.12 was prepared via the thermally initiated polymerization in the presence of PTC [7]. The aminolysis and alcoholysis of the polymer were carried out under conditions that were previously described in [6] for the model hydrolysis of PTC. To avoid the oxidation of thiol groups, all operations were performed in inert atmosphere and the reaction mixture was acidified with a small excess of degassed acetic acid before precipitation. The aminolysis of polystyrene with 1-hexylamine at 100°ë in toluene afforded a polymer with Mn = 4.0 × 103 and Mw /Mn = 1.13 (GPC), in accordance with the expected Mn = 4.10 × 103. The unimodal molecularmass distribution of the polymer (Fig. 2a) and its narrow polydispersity that is nearly equal to its initial value indicate that all trithiocarbonate groups were subjected to hydrolysis. We failed to analyze the content of thiol groups in the polymer on the basis of 1H NMR data because of the superposition of signals from tertiary CH protons of polystyrene and thiol groups (~2.02 ppm [6]). However, as will be shown below, the amount of these groups may be estimated from the 1H NMR measurements of adducts with MA or MMA. From the data on the model aminolysis of PTC, the maximum expected degree of transformation is ~79% [6]. NaOH-catalyzed alcoholysis in 1,4-dioxane at 60°ë for a time two times longer than the time necessary for hydrolysis of the low-molecular-mass analog of PTC yielded the polymer with Mn = 4.2 × 103 (Fig. 2b) and the bimodal molecular-mass distribution (Fig. 2b) and Mw /Mn = 1.19 apparently owing to the incomplete hydrolysis of trithiocarbonate groups. Even though the reaction solution looked transparent and homogeneous, it is suggested that the incomplete hydrolysis is most likely related to difficulties arising during penetration of the hydrophilic reagent into the hydrophobic polymer medium. Addition of 1-Phenylethylthiol and Thiol-Ended Polystyrene to Acrylates This reaction was catalyzed by bases since the addition of thiols to α, β-unsaturated conjugated carbonyl POLYMER SCIENCE

Series A

Vol. 50

No. 4

2008

391

(a)

4

6

8

10

12

(b)

4

6

8 10 Times, min

12

Fig. 2. Chromatogram of PST prepared via (a) aminolysis and (b) alcoholysis of PTC.

compounds to MA and MMA follows the same mechanism as the addition of carboxylic acids via the Michael reaction [8]. Before experiments on the addition of PST, we evaluated the reactivity of the lowmolecular-mass secondary 1-phenylethylthiol, as a compound modeling the end group of PST. The reaction may be described by the following scheme: R SH + Ph

CO2Me

R –

S

MeO

CO2Me .

Ph R = H, Me

392

KOCHEV, KABACHII

Addition of 1-phenylethylthiol to MA and MMA Reagent PET PTB PX

Yield, %

[Bu4N]OH, equiv

Time, min

PET–MA

PET–MMA

0.1 0.1 0.6 1.0 0.3 1.0

15 75 140 180 120 90

99 100 61 56 84 45

97 100 – – – –

Note: THF, 25°C, [MA]0 = [MMA]0 = 0.10 mol/l; and [PET]0 = [PTB]0 = [PX]0 = 0.05 mol/l.

The reaction was carried out in dilute (0.05 M in THF) solution corresponding to the low concentration of end thiol groups in a polymer solution. Furthermore, according to theory, a base (in our case, [Bu4N]OH) that serves as a catalyst of the above reaction and, simultaneously, as a catalyst of thioesters alcoholysis should not be consumed in the course of alcoholysis, we studied whether MA-based adducts may be prepared directly from dithioesters eliminating the stage of alcoholysis. As dithioesters, PTB and PX that are in wide use as chain-transfer agents were employed. The yields of adducts and their preparation conditions are summarized in the table. It is clear that despite dilution the addition of the secondary thiol to both acrylates proceeded rapidly and quantitatively. It was of interest to investigate the direct use of dithioester PTB in the reaction under study primarily due to the fact that it was impossible to isolate 1-phenylethylthiol with a yield

