Versatile metal complexes of 2,5-bis{N-(2,6-di isopropylphenyl)iminomethyl}pyrrole for epoxide–CO₂ coupling and ring opening polymerization of ε-caprolactone

Abstract

The catalytic activities of metal complexes of 2,5-bis{N-(2,6-diisopropylphenyl)iminomethyl}pyrrole 1 for epoxide–CO₂ coupling and ring opening polymerization (ROP) of ε-caprolactone (CL) were explored. The Zn(II) 2, Cd(II) 3,4 and Cu(II) 5 complexes of 1, in the presence of tetrabutylammonium bromide (TBAB) as co-catalyst, effectively catalyzed the epoxide–CO₂ coupling reaction at 1 atm of CO₂ to produce cyclic carbonates. In addition, the catalytic activities of these complexes (2–5) were investigated for the ROP of CL in the presence and absence of benzyl alcohol (BnOH). All these complexes were active for ROP of CL at mild temperature. In particular, the complexes 2 and 5 exhibited remarkable catalytic activities for ROP of CL at 25 °C in toluene in the presence of BnOH. The ROP of CL catalyzed by 2 and 5 complexes, not only proceeded through living polymerization but also immortal.

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Introduction

Over the past two decades, there has been much interest in producing biodegradable and biocompatible materials using renewable resources such as CO₂ and carbohydrates.¹ For example, cyclic and polycarbonates were synthesized by epoxide–CO₂ coupling reactions.² Likewise, poly(ε-caprolactone) (PCL) and polylactide (PLA) were prepared by ROP of ε-caprolactone (CL) and lactide (LA) respectively. The CL and LA can be produced from carbohydrates – D-fructose and starch respectively.⁴ Cyclic and polycarbonates are used as electrolytes, aprotic polar solvents, packaging materials and exterior thermoplastics for engineering parts.³ Because of the biocompatibility, PCL is used in pharmaceutical and biomedical fields, especially as scaffolds in tissue engineering and drug delivery and used as packaging material.⁵ Therefore, it is necessary to control the properties of PCL for a wide utility.

The polymer properties like molecular weight, polydispersity index (PDI), and microstructure are good when PCL is prepared by a ROP rather than through a condensation polymerization.⁶ The ROP is performed in the presence of metal catalysts/initiators, organocatalysts or enzymes. Complexes of alkaline,⁷ alkaline-earth,⁸ Al,⁹ Ti,¹⁰ Zr,¹¹ V,¹² Cr,¹³ Fe,¹⁴ Co,¹⁵ Zn,¹⁶ Sn,¹⁷ Bi¹⁸ and rare earth metals¹⁹ with various ligands including ketiminate, β-diketiminates, imine phenoxide, amine phenoxide, anilido-imine and cyclopentadienyl are used as catalysts for the ROP of CL. Among these catalysts, Mg, Al, Zn, Sn and rare earth metal complexes displayed moderate to excellent catalytic activities by yielding the polymers with narrow polydispersities and good thermal stabilities. These reports explain the catalytic properties of metal complexes can be altered by changing ligands and the electronic and steric features on the ligands.^9b,20 Therefore, finding a suitable ligand is also important to achieve needed polymer properties.

Although many catalysts are independently known for epoxide–CO₂ coupling and for ROP of CL, there is always demand to find an efficient, versatile catalyst with improved efficiency. For example, Al and Cr based tetraphenylporphyrinato complexes; Al, Cr and Co based salen complexes are efficient for both epoxide–CO₂ coupling and ROP of CL.^2c Therefore, we explored the Zn(II), Cd(II) and Cu(II) complexes of 2,5-bis{N-(2,6-diisopropylphenyl)iminomethyl}pyrrole 1 (Fig. 1) as catalysts for the efficient epoxide–CO₂ coupling and ROP of CL. Bis(aryliminomethyl)pyrrole derivatives are much attractive ligands because of their diverse bonding modes and easy coordination with metals. Changing the substituents on these ligands would tune electronic environment, also influence the preferential formation of particular structure of metal complexes.²¹

Fig. 1 Structure of the ligand 1 and its complexes 2–5.

