Ring-opening polymerization of isopropylglycidyl ether (IPGE) with new catalysts of Ti, Sn, Al-alkoxides and comparison of its reactivity

Abstract

The polymerization reactions of isopropylglycidyl ether (IPGE) were carried out by various catalysts potassium hydroxide, potassium tert-butoxide, tetrafluorophthalate aluminum sec-butoxide, tetrafluorophthalate tin tert-butoxide, and tetrafluorophthalate–titanium isopropoxide complexes. Tetrafluorophthalate–aluminum, tin, and titanium have been prepared from aluminum sec-butoxide, tin tert-butoxide, and titanium isopropoxide in alcohol and tetrafluorophthalic acid. Poly-IPGE and catalysts were characterized by ¹H, ¹³C NMR, FTIR spectroscopy, elemental analysis, and gel permeation chromatography. IPGE is less reactive toward OH⁻ nucleophiles than 3-glycidyloxypropyltrimethoxysilane and propylene oxide.

KEYWORDS:

• polymer
• catalyst
• synthesis
• NMR
• ring opening

1. Introduction

Ring-opening polymerization of epoxide monomers has been gaining attention in both academic and industrial circles due to its wide range of applications such as production of detergents, nonionic surfactants, washing agents, the heavy metal-free coating, filler, extenders, in drinking water distribution systems, antireflective composition for photoresists, reinforced plastics, adhesives, and polyurethane production process [1-6].

Propylene oxide (PO), styrene oxide (SO), 3-glycidyloxypropyltrimethoxysilane (GPTS), and phenylglycidyl ether (PGE) of cyclic ethers have been polymerized by both anionic and cationic catalysts and also by more selective catalyst system and their products characterized by different spectroscopic techniques [6-13]. As in the mechanism of ring-opening polymerization of PO initiated with porphyrin aluminum chloro complex [9,10],

I have recently suggested that the polymerization of GPTS initiated with tetrafluorophthalate zirconium complex proceeds as follows [13].

Although there is some spectroscopic evidence (¹H NMR) for the formation of poly-isopropylglycidyl ether (IPGE) [14-16], nothing is known about the effect of different catalysts on the epoxide ring-opening and polymerization degree of IPGE in polymerization reactions. The first purpose was to prepare and use new catalysts in the ring-opening polymerization of IPGE. The second purpose of this study was to synthesize and characterize the polymers of IPGE prepared with various catalysts. The third purpose was to see whether the side isopropoxymethyl group (CH₂OC₃H₇) of oxirane will affect both the reactivity toward basic catalysts and the regioselectivity and stereoselectivity of polymerization when compared with other groups such as methyl, phenoxymethyl, and trimethoxysilylpropoxymethyl on epoxides.

2 Experimental

Some syntheses and solvent manipulations were carried out under a nitrogen atmosphere. IPGE (Aldrich), potassium hydroxide reagent (ACS, assay 85% min, GFS Chemicals), potassium tert-butoxide (97%, Alfa Aesar), titanium(IV) isopropoxide (98%, Merck), aluminum(III) sec-butoxide (97%, Alfa Aesar), tin(IV) tert-butoxide (99.9%, Aldrich), and tetrafluorophthalic acid (TFPA) (98%, Alfa Aesar) were used as received.

¹H and ¹³C{¹H}NMR experiments were carried out with a Varian 500 MHz and Bruker 300 MHz Ultra shield TM (5 mm quaternary nuclei probe) spectrometer. Varian 500 NMR operates at proton Larmor frequencies of 500 MHz and carbon frequencies of 125 MHz. Infrared spectra of complexes were recorded on Shimadzu 8201/86601 PC spectrometer. The elemental analyses were carried out on a LECO CHNS-932 elemental analyzer. Gel permeation chromatographic (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 1000 Å and PSS SDV 5 μL 50 Å. Tetrahydrofuran (THF) (HPLC grade) was used as the mobile phase at 1.0 mL/min. The sample concentration was 2 mg/1 mL, and the injection volume was 10 μL. The calibration curve was made with seven polystyrene standards covering the molecular weight range from 162 to 34,300 Da. Glass transition temperature of IPGE polymers was determined with a Perkin Elmer DSC 4000 differential scanning calorimeter. The cooling rate was 5 °C/min.

