Synthesis of poly(styrene oxide) with different molecular weights using tin catalysts

Abstract

The polymerization reactions of styrene oxide (SO) at different temperatures for different time intervals were performed by four different tin catalysts, tin(IV) perfluorooctanoate (PFO-Sn), tin(IV) methacrylate (MAc-Sn), tin(II) chloride (SnCl₂), and SnCl₂/hexamethyldisilazane. Tin(IV) perfluorooctanoate and methacrylate compounds have been prepared from perfluorooctanoic acid, methacrylic acid, and tin tert-butoxide in tert-butanol. The Sn-PFO and Sn-MAc compounds were characterized by ¹H, ¹³C NMR, FTIR, and elemental analysis. These four compounds were tested as catalysts in polymerization of SO and were effective. Polystyrene oxide (PSO) was characterized by ¹H, ¹³C NMR, and gel permeation chromatography. High molecular weight of PSO (~138,000 Da) was recorded for the first time by this study.

Keywords:

• polymers
• styrene oxide
• inorganic compounds
• nuclear magnetic resonance (NMR)
• infrared spectroscopy (IR)

1. Introduction

Poly(styrene oxide) homopolymer has been produced from ring-opening polymerization of the styrene oxide by cationic catalysts,[1] anionic catalysts,[2] and mostly coordination catalysts.[3,4] The polymerization of styrene oxide (SO) has been conducted under severe polymerization conditions, such as higher temperature, longer polymerization time, and vacuum conditions.[5,6]

A few reports were published on the applications of poly(styrene oxide) in versatile areas such as hydrophobic nanocarriers for drug delivery,[7] block copolymer micelle formation in water,[7] coating materials, surface modifiers,[8] and in the production of polystyrene carbonate.[9]

There is not enough information and comparison on the ring-opening polymerization of styrene oxide with tin perfluorooctanoate, tin methacrylate catalysts, and tin chloride in the presence of hexamethyldisilazane (HMDS).

In this study, PFO-Sn/MAc-Sn catalysts were synthesized, characterized and used in the polymerization of styrene oxide. In addition to PFO-Sn/MAc-Sn catalysts, tin(II) chloride and tin(II) chloride/HMDS catalysts were also investigated in the polymerization of styrene oxide. The first goal of this study was to prepare and characterize new Sn-catalysts and to investigate their potential application as catalysts in the ring-opening of styrene oxide. The second goal was to prepare high molecular weight polystyrene oxide (PSO) because there is no report about the synthesis of a high molecular weight PSO. The reactivities of the catalysts to ring-opening polymerization of styrene oxide were compared in this study.

2. Experimental

2.1. Materials and instrumentation

Tin(IV) tert-butoxide (99.99%, Aldrich)_, pentadecafluorooctanoic acid (Merck KGaA, PFOH), (±) styrene oxide (98%, Alfa Aesar), methacrylic acid (98%, Fluka, MAcH), tin(II) chloride (100%, Merck), and hexamethyldisilazane (99%, Fluka, HMDS) were used as received. Tetrahydrofuran (THF) (99.9%, Merck) and tert-butanol (99%, Merck) were dried over activated molecular sieves before use. Polymer syntheses were carried out under nitrogen atmosphere. Tin compounds and polymer were characterized by ¹H NMR (Bruker DPX, 400 MHz) and ¹³C NMR spectroscopy (Bruker DPX, 100 MHz). Infrared spectra of complexes were recorded on a Shimadzu 8201/86601 PC spectrometer. The elemental analysis was carried out with a LECO CHNS-932 elemental analyzer. Gel permeation chromatography (GPC) analysis was performed at 30 °C on a Shimadzu prominence GPC system equipped with a RID-10A refractive index detector, a LC-20AD solvent delivery unit, a CTO-10AS column oven, and a set of two columns, PSS SDV 5 μL and PSS SDV 5 μL . THF (HPLC grade) was used as the mobile phase at 1.0 mL/min. The sample concentration was 2 mg/mL, and the injection volume was 50 μL. The calibration curve was made with seven polystyrene standards covering the molecular weight range from 162 to 100,000 Da.