Results and discussion

Synthesis and properties

Recently, we reported efficient catalytic activities of Zn(II) 2, Cd(II) 3,4 and Cu(II) 5 complexes of the ligand 1 (Fig. 1).²² All these metal complexes demonstrated good catalytic activities for coupling of epoxide and CO₂ (1 atm) to give cyclic carbonates in the presence of TBAB under mild temperature. They were highly effective for conversion of monosubstituted terminal epoxides, disubstituted terminal and internal epoxides. The present work is to prove the versatility of these catalysts (2–5) for ROP of CL. For the present study, the ligand 1 was prepared by following a literature procedure through a condensation reaction of pyrrole-2,5-dicarbaldehyde with 2,6-diisopropylaniline.^19a The metal complexes were prepared according to our earlier reported procedures.²²

The UV-visible absorption spectrum of ligand 1 revealed absorptions at 325 and 317 nm, corresponding to n–π* and π–π* transitions (Fig. 2). The complexes (2–5) showed absorption similar to ligand 1 without any shift of absorption maxima. The complex 5 absorbed also at 378 nm because of metal to ligand charge transfer (MLCT). This was confirmed from the spectra recorded using solvents of varying polarities. It was observed that the absorption maximum was also varying.²³ Thermogravimetric analyses (Fig. 3) revealed the complexes were highly stable and their decomposition temperatures ranged between 330 and 370 °C. Despite having same ligands, the Cd(II) complexes 3 and 4 showed a difference in their thermal stabilities because of dissimilarity of the structures.

Fig. 2 UV visible absorption spectra of 1 and its complexes 2–5 in hexane.

Fig. 3 TGA plots of 1 and its complexes 2–5.

Epoxide–CO₂ coupling

To explore the broad utility of our metal complexes 2–5, cycloaddition of monosubstituted phosphorus containing epoxide 2-{[(diphenylphosphoryl)methoxy]methyl}oxirane 6 with CO₂ was investigated in the presence of TBAB as co-catalyst (Scheme 1). For good yields from the reactions, various molar ratios of catalyst and co-catalyst to 6 were optimized and fixed as 2.5 mol% and 5 mol% respectively. The results of these reactions are summarized in the Table 1. Controlled reactions in the presence and absence of catalyst or TBAB showed the importance of both for effective reactions (Table 1, entries 1–4). Though few reports were on TBAB catalyzed cycloaddition, our catalysts improved the efficiency of the reactions. Thus, the compound 6 was efficiently converted to 4-{[(diphenylphosphoryl)methoxy]methyl}-1,3-dioxolan-2-one 7 (yield 84%) at 105 °C within 3 h in the presence of 2 and TBAB under 1 atm of CO₂ (Table 1, entry 4). Similarly, the complex 5 catalyzed the reaction effectively with a maximum yield of 93% within 5 h (Table 1, entry 11).

Scheme 1 Synthesis of phosphorus containing cyclic carbonate 7.

Table 1 Screening studies for the synthesis of 7^a

Entry	Catalyst	Co-catalyst	T (°C)	Time (h)	Yield^b (%)
a Reaction condition: epoxide 6 (1 mol), catalyst (2.5 mol%), co-catalyst (5 mol%), CO2 (1 atm, balloon).b Purification by flash column chromatography.
1	2	—	105	3	—
2	—	TBAB	105	3	31
3	2	TBAB	60	20	42
4	2	TBAB	105	3	84
5	2	DMAP	60	20	41
6	2	DMAP	105	3	76
7	2	PPNCl	60	20	38
8	2	PPNCl	105	3	74
9	3	TBAB	105	3	59
10	4	TBAB	105	3	54
11	5	TBAB	105	5	93
12	5	DMAP	105	5	82
13	5	PPNCl	105	5	78

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To find a better combination, the cycloaddition reactions were tested using 4-dimethylaminopyridine (DMAP) and bis(triphenylphosphine)iminium chloride (PPNCl) as co-catalysts. The Table 1 displays all these results (entries 5–8). Among the combinations of 2, 2/TBAB delivered the best result. Similarly, among the combinations of 5, 5/TBAB gave the maximum yield (Table 1, entries 11–13). It is worth stating here is since the compound 7 is a cyclic carbonate having phosphorus atom within the molecule, it would give flame-retardant property when used as an electrolyte in secondary batteries.