2.1 Preparation of [(C₆F₄(COO)₂)₂Ti₄O₄(OⁱPr)₄]

$\text{Ti}{({\text{O}}^{\text{i}}\text{Pr})}_4+{\text{C}}_6{\text{F}}_4{(\text{COOH})}_2\to [{({\text{C}}_6{\text{F}}_4{(\text{COO})}_2)}_2{\text{Ti}}_4{\text{O}}_4{({\text{O}}^{\text{i}}\text{Pr})}_4]$

0.002 mol (0.502 g) TFPA was added to 0.002 mol (0.600 g) titanium(IV) isopropoxide complex in 30 mL isopropanol. The reaction mixture was stirred at room temperature for 2 h. Then, the solvent and volatile fractions were removed from light green solid product under low pressure by vacuum pump. Product was washed again with solvent isopropanol and dried in vacuum. ¹H NMR (25 °C, DMSO, ppm) δ: 1.03–1.05 (d, gem-Me₂, OⁱPr), 3.76 (sept., CH, OⁱPr). ¹³C NMR (DMSO, ppm): 163.15 (COO), 143.51 (d, ¹J_CF = 241.5 Hz, CF, C3, C6, C₆F₄), 139.67 (d, ¹J_CF = 254.5 Hz, CF, C4, C5, C₆F₄), 122.26 (C1, C2, C₆F₄), 72.52 (OCH), 25.89 (gem-Me₂, OⁱPr). Elemental analysis (C₂₈H₂₈O₁₆F₈Ti₄, M _w = 963.97 g/mol): Calc. C, 34.9; H, 2.93%. Found: C, 34.8; H, 2.88%. FTIR (KBr, cm⁻¹): 1630 (strong, ν_as CO₂), 1585, 1520, 1477, 1418 (strong, ν_s CO₂), 1281, 1124, 1078, 1014, 958, 744.

2.2 Preparation of [(C₆F₄(COO)₂)₂Sn₄O₅(O^tBu)₂]

${\text{Sn(O}}^{\mathrm{t}}\mathrm{Bu}{)}_4+{\mathrm{C}}_6{\mathrm{F}}_4{(\mathrm{COOH})}_2\to [{({\mathrm{C}}_6{\mathrm{F}}_4{(\mathrm{COO})}_2)}_2{\mathrm{Sn}}_4{\mathrm{O}}_5{({\mathrm{O}}^{\mathrm{t}}\mathrm{Bu})}_2]$

0.0026 mol (0.6353 g) TFPA was added to 0.0026 mol (1.075 g) tin(IV) tert-butoxide complex in 25 ml THF. The reaction mixture was stirred at room temperature for 2 h. Then, the solvent and volatile fractions were removed from white solid product under low pressure by vacuum pump. Product was washed again with solvent THF and dried in vacuum. ¹H NMR (25 °C, CDCl₃, ppm) δ: 1.52 (s, CH₃ of O^tBu). ¹³C NMR (CDCl₃, ppm): 163.10 (COO), 145.04 (d, ¹J_CF = 247.6 Hz, CF, C3, C6 of C₆F₄), 141.60 (d, ¹J_CF = 250.5 Hz, CF, C4, C5 of C₆F₄), 118.56 (²J_CF = 16.6 Hz, C1, C2 of C₆F₄), 84.57 (OC, O^tBu), 31.7 (CH₃, O^tBu). Elemental analysis (C₂₄H₁₈O₁₅F₈Sn₄, M _w = 1173.22 g/mol): Calc. C, 24.57; H, 1.55%. Found: C 24.62%, H, 1.37%. FTIR (KBr, cm⁻¹): 1720, 1632 (strong, ν_as CO₂), 1520, 1479, 1394 (strong, ν_s CO₂), 1325, 1252, 1126, 1074, 952, 837, 754, 707.