2.2. Preparation of [(C₄H₅O₂)₄Sn₂(OC₄H₉)₂O]

Methacrylic acid (2.44 mmol, 0.214 g) was added to a solution of tin tert-butoxide (1.22 mmol, 0.50 g) in 20 mL tert-butanol. The reaction mixture was stirred for 3 h at room temperature. Then, the volatile parts were removed by vacuum evaporator at 50 °C and dried. The resultant product was white solid. Elemental analysis (dimer, C₂₄H₃₈O₁₁Sn₂, M_w = 739.97 g/mol), [(C₄H₅O₂)₄Sn₂(OC₄H₉)₂O] Calcd.: C, 38.96; H, 5.18%. Found: C, 38.10; H, 4.96%. ¹H NMR, CDCl_3, ppm, δ: 1.29 (s, CH₃, O^tBu), 1.95 (s, CH₃, MAc), 5.67 (s, H, CH₂, MAc), 6.24 (s, H, CH₂, MAc). ¹³C NMR, CDCl₃, ppm, δ: 17.9 (CH₃, MAc), 31.4 (CH₃, O^tBu), 74.0 (C, O^tBu), 127.5 (H₂C=, MAc), 135.9 (CMe=, MAc), 172.7 (COO). FTIR (KBr, cm⁻¹): 1646 (C=C), 1556 (strong, ν_as COO), 1460 (strong, ν_s COO), 1298, 1264, 1161, 1120, 1082, 940, 850, 752, 652, 590.

2.3. Preparation of [(C₈F₁₅O₂)₄Sn₂(OC₄H₉)₂O]

Perfluoroctanoic acid (1.90 mmol, 0.785 g) was added to a solution of tin tert-butoxide (0.95 mmol, 0.39 g) in 20 mL tert-butanol. The reaction mixture was stirred for 3 h at room temperature. Then, the volatile parts were removed by vacuum evaporator at 50 °C and dried. The resultant product was liquid. Elemental analysis (dimer, C₄₀H₁₈O₁₁F₆₀Sn₂, M_w = 2051.89 g/mol), [(C₈F₁₅O₂)₄Sn₂(OC₄H₉)₂O] Calcd.: C, 23.41; H, 0.88%. Found: C, 22.38; H, 0.61%. ¹H NMR, DMSO_, ppm, δ: 1.39 (s, CH₃, O^tBu). ¹³C NMR, DMSO, ppm, δ: 26.1 (CH₃, O^tBu), 31.0 (CH₃,), 66.9 (C, O^tBu), 107.8 (CF₂), 110.2 (CF₂), 117.8 (CF₂), 122.6 (CF₃), 159.0 (COO). FTIR (NaCl cell, cm⁻¹): 1674 (υ_asCOO), 1445 (υ_s COO), 1360 (CH, O^tBu)), 1258 (CF₂), 1215, 1157, 1068, 663.

2.4. Polymerization of styrene oxide with tin catalysts

A typical procedure for the polymerization was as follows: each catalyst SnCl₂ (15 mg), [(C₈F₁₅O₂)₄Sn₂(OC₄H₉)₂O] (20 mg), [(C₄H₅O₂)₄Sn₂(OC₄H₉)₂O] (15 mg), and SnCl₂/HMDS (15 mg/13 mg) were mixed with styrene oxide (1.2 mL) in a vial under nitrogen. The solvent free mixture was stirred at different temperatures and hours as indicated in Table 1. The monomer conversion was determined by the GPC and NMR measurements. The polymer prepared with SnCl₂ catalyst: ¹H NMR (CDCl₃, ppm), δ: 7.5–7.0 (m, Ph), 4.2–3.39 (t, CH polymer backbone), 3.55–3.50; 3.40–3.37 (m, CH₂O polymer backbone).¹³C NMR (CDCl₃, ppm), δ: 139.7, 128.1, 127.5, 126.9 (Ph), 81.8 (s, CH polymer backbone), 74.2 (s, CH₂ polymer backbone).