Ring opening polymerization

The ligands having only nitrogen donors exhibited better catalytic activity for ROP than the ligands with both nitrogen and oxygen donors.^1a The metal complexes 2–5 comprising pyrrolyl ligands produced in our lab possessed only nitrogens as donating atoms. Therefore, we explored the catalytic activities of complexes 2–5 for ROP of CL under various conditions for a detailed study. We have carried out the ROP in two different methods, viz., bulk polymerization (BP-ROP) and solution polymerization (SP-ROP).

Bulk and solution phase ring opening polymerization (BP-ROP and SP-ROP)

First, we examined the ROP of CL using 2 as a catalyst without alcohol as initiator and solvent (BP-ROP) (Scheme 2). The Table 2 illustrates results of these reactions. By varying catalyst loading, temperature and time, the reaction conditions were optimized to achieve good yield and higher molecular weight with narrow PDI (Table 2, entries 1–7). Among different catalyst loadings tried (Table 2, entries 1–5), the ratio 100 : 1 ([CL]₀ : [C]₀) yielded a polymer with moderate properties (Table 2, entry 3). The BP-ROP of CL was also examined using catalysts 3, 4 and 5 at different temperatures in neat conditions (Table 2, entries 8–16). The reaction proceeded faster in the presence of 2 than other catalysts (3–5) as noted. The SP-ROP of CL was also carried out in toluene in the presence of 2–5 (but without the alcohol) at different temperatures (Table 3). Few reactions were performed by varying catalyst loadings (Table 3, entries 1–5) of 2, and also changing the catalysts to 3–5 (Table 3, entries 7–12). Conversion in SP-ROP was roughly equal to that in BP-ROP (Table 2). But, the PDI became narrower while performing the reaction in a solvent (Table 3). Based on the data listed in Tables 2 and 3, the catalytic activity of complexes for BP-ROP and SP-ROP followed the order 2 > 5 > 4 > 3.

Scheme 2 ROP of CL.

Table 2 Optimization studies for BP-ROP of CL^a

Entry	Catalyst (C)	[CL]0 : [C]0	T (°C)	Time (h)	Conversion^b (%)	Mn^c (exp)	Mn^d (theo)	PDI^e
a Reaction condition: reactions were carried out in neat condition (without solvent).b Obtained from ^1H NMR.c Molecular weight measured by GPC and calibrated against polystyrene standard.d Calculated from ([CL]0/[C]0) × 114.14 × conversion.e Obtained from GPC analysis.
1	2	100 : 0.1	25	8	8	10014	9131	1.32
2	2	100 : 0.5	25	8	24	6358	5478	1.27
3	2	100 : 1	25	8	71	10421	8104	1.30
4	2	100 : 1.5	25	8	73	6145	5554	1.28
5	2	100 : 2	25	8	78	5223	4451	1.26
6	2	100 : 1	60	5	82	10968	9359	1.32
7	2	100 : 1	100	2	89	11654	10158	1.38
8	3	100 : 1	25	8	57	7863	6506	1.43
9	3	100 : 1	60	5	66	9028	7533	1.51
10	3	100 : 1	100	2	69	8956	7876	1.56
11	4	100 : 1	25	8	60	7912	6848	1.39
12	4	100 : 1	60	5	71	9755	8104	1.45
13	4	100 : 1	100	2	75	9881	8561	1.48
14	5	100 : 1	25	8	62	7832	7076	1.35
15	5	100 : 1	60	5	74	9848	8446	1.42
16	5	100 : 1	100	2	77	9864	8788	1.46

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Table 3 Optimization studies for SP-ROP of CL^a

Entry	Catalyst (C)	[CL]0 : [C]0	T (°C)	Time (h)	Conversion^b (%)	Mn^c (exp)	Mn^d (theo)	PDI^e
a Reaction condition: reactions were carried out in toluene, [CL]0 = 0.5 M.b Obtained from ^1H NMR.c Molecular weight measured by GPC and calibrated against polystyrene standard.d Calculated from ([CL]0/[C]0) × 114.14 × conversion.e Obtained from GPC analysis.
1	2	100 : 0.1	25	8	7	8726	7990	1.29
2	2	100 : 0.5	25	8	22	5873	5022	1.18
3	2	100 : 1	25	8	69	8692	7876	1.15
4	2	100 : 1.5	25	8	72	5994	5479	1.14
5	2	100 : 2	25	8	78	5017	4451	1.14
6	2	100 : 1	60	5	84	9859	9588	1.18
7	3	100 : 1	25	8	54	6983	6164	1.24
8	3	100 : 1	60	5	71	9027	8104	1.28
9	4	100 : 1	25	8	59	7521	6734	1.21
10	4	100 : 1	60	5	76	9864	8675	1.26
11	5	100 : 1	25	8	60	7198	6848	1.18
12	5	100 : 1	60	5	79	9824	9017	1.24