2.3 Preparation of [(C₆F₄(COO)₂)₂Al₄O₃(O^sBu)₂]

$\text{Al}{({\text{O}}^{\text{s}}\text{Bu})}_3+{\text{C}}_6{\text{F}}_4{(\text{COOH})}_2\to [{({\text{C}}_6{\text{F}}_4{(\text{COO})}_2)}_2{\text{Al}}_4{\text{O}}_3{({\text{O}}^{\text{s}}\text{Bu})}_2]$

0.002 mol (0.503 g) TFPA was added to 0.002 mol (0.526 g) aluminum(III) sec-butoxide complex in 30 mL sec-butanol. The reaction mixture was stirred at room temperature for 2 h. Then, the solvent and volatile fractions were removed from white solid product under low pressure by vacuum pump. Product was washed again with solvent sec-butanol and dried in vacuum. ¹H NMR (25 °C, CDCl₃, ppm) δ: 3.50 (m, CH, O^sBu), 1.32 (m, CH₂, O^sBu), 1.03 (d, CH₃, O^sBu), 0.83 (t, CH₃, O^sBu). ¹³C NMR (DMSO-d₆, ppm): 161.78 (COO), 143.55 (d, ¹J_CF = 249.07 Hz, CF, C3, C6 of C₆F₄), 139.50 (d, ¹J_CF = 249.00 Hz, CF, C4, C5 of C₆F₄), 122.07 (²J_CF = 15.85 Hz, C1, C2 of C₆F₄), 67.62 (OCH₂, O^sBu), 32.15 (CH₂, O^sBu), 23.51 (CH₃, O^sBu), 10.52 (CH₃, O^sBu). Elemental analysis (C₂₄H₁₈O₁₃F₈Al₄, M _w = 774.31 g/mol): Calc. C, 37.23; H, 2.34%. Found: C 35.72%, H 2.61%. FTIR (KBr, cm⁻¹): 1632 (strong, ν_as CO₂), 1520, 1476, 1431 (strong, ν_s CO₂), 1281, 1129, 1080, 962, 846, 760.

2.4 Polymerization of IPGE with t-BuOK

The catalyst (20 mg, t-BuOK) was taken in a vial, and 1.2 mL of IPGE was added under nitrogen. The mixture was stirred at two different temperatures (25 and 50 °C) for 1 and 2 days. The conversion of monomer (IPGE) to polymers was 17% for stirring at 25 °C and 84% for 50 °C for 2 days. The following data belong to the stirring at 50 °C and 2 days. ¹H NMR (CDCl₃, ppm), δ: 3.58–3.50 (m, CH₂), 3.49–3.43 (m, CH₂OⁱPr), 3.37–3.32 (m, CH), 1.09–1.07 (t, small, CH_3, OⁱPr), 1.05–1.03 (dd, CH₃, OⁱPr). ¹³C NMR, (CDCl₃, ppm), δ: for CH region: 79.07–78.96–78.84 (CH, triad, ii, is-si, ss), 71.77–71.76 (CH, diad, i, s, CHMe₂), 70.19, 69.95–69.85 (CH₂, diad, i, s), 68.29–68.20–68.12 (CH₂OⁱPr, triad, ii, is-si, ss), 22.08–22.03 (doublet, CH₃, OⁱPr), (Figure 1).

Figure 1 ¹³C DEPT spectrum of polymers of IPGE obtained from polymerization with KO^tBu at 50 °C.

2.5 Polymerization of IPGE with KOH

The catalyst (15 mg, KOH) was taken in a vial, and 1.2 mL of IPGE was added under nitrogen in dry box. The mixture was stirred at 75 °C for 3 days. The conversion of monomer to polymer was followed by GPC and ¹H NMR spectroscopy. ¹H NMR (CDCl₃, ppm), δ: 3.61–3.57 (m, CH₂), 3.54–3.46 (m, CH₂O), 3.41–3.37 (m, CH), 1.14–1.11 (t, small, CH₃, OⁱPr), 1.09–1.08 (dd, CH₃), (Figure 2). ¹³C NMR, (CDCl₃, ppm), δ: for CH region: 79.25–79.13–79.00 (CH, triad, ii, is-si, ss), 72.23–72.08 (CH, diad, i, s, CHMe₂), 70.36, 70.10–69.99 (CH₂, diad, i, s), 68.47–68.39–68.29 (CH₂OⁱPr, triad, ii, is-si, ss), 22.28–22.26(d), 22.24–22.21–22.17(t), 22.00 (s).

Figure 2 ¹H NMR spectrum of polymers of IPGE obtained from polymerization with KOH at 75 °C.