greater than 50% because of the occurrence of side reactions during the hydrolysis of PTB [6]. When this dithioester was directly involved in the addition reaction, the yield of the adduct increased to 56–61%. A low yield in both cases is apparently associated with the side process that hampers the quantitative synthesis of thiol from this dithioester and proceeds simultaneously with hydrolysis during the attack of nucleophile at dithioester. The alkaline hydrolysis of PX to dithioester proceeds with a yield close to quantitative (94%) [6]). Furthermore, the MA-based adduct may be prepared with a high yield directly from dithioester even though this yield is smaller than the yield of thiol in the course of dithioester hydrolysis. It is of interest that the highest yield of the adduct is achieved when the catalytic amount of the base is used, as in the case of PTB. This tendency is probably related to the reversible character of thiolate ion addition at high concentrations of the base. Nevertheless, our data suggest that the preliminary hydrolysis of polystyrene is preferable for the quantitative addition to acrylates. The addition of polystyrene carrying end thiol groups to MA and MMA afforded polystyrene with 2-methoxycarbonylethyl- and 2-methoxycarbonylpropyl end groups, respectively. The degree of transformation was checked by 1H NMR spectroscopy from measurements of the intensity of a signal at 3.6 ppm due to methoxy group (Fig. 1). The as-calculated numberaverage degree of polymerization was 50.6 in both cases, whereas the calculated value was 39.4 for PST with Mn ~ 4.1 × 103. As follows from these data, the amount of end thiol groups of PST transformed into 2-methoxycarbonylethyl or 2-methoxycarbonylpropyl groups is 78% and is close to the expected value (79%). As is seen, in the case of PST, the addition to acrylates likewise proceeds in the quantitative manner and the efficiency of obtaining PST adducts with acrylates is determined solely by the efficiency of hydrolysis of thioesters groups during the synthesis of PST. The addition of PST to the polyfunctional substrate (pentaerythritol tetraacrylate) carried out in the presence of a base produced a star polymer O C

O

S Ph

x 4

4

6

8 Time, min

10

12

Fig. 3. Chromatogram of the product of PST addition to pentaerythritol tetracrylate.

As evidenced by GPC (Fig. 3), this polymer had Mn = 14.4 × 103 (the expected calculated value was 16.4 × 103), a polydispersity coefficient of 1.1, and a peak area of 1460. In addition, the chromatogram exhibits a peak due to the intact PST with an area of 507 corresponding to a PST conversion of ~72%. Thus, polystyrene carrying end thiol groups has been prepared through the aminolysis of polystyrene POLYMER SCIENCE

Series A

Vol. 50

No. 4

2008

ADDITION OF THIOL-ENDED POLYSTYRENE TO ACRYLATES

synthesized via the controlled radical polymerization in the presence of bis(1-phenylethyl) trithiocarbonate. The addition of 1-phenylethylthiol and thiol-ended PS to MA and MMA via the Michael reaction has been studied. It has been shown that the addition proceeds quantitatively even at room temperature. It has been established that, on the whole, dithioesters and the catalytic amount of the base may be used for addition and the stage of thiol addition may be eliminated. The addition of PS with end thiol groups to polyfunctional α,β-unsaturated conjugated carbonyl compounds is useful for the design of polymers with a complex architecture. REFERENCES 1. J. Chiefari, Y. K. Chong, F. Ercole, et al., Macromolecules 31, 5559 (1998).

POLYMER SCIENCE

Series A

Vol. 50

No. 4

2008

393

2. G. Moad, J. Chiefari, Y. K. Chong, et al., Polym. Int. 49, 993 (2000). 3. J. Liu, C.-Y. Hong, and C.-Y. Pan, Polymer 45, 4413 (2004). 4. R. T. A. Mayadunne, E. Rizzardo, J. Chiefari, et al., Macromolecules 33, 243 (2000). 5. T. M. Roper, C. E. Hoyle, and C. A. Guymon, Polym. Prepr. (Am. Chem. Soc., Div. Polym. Chem.) 44, 111 (2003). 6. Yu. A. Kabachii and S. Yu. Kochev, Polymer Science, Ser. A 48, 717 (2006) [Vysokomol. Soedin., Ser. A 48, 1108 (2006)]. 7. T. P. Le, G. Moad, E. Rizzardo, and S. H. Tang, WO Patent No. 98/01478 A1 (1998). 8. K. V. Vatsuro and G. L. Mishchenko, in Name Reactions in Organic Chemistry, Ed. by M. N. Pastushenko and G. N. Gosteva (Khimiya, Moscow, 1976), p. 283.