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Significance of alcohols as initiator

It is also important to probe the role of alcohol in ROP because a closer look in literature showed an alcohol was used as initiator in various occasions. To understand the significance of alcohol as initiator, we conducted ROPs in the presence of various alcohols using 2 or 5 at 25 °C (Table 4). The rate of SP-ROP in toluene initiated by BnOH and catalyzed by 2 was remarkably high even at 25 °C (Table 4, entry 5). The reaction proceeded in a controlled manner and achieved good control in PDI (1.12) with an improved molecular weight of producing polymers. In the presence of EtOH, good conversion achieved whereas lesser control in molecular weight distribution noted. However, i-PrOH yielded polymers with similar conversion rate but with narrow PDI. Because of steric hindrance and lesser nucleophilicity of t-BuOH, lesser conversion rate for CL to polymer was witnessed. The trend implied that the ROP was accelerated by an alcohol having more nucleophilic character and moderate steric feature. These results encouraged us to perform BP-ROP and SP-ROP in the presence of BnOH for a detailed study.

Table 4 SP-ROP of CL in presence of various initiators^a

Entry	Catalyst (C)	Initiator (I)	Conversion^b (%)	Mn^c (exp)	Mn^d (theo)	PDI^e
a Reaction condition: [CL]0 : [C]0 : [ROH]0 = 100 : 1 : 1 in toluene at 25 °C for 6 h. [CL]0 = 0.5 M.b Obtained from ^1H NMR.c Molecular weight measured by GPC and calibrated against polystyrene standard.d Calculated from ([CL]0/[C]0) × 114.14 × conversion.e Obtained from GPC analysis.
1	2	MeOH	74	9054	8446	1.23
2	2	EtOH	89	10947	10158	1.30
3	2	i-PrOH	87	10321	9930	1.15
4	2	t-BuOH	68	8798	7762	1.29
5	2	BnOH	98	11956	11185	1.12
6	5	MeOH	54	6826	6164	1.21
7	5	EtOH	71	9773	8104	1.28
8	5	i-PrOH	70	8434	7990	1.20
9	5	t-BuOH	52	6967	5935	1.27
10	5	BnOH	80	9928	9359	1.19

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Effect of BnOH in BP-ROP and SP-ROP

First, the BP-ROP of CL was tested using BnOH as initiator at different temperatures using 2–5 (Table 5). The molar ratio of [CL]₀ : [C]₀ : [BnOH]₀ used for reactions was varied and optimized as 100 : 1 : 1 (Table 5, entries 1–5). When BP-ROP was performed without BnOH, the PDI of the resulting polymers were broad (Table 2). But, the reactions initiated by BnOH progressed much faster and yielded polymers with fairly narrow PDI (Table 5). The catalysts 2–5 were soluble in CL that promoted for increased rate of reactions and good conversions even in neat condition. Despite higher reaction temperatures (60 and 100 °C), PDI of resulting polymers was within a narrow range probably because of minimum intermolecular and intramolecular trans esterification reactions. This represented that the steric hindrance around the metal center played an important role in ROP by preventing chain aggregation/transfer. So the reactions maintained selectivity at higher temperatures also. Thus, bulky groups were not only helpful to protect the metal center but also in controlling PDI.