2.6 Polymerization of IPGE with [(C₆F₄(COO)₂)₂Ti₄O₄(OⁱPr)₄]

The catalyst [35 mg, (C₆F₄(COO)₂)₂Ti₄O₄(OⁱPr)₄] was taken in a vial, and 1.2 mL of IPGE was added under nitrogen. The mixture was stirred at 75 °C for 2 and 3 days. ¹H NMR (CDCl₃, ppm), δ: 3.82 (m, small, CH), 3.75–3.20 (m, CH₂, CH₂O, OCH), 1.08 (s, CH₃). ¹³C NMR, (125 MHz,CDCl₃, ppm), δ: for CH region: 79.21–79.04–78.96 (CH, triad, ii, is-si, ss), 73.02–72.91 (CH, diad, i, s, CHMe₂), 72.27–72.12 71.96 (CH₂, triad, ii, is-si, ss), 69.17, 68.65–68.55- 68.33–68.23 (CH₂OⁱPr, diad, i, s), 22.12 (s, CH₃) (Figure 3).

Figure 3 ¹³C NMR spectrum of polymers of IPGE obtained from polymerization with (C₆F₄(COO)₂)₂Ti₄O₄(OⁱPr)₄ at 75 °C.

2.7 Polymerization of IPGE with [(C₆F₄(COO)₂)₂Sn₄O₅(O^tBu)₂]

The catalyst [35 mg, (C₆F₄(COO)₂)₂Sn₄O₅(O^tBu)₂] was taken in a vial, and 1.2 mL of IPGE was added under nitrogen. The mixture was stirred at 75 °C for 2 and 3 days. ¹H NMR (CDCl₃, ppm), δ: 3.83 (s, broad, CH), 3.75–3.20 (m, CH₂, CH₂O, OCH), 1.08 (s, CH₃), (Fig. Figure 4).¹³C NMR, (125 MHz, CDCl₃, ppm), δ: for CH region: 79.20–79.03–78.95 (CH, triad, ii, is-si, ss), 73.00–72.90 (CH, diad, i, s, CHMe₂), 72.28–72.13–71.98 (CH₂, triad, ii, is-si, ss), 69.18, 68.53–68.30 (CH₂OⁱPr, diad, i, s), 22.12 (s, CH₃).

Figure 4 ¹H NMR spectrum of polymers of IPGE obtained from polymerization with (C₆F₄(COO)₂)₂Sn₄O₅(O^tBu)₂ at 75 °C.

2.8 Polymerization of IPGE with [(C₆F₄(COO)₂)₂Al₄O₃(O^sBu)₂]

The catalyst [35 mg, (C₆F₄(COO)₂)₂Al₄O₃(O^sBu)₂] was taken in a vial, and 1.2 mL of IPGE was added under nitrogen. The mixture was stirred at 75 °C for 2 and 3 days. ¹H NMR (CDCl₃, ppm), δ: 3.70–3.20 (m, CH₂, CH₂O, CH), 1.09–1.08 (s, CH₃). ¹³C NMR, (125 MHz, CDCl₃, ppm), δ: for CH region: 79.22–79.10–78.98 (CH, triad, i, is-si, s), 73.01–72.90 (CH, diad, i, s, CHMe₂), 72.26–72.13–72.00 (CH₂, triad, ii, is-si, ss), 69.18, 68.53–68.30 (CH₂OⁱPr, diad, i, s), 22.12 (s, CH₃) (yield: 12%).

3 Results and discussion

Reactions of M(OR)_m (M-OR: Ti-OⁱPr, Sn-O^tBu, Al-O^sBu, m: 4 for Ti, Sn, m: 3 for Al) with tertafluorophthalic acid in 1:1 molar ratio in alcohol at room temperature gave the products of [(C₆F₄(COO)₂)₂Ti₄O₄(OⁱPr)₄], [(C₆F₄(COO)₂)₂Sn₄O₅(O^tBu)₂], and [(C₆F₄(COO)₂)₂Al₄O₃(O^sBu)₂], respectively. In the presence of strong acid like TFPA, some of the alkoxy groups undergo condensation to form oxo group. The amount of oxo group determined by elemental analysis and NMR measurement changes depending on reaction time and temperature. In the FTIR spectrum of tetrafluorophthalate–Ti(IV), Sn(IV), and Al(III) complexes the bands indicate presence of chelating carboxylic groups. The FTIR spectrum of free TFPA exhibits intense bands at 1730–1720, 1636, and 1424 cm^–1 corresponding to asymmetrical and symmetrical stretching vibrations of the carboxyl groups. After coordination of tetrafluorophthalate to metal alkoxides, the bands at high wave numbers shift to low wave numbers to approximately, 1630, 1520, 1478, and 1430 cm⁻¹. The carboxyl bands appear at 1630 cm⁻¹ for νCOO_asym and 1430 cm⁻¹ for νCOO_sym. Tetrafluorophthalate–Sn(IV) complex also includes monodentate carboxylic group at 1720 cm⁻¹. This can be related to the large size of tin atom. All of these values are consistent with those detected in a number of carboxylate-metal(IV or III) derivatives [13,15-22].