Table 1. Data for PSO obtained from GPC measurements.

Catalyst	T (°C)	Time (h)	Mw	Mn	(Mw/Mn)	Conversion (%)
[(C8F15O2)4Sn2(O C4H9)2O]	60	24	1350; 560	1075; 560	1.3; 1.1	66; 30
[(C4H5O2)4Sn2(OC4H9)2O]	60	24	1560; 650	1280; 640	1.2; 1.0	38; 32
[(C4H5O2)4Sn2(OC4H9)2O]	80	24	1170; 560	990; 560	1.2; 1.0	40; 50
SnCl2	80	12	72,900; 3670	54,080; 2110	1.3; 1.7	55; 40
SnCl2/Me6Si2NH	80	24	137,980; 630	86,240; 590	1.6; 1.1	20; 75

The polymer prepared with PFO-Sn catalyst: ¹H NMR (CDCl₃, ppm), δ: 7.35 (m, Ph), 4.56 (m, CH polymer backbone), 3.68 (m, CH₂O polymer backbone). Similar ¹H NMR result was obtained for PSO prepared with MAc-Sn catalyst.

3. Results and discussion

Two new metal-organic compounds were synthesized by reactions of Sn(O^tBu)₄ with 1:2 mol ratio of perfluorooctanoic acid and methacrylic acid in alcohol at room temperature to give the products of [(C₇F₁₅COO)₄Sn₂(O^tBu)₂O] and [(C₄H₅O₂)₄Sn₂(OC₄H₉)₂O] in accordance with the following reaction:

The reactions were carried out similarly to the preparations of β-diketonate tin(IV) propoxide and tetrafluorophthalate tin(IV) butoxide complexes.[10,11] The formulations of compounds were determined by elemental analysis, FTIR, and NMR measurements. The number of oxo group was one for both compounds. The FTIR spectrum of free perfluorooctanoic acid (PFOH) exhibits intense band at 1775 cm⁻¹ corresponding to asymmetrical stretching vibrations of the carboxyl group.[12] After coordination of PFOH to tin tert-butoxide, the band shifts to low wave numbers ~1674 for νCOO_asym and 1445 cm⁻¹ for νCOO_sym. In the FTIR spectrum of perfluorooctanoate–Sn(IV) complex the band at ~1674 cm⁻¹ indicates the bonding of carboxylic group. These values are consistent with those detected in a number of carboxylate-metal compounds.[13–15]

The ¹H NMR spectra of perfluorooctanoate-Sn and methacrylate-Sn complexes showed singlet at ~1.39 and 1.29 ppm, respectively, because of methyl protons of tert-butoxy groups in Sn-complex.[16] The signal for the COOH proton is absent. Lack of a carboxylic acid proton signal at ~11 ppm indicates that PFOH and MAcH are completely coordinated to tin. The ¹³C NMR spectra of perfluorooctanoate-Sn and methacrylate-Sn complexes showed characteristic peaks for bonded perfluorooctanoate and methacrylate groups. For example, carboxylate group appeared at 159 ppm for PFO-Sn and 172.7 for Mac-Sn. These values are very characteristic for bonded PFO and Mac groups.[17,18]

SO was polymerized with PFO-tin catalyst at 60 °C for 24 h. The conversion was 96%. When MAc-tin catalyst was used instead of PFO-tin catalyst, a low conversion (70%) was obtained. Polymerization temperature was very effective on the conversion of SO. The conversion increased with the temperature rising from 60 °C to 80 °C and reached a high conversion of 90% at 80 °C for MAc-Sn catalyst after 24 h stirring. These catalysts have covalent and acidic character. Therefore, as expected for acidic catalysts, the molecular weights of polymers (~1500 Da) were very small when compared with basic and ionic catalyst.