Table 5 Optimization studies for BP-ROP of CL in the presence of BnOH^a

Entry	Catalyst (C)	[CL]0 : [C]0 : [BnOH]0	T (°C)	Time (h)	Conversion^b (%)	Mn^c (exp)	Mn^d (theo)	PDI^e
a Reaction condition: reactions were carried out in neat condition (without solvent).b Obtained from ^1H NMR.c Molecular weight measured by GPC and calibrated against polystyrene standard.d Calculated from ([CL]0/[C]0) × 114.14 × conversion.e Obtained from GPC analysis.
1	2	100 : 0.1 : 0.1	25	6	8	10006	9131	1.24
2	2	100 : 0.5 : 0.5	25	6	31	7953	7070	1.21
3	2	100 : 1 : 1	25	6	94	10965	10729	1.19
4	2	100 : 1.5 : 1.5	25	6	95	8086	7229	1.18
5	2	100 : 2 : 2	25	6	95	5848	5422	1.16
6	2	100 : 1 : 1	60	3	90	10876	10272	1.23
7	2	100 : 1 : 1	100	2	92	11257	10500	1.29
8	3	100 : 1 : 1	25	6	78	9356	8903	1.20
9	3	100 : 1 : 1	60	3	80	10083	9131	1.25
10	3	100 : 1 : 1	100	2	75	9943	8561	1.31
11	4	100 : 1 : 1	25	6	71	9037	8104	1.22
12	4	100 : 1 : 1	60	3	76	9198	8674	1.26
13	4	100 : 1 : 1	100	2	73	9065	8332	1.33
14	5	100 : 1 : 1	25	6	83	10523	9474	1.20
15	5	100 : 1 : 1	60	3	81	9725	9245	1.24
16	5	100 : 1 : 1	100	2	78	9421	8903	1.29

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The SP-ROP of CL in toluene in the presence of BnOH and 2–5 was examined at different temperatures. The results (Table 6) revealed the polymerization initiated by BnOH progressed rapidly in toluene and achieved maximum conversion with excellent control in PDI. In the presence of BnOH, efficiency of complexes in catalyzing BP-ROP and SP-ROP of CL was in the order 2 > 5 > 3 > 4 which was different from reactions conducted without BnOH. Without initiator, the complex 4 displayed better conversion than the complex 3. This was because the complex 4 had a solvent molecule methanol within a structure which probably acted as an initiator speeding up the reaction rate. With initiator, the complex 3 achieved more conversion than the complex 4. In BP-ROP and SP-ROP conducted in the presence of BnOH and 4, it seems a competition between methanol and BnOH to initiate the reaction impeded the reaction rate. Catalytic activities of few homoleptic zinc complexes with multi-dentate nitrogen containing ligands for ROP were reported earlier.²⁴ Compared with those complexes, the complexes 2–5 displayed high catalytic activity for the ROP of CL under mild temperature.

Table 6 Optimization studies for SP-ROP of CL in the presence of BnOH^a

Entry	Catalyst (C)	[CL]0 : [C]0 : [BnOH]0	T (°C)	Time (h)	Conversion^b (%)	Mn^c (exp)	Mn^d (theo)	PDI^e
a Reaction condition: reactions were carried out in toluene, [CL]0 = 0.5 M.b Obtained from ^1H NMR.c Molecular weight measured by GPC and calibrated against polystyrene standard.d Calculated from ([CL]0/[C]0) × 114.14 × conversion.e Obtained from GPC analysis.
1	2	100 : 0.1 : 0.1	25	6	9	11014	10273	1.19
2	2	100 : 0.5 : 0.5	25	6	41	10092	9360	1.14
3	2	100 : 1 : 1	25	6	98	11956	11185	1.12
4	2	100 : 1.5 : 1.5	25	6	98	7844	7457	1.12
5	2	100 : 2 : 2	25	6	98	5958	5593	1.10
6	2	100 : 1 : 1	60	3	98	12145	11185	1.14
7	3	100 : 1 : 1	25	6	64	7956	7305	1.19
8	3	100 : 1 : 1	60	5	76	9268	8674	1.20
9	4	100 : 1 : 1	25	6	57	7914	6506	1.21
10	4	100 : 1 : 1	60	5	65	8547	7419	1.24
11	5	100 : 1 : 1	25	6	80	9928	9359	1.19
12	5	100 : 1 : 1	25	9	95	11423	10843	1.19
13	5	100 : 1 : 1	60	5	85	9680	9701	1.16

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Mechanism of SP-ROP

For all polymers, the molecular weight [M_n(exp)] measured by GPC matched well with the molecular weight [M_n(theo)] calculated from initial monomer/catalyst ratio. Also, their PDIs were in narrow ranges. These observations exposed the control in ROP and the nature of catalysis was single site catalysis. Complexes 2 and 5 were utilized for studying the mechanism of polymerization since they showed good catalysis yielding polymers with narrow PDI. To understand the order of ROP, reactions were monitored with time in the presence of either 2 or 5 and BnOH at 25 °C. Semi logarithmic plot of ln[CL]₀/[CL]_t versus time revealed linearity proving the first order dependence of polymerization on monomer concentration (Fig. 4).