The ¹H NMR spectra of tetrafluorophthalate–Ti, Sn, and Al complexes show the expected peaks and peak multiplicities. For example, ¹H NMR spectrum of Ti-complex showed doublets at 1.04 ppm because of gem-dimethyl protons and a septet at 3.76 ppm for CH proton of isopropoxy groups in Ti-complex. The ¹³C NMR spectra of the metal complexes show shifts for the carboxylate carbons and C1-C2 resonances compared with that of the free tertafluorophthalic acid molecule. The single resonances at δ = 163.15, 163.10, and 161.78 ppm are attributed to the COO– groups in Ti, Sn, and Al complexes, respectively. The chemical shifts and coupling constants given in experimental section are in agreement with reported values [13,20].

Polymer samples obtained from ring-opening polymerization of IPGE were characterized by ¹H, ¹³C NMR, GPC, FTIR, and DSC. ¹³C DEPT, ¹³C{¹H}, and ¹H NMR spectra were obtained at ¹³C frequencies of 125 MHz and ¹H frequencies of 500 and 300 MHz. The ¹H NMR assignments of pure IPGE are as follows: CH₃: 0.92–0.96(t, OⁱPr), ring-CH₂: 2.35 (1H, dd), CH₂: 2.50 (1H, t), ring-CH: 2.89 (m), CH: 3.12–3.16(dd, OⁱPr), CH₂: 3.45 (m, CH₂OⁱPr) ppm. ¹H NMR spectrum shows that main change occurs at the CH and CH₂ protons of epoxy ring when IPGE is polymerized. The CH and CH₂ protons shift to higher values 3.20–3.83 ppm from 2.35–2.89 ppm. These values are consistent with those of GPTS and PO in the literature [13,23,24]. ¹H NMR spectrum of IPGE polymers (prepared with KOH) gives peaks at 3.61–3.57 (m, CH₂), 3.54–3.46 (m, CH₂OⁱPr), 3.41–3.37 (m, CH), 1.09–1.08 (dd, CH_3, OⁱPr). The proton NMR spectrum also includes small peaks at 2.35–2.89 ppm for monomer IPGE.

The ¹³C NMR spectrum of pure IPGE shows six resonances which are attributed to the carbon atoms as follows: 21.57, 21.78 (CH₃, OⁱPr), 43.97 (ring-CH₂), 50.69 (ring-CH), 68.62 (CH₂O), 71.68 (CH, OⁱPr) ppm. The CH, CH₂, and OCH₃ carbons of IPGE polymers are easily determined from ¹³C DEPT spectra. ¹³C DEPT-135 gives all CH and CH₃ in a phase (peaks up) opposite to CH₂ (peaks down). ¹³C NMR spectrum of IPGE polymers includes more peaks than that of pure IPGE. ¹³C NMR data of IPGE polymers were given in the experimental section. For example, as seen from the ¹³C NMR data of IPGE polymers prepared with KO^tBu catalyst, the regioisomer of –(CHRCH₂O) _n – was formed. The methine carbon resonances in ¹³C NMR spectrum at 79.07–78.96–78.84 ppm region appear as three signals readily assignable to the triads (ii, is-si, ss). This is attributed to different stereosequences due to its asymmetric methine carbon. The side propoxymethyl group of IPGE has similar stereoselective effect on stereosequences when compared to the methyl group of PO and trimethoxysilylpropoxymethyl group of GPTS [7,13,23,24].

When KOH and KO-t-Bu are used as catalysts, the reaction pathway leading to polymers formation can be shown in Scheme 1. The strong nucleophiles OH⁻ and t-BuO⁻ ions attack CHR carbon atom as seen in Scheme 1 (c. 95%).