In the ¹H NMR spectrum of the PSO prepared with SnCl₂, the major signals appear at 4.20–3.39 ppm for methine proton (t, CH polymer backbone), and 3.55–3.50; 3.40–3.37 for methylene protons (m, CH₂O polymer backbone), respectively. The other signals at 7.5–7.0 ppm are assigned to the aromatic protons of the PSO side chain. There is a peak around ~4.05 ppm because of OCH proton of isopropanol which was used as a washing solvent. The ¹³C NMR spectrum of PSO showed the expected peaks due to the methine and methylene carbons of the PSO backbone (Figure 1). The main peaks of methine and methylene (due to the head-to-tail linkage) appeared at 81.8 (s, CH polymer backbone) and 74.2 (s, CH₂ polymer backbone).

Figure 1. ¹³C NMR spectrum of PSO prepared with SnCl₂ catalyst.

Figure 2. Gel permeation chromatogram of PSO prepared with SnCl₂/HMDS.

¹³C NMR result shows that polymer has a regular structure as both region and stereo. These values are consistent with those reported in a number of articles about PSO.[19,20]

Styrene oxide is notably less reactive than aliphatic and other alicyclic epoxides, but is known to undergo ring-opening with acids and bases.[4] Therefore, it is important to find suitable catalysts for styrene oxide to undergo ring opening polymerization through forming large molecular weights of polymers. The catalyst, SnCl_2, is suitable for above purposes and has ionic and basic character. Therefore, as expected for basic catalysts, the molecular weights of polymers (~73,000 Da) were very large when compared with above acidic catalysts, PFO-Sn/MAc-Sn (Table 1). However, The SnCl₂ catalyst produces polymer having relatively broad molecular distribution and having long tails.

In the presence of co-catalyst, SnCl₂/HMDS (15/13 mg, in 1:1 mol ratio), polymers were obtained with more regular structure and higher molecular weight (138,000 Da). This can be attributed the single site properties of catalyst and basicity of the hexamethyldisilazane which coordinates to tin atom and enhances the catalytic activity and the degree of control on polymerization process. The reaction of tin(II) chloride with 1 equiv of hexamethyldisilazane led to the formation of (Me₃Si)₂NSnCl as shown in Scheme 1.[21,22] The SnCl₂/HMDS catalyst produces polymer having very high molecular weight and having no tail (Figure 2).

Scheme 1. Coordination of hexamethyldisilazane to tin(II) chloride.

All PSO obtained by different catalysts have similar IR spectra. They all show strong characteristic absorption bands of benzene ring (1605, 1582, 1490 cm⁻¹) and C–O group (1080 cm⁻¹), which are similar to PSO obtained with other catalysts.[23]

The ring-opening polymerization of cyclic esters by metal complexes via coordination-insertion mechanism represents the most effective and versatile method for preparing aliphatic polyethers with a good control in terms of molecular weight and stereoselectivity.

PFO is an ancillary ligand and O^tBu a nucleophilic group. The presence of chelate ligands (PFO and MAc) tunes the steric properties of the metal center, adjusts its Lewis acidity, and enhances the catalytic activity. NMR data suggest that SO attacks the metal center first and then the nucleophile O^tBu ion attacks CH₂ carbon atom in SO. Small molecular weights of PSO were obtained with these two catalysts. Acid catalysts generate low molecular weight polymers. Therefore, it can be attributed to their acidic behavior.

4. Conclusion

High molecular weights of the PSO were prepared for the first time with this study. The catalysts SnCl₂ and SnCl₂/HMDS were more effective in the high molecular weights PSO preparation as compared to the conventional cationic and anionic catalysts. As expected, polymers prepared with acidic catalysts, PFO-Sn and MAc-Sn have low molecular weights. Polymers prepared with tin catalysts (PFO-Sn, MAc-Sn, and SnCl₂/HMDS) were more regular than that of prepared with SnCl₂. New compounds are suitable as catalysts for ring-opening polymerization of epoxides and still needs further research.

Disclosure statement

No potential conflict of interest was reported by the author.

Additional information

Funding

This work was supported by the research foundation of Kocaeli University [project number 17/2014].