Fig. 4 Plot of ln([CL]₀/[CL]_t) versus time for the polymerization of CL catalyzed by 2 and 5 in presence of BnOH. Reaction condition: [CL]₀ : [C]₀ : [BnOH]₀ = 100 : 1 : 1 in toluene at 25 °C, [CL]₀ = 0.5 M.

Few reactions were conducted to check the living character of polymerization by changing CL/C molar ratios (Table 6, entries 3 and 12; Table 7, entries 1–6). Increasing CL/C molar ratio increased the molecular weights of resultant polymers but the polymers were obtained with narrower PDI (in the presence of 2, PDI = 1.12–1.20; in the presence of 5, PDI = 1.19–1.29). Further, the molecular weights of resultant polymers were increased linearly with increasing monomer conversion which inferred that the polymerization proceeded through a living polymerization manner (Fig. 5). The living polymerization behaviour was further confirmed from the linear relationship of molecular weight with the initial monomer/catalyst ratio [CL]₀/[C]₀ (Fig. 6).

Table 7 Results of living and immortal SP-ROP of CL^a

Entry	Catalyst (C)	[CL]0 : [C]0 : [BnOH]0	Conversion^b (%)	Mn^c (exp)	Mn^d (theo)	PDI^e
a Reaction condition: reactions were carried out in toluene at 25 °C either for 6 h (2) or 9 h (5), [CL]0 = 0.5 M.b Obtained from ^1H NMR.c Molecular weight measured by GPC and calibrated against polystyrene standard.d Calculated from ([CL]0/[C]0) × 114.14 × conversion or ([CL]0/[BnOH]0) × 114.14 × conversion.e Obtained from GPC analysis.
1	2	200 : 1 : 1	86	20145	19632	1.15
2	2	300 : 1 : 1	81	29654	27736	1.18
3	2	400 : 1 : 1	74	34043	33785	1.20
4	5	200 : 1 : 1	82	19854	18719	1.22
5	5	300 : 1 : 1	73	26225	24996	1.25
6	5	400 : 1 : 1	68	32852	31046	1.29
7	2	100 : 1 : 2	97	6012	5535	1.12
8	2	100 : 1 : 4	98	2958	2796	1.08
9	2	100 : 1 : 6	98	2160	1863	1.06
10	5	100 : 1 : 2	93	5754	5307	1.16
11	5	100 : 1 : 4	94	2815	2682	1.12
12	5	100 : 1 : 6	95	1967	1806	1.10

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Fig. 5 Plot of molecular weight (M_n) and polydispersity (PDI) versus monomer conversion for the ROP of CL catalyzed by 2 (top) and 5 (down) in presence of BnOH. Reaction condition: [CL]₀ : [C]₀ : [BnOH]₀ = 100 : 1 : 1 in toluene at 25 °C, [CL]₀ = 0.5 M.

Fig. 6 Plot of molecular weight (M_n) and polydispersity (PDI) versus [CL]₀/[C]₀ for the ROP of CL catalyzed by 2 (top) and 5 (down) in presence of BnOH. Reaction condition: [CL]₀ : [C]₀ : [BnOH]₀ = 100 : 1 : 1 in toluene at 25 °C, [CL]₀ = 0.5 M.

The ROP was examined by varying CL/BnOH molar ratios ([CL]₀/[BnOH]₀) in the presence of 2 or 5 (Table 6, entries 3 and 12; Table 7, entries 7–12). The decreasing [CL]₀/[BnOH]₀ ratio decreased the molecular weights of polymers and they were obtained with narrow PDI (2, 1.06–1.12; 5, 1.19–1.10). These studies revealed the immortal character of ROP of CL. The ROP in the presence of 2 using BnOH and toluene was progressing by coordination–insertion mechanism. The ¹H NMR spectrum of PCL is shown in Fig. 7. The presence of phenyl ring protons (a, 7.37 ppm) and CH₂ (b, 5.12 ppm) signal of BnO group suggested that the polymerization was initiated by the insertion of BnO group to CL.

Fig. 7 ¹H NMR spectrum of PCL. Reaction condition: [CL]₀ : [C]₀ : [BnOH]₀ = 100 : 1 : 1 in toluene at 25 °C, 6 h, [CL]₀ = 0.5 M.