In addition to the main regioregular triads, additional regioregular polymer is possible with small intensity (as seen from ¹³C NMR spectrum, <5%) due to CH₂–O bond cleavage.

When tetrafluorophthalate–Ti, Sn, and Al catalysts were used instead of OR⁻ catalysts, the maximum recovery rate of polymer and the maximum polymerization rate of IPGE occur at 75 °C. The conversion of monomer IPGE to polymer was followed by GPC measurements. The polymerization rate of GPTS decreases in the order of KO-t-Bu > KOH > [(C₆F₄(COO)₂)₂Ti₄O₄(OⁱPr)₄] > [(C₆F₄(COO)₂)₂Sn₄O₅(O^tBu)₂] > [(C₆F₄(COO)₂)₂Al₄O₃(O^sBu)₂] catalysts. The reaction pathway leading to polymers formation can be shown in Scheme 2 when tetrafluorophthalate–Ti, Sn, and Al complexes are used as catalysts.

Experimental data support that the alkoxide groups bound to metal are active groups to ring opening of epoxide. Catalysts without alkoxide groups prepared at high temperature were not active to ring opening of epoxide. As seen from the ¹³C NMR data of IPGE polymers prepared with tetrafluorophthalate–Ti complex, both regioisomers are formed but that the major regioisomer –(OCH₂CHR) _n – predominates by c. 9:1 (or major product 90%) (R:CH₂OCHMe₂). When both OR⁻ and tetrafluorophthalate metal complexes were used as catalysts, there were similarities in region regularity of IPGE polymers.

As seen from ¹³C NMR spectrum (Figure 3), the methine carbon shows three peaks due to triad sensitivity. The higher methine peak at 79.21 ppm corresponds to the isotactic triad as in the literature for poly-PGE [12]. The heterotactic(is-si) triads appear at 79.04 ppm as a single peak because of their close chemical shifts. The peak at 78.96 ppm is attributed to the syndiotactic triad. The main chain methylene carbon appears as three signals which correspond to a triad sensitivity. The peak at 72.27 ppm is attributed to the isotactic triad. The heterotactic(is-si) triads appear at 72.12 ppm as a single peak. The peak at 71.96 ppm is attributed to the syndiotactic triad. The number of methyl peaks (in ¹H, ¹³C NMR spectra) of isopropoxide group bound to glycidyl shows that tetrafluorophthalate-metal catalysts are more stereoselective in polymerization of IPGE.

These data suggest that first IPGE attacks to Ti center and then the nucleophile OⁱPr⁻ ion attacks CHR carbon atom in IPGE as seen in Scheme 2. When reaction time is increased from three days to five days, just molecular weight of polymer increases without any change in regio- and stereoselectivity. Tetrafluorophthalate metal complexes produce low molecular weight polymers as seen from Table 1.

Table 1. Data for IPGE polymers obtained from GPC measurements.

Catalyst	T (°C)	Time (days)	M w	M n	M w/M n	Conversion (%)
KO^tBu	RT	2	3989	3115	1.28	17
KO^tBu	50	1	19,427, 8816	18,518, 8539	1.05, 1.03	63
KO^tBu	50	2	27,517, 11,852, 5642	26,778, 11,743, 5439	1.03, 1.01, 1.04	84
KOH	75	2	19,632, 6575	18,989, 5775	1.03, 1.14	64
Tetrafluorophthalate–Ti	75	3	2775, 770,552	1871, 767,550	1.48, 1.00, 1.00	44
Tetrafluorophthalate–Sn	75	3	2221, 585, 406	1822, 583, 404	1.22, 1.00, 1.01	31
Tetrafluorophthalate–Al	75	3	647, 423	647, 421	1.02, 1.00	12

GPC was also used to determine molecular weight and the molecular weight distribution index of polymers. By varying the reaction time and catalyst, the polymers with different average molecular weights or number average molecular weights were obtained (Figures Figure 5 and Figure 6). Polymers of IPGE prepared with KO^tBu by stirring at 50 °C for 2 days, the main peak appeared at weight average molecular weight (M _w) 27,517 Da or number average molecular weight (M _n) 26,778 Da. The ratio of the weight average molecular weight to the number average molecular weight (M _w/M _n) was 1.03. Data for IPGE polymers prepared at different times and temperatures are given in Table 1. As seen from Table 1, polymers prepared at room temperature have lower molecular weight and smaller molecular weight distribution index for KO^tBu catalyst. When the temperature increases to 50 °C, the molecular weights and the conversion of monomer to polymer increase.

Figure 5. GPC of IPGE polymers prepared at 50 °C with KO^tBu.

Figure 6. GPC of IPGE polymers prepared at 75 °C with (C₆F₄(COO)₂)₂Ti₄O₄(OⁱPr)₄.

The stereosequence of IPGE polymers prepared with OH⁻ and t-BuO⁻ was the same for 50 and 75 °C. Polymers of IPGE prepared with tetrafluorophthalate–Ti complex by stirring at 75 °C for 3 days, the main peak appeared at weight average molecular weight (M _w) 2775 Da or number average molecular weight (M _n) 1871 Da. The ratio of the weight average molecular weight to the number average molecular weight (M _w/M _n) was 1.48. There were also small peaks of average molecular weight at 770 and 767 Da (with c. M _w/M _n 1.0). These results indicate that the polymerization has a characteristic of living polymerization. The polymer structure can be further verified by DSC analysis. According to the DSC test, IPGE polymers showed a single glass transition temperature (Tg) below −33 °C. Tg was taken as the midpoint of the glass transition.

FTIR studies support the polymerization of IPGE. The monomer of IPGE shows characteristic band at 1255 cm⁻¹ (epoxide ring breathing). When IPGE undergoes polymerization, the peak at 1255 cm⁻¹ disappears in the FTIR spectrum. This disappearance supports the fact that the epoxy ring of IPGE undergoes a ring-opening reaction [13,25]. These all data support the polymerization of IPGE.

Scheme 1 Attacking of R₁O⁻ to CHR carbon of IPGE.

The reactivity of epoxides towards OH⁻ and O^tBu⁻ ions is investigated by both ¹H NMR spectroscopy and GPC. The polymerization rate of epoxides decreases in the order of PO > GPTS > IPGE. In basic condition (OH⁻), PO and GPTS undergoes the ring-opening polymerization reactions at room temperature for one day with high conversion 95 and 35%, respectively. However, IPGE does not undergo the ring-opening reaction with OH⁻ ions at room temperature. IPGE undergoes the ring-opening polymerization at 75 °C for 2 days with just 64% conversion. This inertness towards basic catalysts can be attributed to both electronic and steric effects of the side isopropoxymethyl group of isopropylglycidyl.

Scheme 2 Attacking of IPGE to Ti center of catalyst tetrafluorophthalate–Ti complex.

4 Conclusion

The five catalysts used in this study are effective for the polymerization of IPGE and have somewhat different influence on the selective synthesis of polymers. Mainly one regioisomer is formed due to the ring opening of epoxide from CHR–O bond. The structures of polymers were characterized by NMR, FTIR, GPC, and DSC analysis. The side isopropoxymethyl group of IPGE has similar stereoselective effect on stereosequences when compared to the methyl group of PO. Isotactic, iso/syn-tactic, and syndiotactic polymers are obtained. As seen from ¹H- and ¹³C NMR spectra and experimental data, there are less peaks in methyl (OCHMe₂) region when tetrafluorophthalate metal catalysts were used as catalysts. This is evidence that tetrafluorophthalate-metal catalysts are somewhat more stereoselective in polymerization of IPGE.

The polymerization rate of IPGE decreases in the order of KO-t-Bu > KOH > [(C₆F₄(COO)₂)₂Ti₄O₄(OⁱPr)₄] > [(C₆F₄(COO)₂)₂Sn₄O₅(O^tBu)₂] > [(C₆F₄(COO)₂)₂Al₄O₃(O^sBu)₂] catalysts. When tetrafluorophthalate–Al complex was used as catalyst, slow polymerization was performed and lower molecular weight polymers were obtained. GPC measurements show that the polymerization has a characteristic of living polymerization with tetrafluorophthalate metal catalysts. Low molecular weight polymers of IPGE were prepared by using tetrafluorophthalate-metal catalysts. The polymerization rate of epoxides decreases in the order of PO > GPTS > IPGE.

Acknowledgement

This work was supported by the research foundation of Kocaeli University (Project No: 2009/034).