Conclusion

The Zn(II), Cd(II) and Cu(II) complexes of 2,5-bis{N-(2,6-diisopropylphenyl)iminomethyl}pyrrole (2–5) were examined as catalyst to produce cyclic carbonates from renewable resources and ROP of CL. The complexes were useful to synthesize phosphorus containing (as a pendant group) cyclic carbonate from its epoxide and CO₂ (1 atm). Further, all these complexes were active for ROP of CL, among them 2 and 5 complexes were highly efficient.

The BP-ROP of CL yielded polymers with broad PDI compared with SP-ROP route. In the presence of BnOH, temperature and time of BP-ROP and SP-ROP of CL was lesser than the polymerization performed without BnOH. Complete study on the polymerization behavior revealed, the ROP of CL progressed in a controlled, living fashion and with immortal character. The applications on epoxide–CO₂ coupling and ROP of CL proved the versatility of ligand 1 and its complexes (2–5).

Experimental section

Materials and instrumentation

All manipulations involving air and moisture sensitive compounds were carried out using standard Schlenk techniques under dry nitrogen. The solvents, toluene, methanol, dimethylacetamide (DMAc) and chloroform were dried and purified using suitable drying agents, sodium benzophenone ketyl, Mg(OMe)₂ and CaCl₂ respectively followed by distillation under nitrogen. The compounds, 2,5-bis{N-(2,6-diisopropylphenyl)iminomethyl}pyrrole,^19a 2–5²² and (diphenyl phosphoryl)methanol²⁵ were synthesized by following procedures reported in literature. ε-Caprolactone, BnOH (Aldrich) and epichlorohydrin (SRL) were used without any further purification. Tetrabutylammonium bromide (Merck) was dried at 50 °C for 4 h under vacuum prior to use.

All products were confirmed by ¹H, ¹³C, ³¹P NMR and HRMS data. The NMR spectra were recorded using a Bruker Avance 400 MHz FT NMR spectrometer at room temperature in CDCl₃. The data are reported in parts per million (δ) with reference to TMS or 85% H₃PO₄ as appropriate. HRMS spectra were recorded on a Bruker maXis instrument using ESI technique. The GPC measurements were performed on a JAI NEXT series recycling preparative HPLC equipped with UV and RI detector. The separation was achieved using a JAIGEL 3H column (600 × 20 mm) operated at 25 °C with a flow rate of 3.5 mL min⁻¹ using chloroform as the eluent. The molecular weight and molecular weight distributions were calculated using polystyrene as a standard.

Experimental procedure

Synthesis of 2-(((diphenylphosphoryl)methoxy)methyl)oxirane (6). Sodium hydride (0.10 g, 1.1 mmol) was added to a solution of (diphenylphosphoryl)methanol (0.50 g, 1 mmol) in DMAc (6.0 mL) at 0 °C under nitrogen. The mixture was stirred for 30 min. at 0 °C. To this, epichlorohydrin (0.34 mL, 2 mmol) was added drop wise. The reaction mixture was then allowed to warm to 25 °C within 1 h. After 5 h stirring, methanol (1 mL) was added and the reaction mixture was poured into water (25 mL). The reaction mixture was neutralized with dilute HCl and extracted with ethyl acetate (30 mL × 3). The organic layer was washed with water (20 mL × 2). The solvent was evaporated to give the crude product which was purified by column chromatography on silica gel eluting with ethyl acetate–methanol (9 : 1) to give colorless liquid product 6. Yield 82%. ¹H NMR (400 MHz, CDCl₃): δ 2.53–2.48 (m, 1H, –OCHCH₂), 2.77–2.72 (m, 1H, –OCHCH₂), 3.12–3.07 (m, 1H, –CH₂CHCH₂), 3.48 (dd, J = 11.8 Hz and J = 6 Hz, 1H, –OCH₂CH), 3.92 (dd, J = 11.8 Hz and J = 2.6 Hz, 1H, –OCH₂CH), 4.33 (dd, J = 13.3 Hz and J = 6.3 Hz, 1H, –PCH₂O), 4.44–4.38 (m, 1H, –PCH₂O), 7.53–7.47 (m, 4H, –C₆H₅), 7.60–7.54 (m, 2H, –C₆H₅), 7.86–7.78 (m, 4H, –C₆H₅). ¹³C NMR (100 MHz, CDCl₃): δ 43.8 (–OCH₂CH), 50.5 (–CH₂CHCH₂), 69.6 (d, J = 86.9 Hz, –PCH₂O), 74.0 (d, J = 9.9 Hz, –OCH₂CH), 128.6 (d, J = 11.8 Hz, –C₆H₅), 130.2 (–C₆H₅), 131.5 (dd, J = 9.3 Hz and J = 3.5 Hz, –C₆H₅), 132.3 (–C₆H₅). ³¹P NMR (162 MHz, CDCl₃): δ 27.93. HRMS (ESI) m/z: calcd for C₁₆H₁₇O₃P [M + Na]⁺ 311.0813, found 311.0813.

Synthesis of 4-(((diphenylphosphoryl)methoxy)methyl)-1,3-dioxolan-2-one (7). A dry, pre-evacuated Schlenk tube was filled with CO₂. To that, 2-(((diphenylphosphoryl)methoxy) methyl)oxirane (6) (1 mol), catalyst (2.5 mol%) and co-catalyst (5 mol%) was then added. The reaction mixture was stirred at needed temperature condition under CO₂ (balloon). After the reaction time, water was poured into the reaction mixture and extracted with chloroform. The organic layer was separated and evaporated to give the crude product which was purified by flash column chromatography on silica gel eluting with ethyl acetate–methanol to yield colorless liquid 7. ¹H NMR (400 MHz, CDCl₃): δ 3.78 (dd, J = 11.1 Hz and J = 3.3 Hz, 1H, –OCH₂CH), 3.89 (dd, J = 11.1 Hz and J = 3 Hz, 1H, –OCH₂CH), 4.13 (dd, J = 8.4 Hz and J = 5.8 Hz, 1H, –OCH₂CH), 4.27 (dd, J = 13.4 Hz and J = 6.5 Hz, 1H, –PCH₂O), 4.34 (d, J = 8.4 Hz, 1H, –OCH₂CH), 4.39 (dd, J = 13.4 Hz and J = 3.9 Hz, 1H, –PCH₂O), 4.78–4.71 (m, 1H, –CH₂CHCH₂), 7.52–7.45 (m, 4H, –C₆H₅), 7.59–7.53 (m, 2H, –C₆H₅), 7.80–7.71 (m, 4H, –C₆H₅). ¹³C NMR (100 MHz, CDCl₃): δ 65.8 (–OCH₂CH), 69.8 (d, J = 84.9 Hz, –PCH₂O), 72.3 (d, J = 8.7 Hz, –OCH₂CH), 74.7 (–CH₂CHCH₂), 129–128.6 (m, –C₆H₅), 130.1 (dd, J = 99.8 Hz and J = 14 Hz, –C₆H₅), 131.5–131 (m, –C₆H₅), 132.6 (d, J = 5.1 Hz, –C₆H₅), 154.7 (–OCO). ³¹P NMR (162 MHz, CDCl₃): δ 28.29. HRMS (ESI) m/z: calcd for C₁₇H₁₇O₅P [M + Na]⁺ 355.0711, found 355.0792.

General procedure for the ring opening polymerization of ε-caprolactone

Typical polymerization reactions were carried out in a nitrogen filled Schlenk tube equipped with a stir bar.

For polymerization in neat condition (BP-ROP). To a stirred mixture of catalyst (0.02 mmol) and BnOH (0.02 mmol), CL (2 mmol) was added. As time progresses, an increase in the viscosity of the solution was observed. After the reaction time, the polymerization was terminated by addition of glacial acetic acid and the resulting viscous solution was poured into cold methanol. The polymer precipitated methanol was collected, washed and dried under vacuum.

For solution polymerization (SP-ROP). To a stirred solution of toluene (4 mL) and catalyst (0.02 mmol), BnOH (0.02 mmol) was added. The CL (2 mmol) was then added to the solution. The reaction mixture was stirred at the needed temperature for the prescribed time. Samples were taken from the reaction mixture at a regular time interval to determine the monomer conversion by ¹H NMR spectroscopy. After the reaction time, the polymerization was quenched by addition of glacial acetic acid into the solution and the resulting solution was poured into cold methanol with stirring. The white precipitate was collected by filtration, washed with cold methanol and dried under vacuum.

Acknowledgements

HVB is thankful to CSIR India for a fellowship.

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: ¹H, ¹³C, ³¹P NMR and HRMS spectra of 6 and 7. See DOI: 10.1039/c3ra44988b

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