Alkoxides of group 4 metals containing the bis(imino)phenoxide ligand: synthesis, structural characterization and polymerization studies

Abstract

A variety of group 4 metal compounds containing the bis(imino)phenoxide backbone were synthesized from the reaction of suitable ligands and group 4 metal alkoxides. They were completely characterized with different spectroscopic techniques and single-crystal X-ray diffraction studies on a few of them. These compounds were found extremely active towards the bulk polymerization of ε-caprolactone (CL), δ-valerolactone (VL), rac-butyrolactone (rac-BL), L-lactide (L-LA) and rac-lactide (rac-LA) yielding polymers with high number average molecular weight (M_n) and controlled molecular weight distribution (MWD). The kinetics and mechanistic studies associated with these polymerizations have been performed. In addition, the catalytic activity of these compounds towards the polymerization of ethylene was investigated.

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Introduction

In the recent years, studies on ring-opening polymerization of cyclic esters and lactides yielding biodegradable polymers have become a topical area of research.¹ This has occurred as a result of depletion of petroleum resources, ultimately initiating research that uses annually renewable feed stock as raw materials leading to the formation of green polymers.^1,2 Aliphatic polyesters have become important for the biomedical industry like delivery medium for the controlled release of medicines and in the manufacturing of biodegradable surgical sutures.³ These polymers have practical applications as a result of the permeability, biocompatibility and biodegradability.⁴ One of the important methods in producing them is the ring-opening polymerization of cyclic esters and lactide using metal catalysts. Among the different mechanistic pathways available, the coordination-insertion mechanism is the most popular because it yields polymer with good molecular weights and narrow molecular weight distributions.⁵Group 4 metals have been popular towards the synthesis of new catalysts for the polymerization of cyclic esters and lactide.^6,7 Our continued interest in pursuing new chemistry with group 4 metals⁸ prompted us to undertake this investigation for synthesizing new catalysts and evaluate their catalytic activity in ring-opening polymerization of cyclic esters and lactide.

The pioneering work contributed by Ziegler and Natta towards the synthesis of high density polyethylene and the discovery of the capability of MAO (methylalumoxane) in activating group 4 metallocene complexes created a very high global impetus in polyolefin research.⁹Metallocenes are active in controlling the molecular weight and molecular weight distribution of the resultant polymers. A huge span of literature is available regarding the synthesis and polymerization activity of homogeneous single-site catalysts.¹⁰ Recent endeavors in this area involves the synthesis of nonmetallocene complexes, creating opportunities to synthesize polymers with novel properties and usage.^11–15

Here, we report the synthesis of a family of catalysts containing the bis(imino)phenoxide ligating backbone and group 4 metals and their activity in the polymerization of cyclic esters, lactide and ethylene. A preliminary communication describing the catalytic activity of bis(imino)phenoxide compounds of zirconium towards the polymerization of cyclic esters and lactide has already appeared from our group.^8b Our previous communication describing the capability of zirconium complexes of the bis(imino)phenoxide ligand shows that the variation of substituents of the aryl ring containing the N center does not have significant effect on the polymerizations of cyclic esters and lactide.^8b Hence, it was required to study the effect of substituents on the central aryl ring contained in the bis(imino)phenoxide backbone.

Results and discussion

Syntheses and structural characterization

Synthesis of group 4 metal complexes was carried out using ligands L¹–L⁴ as shown in Scheme 1. A 2 : 1 stoichiometric reaction between ligands L¹–L⁴ with group 4 metal alcoholates in toluene resulted in the quantitative formation of compounds 1–10 respectively (Scheme 1). Nevertheless, a 1 : 1 stoichiometric reaction between the reactants resulted in the formation of 1–10 along with the presence of unreacted metal alcoholate, as understood by the spectroscopic analysis of the crude reaction mixture. The resulting products were purified by crystallization from toluene and isolated in high yields and purity as orange to yellow solids. Compounds 1–10 were unambiguously characterized using various spectroscopic techniques and single-crystal X-ray diffraction studies on a few of them. Compound 10 was synthesized with the intention of the possibility of growing single crystals suitable for X-ray diffraction studies. Repeated attempts in growing suitable single crystals with 7–9 did not give fruitful results.

Scheme 1 Bis(imino)phenoxide compounds of group 4 metals.

The ¹H NMR of 1–10 reveal all the signals in the correct integration ratio. All these compounds contain a molecule of coordinated toluene. In case of compounds 1–6, the protons from the CH moiety present in the OiPr group appears much downfield shifted as compared to the signals of the CH unit from OiPr group of the ligand. The interesting observation is that the CH protons from the CH=N fragments do not appear as a single peak uniformly for all the compounds. In the case of 1, 2 and 9, the CH=N fragments are present as a single broad peak with the correct integration. For 8, it appears as a set of two signals in 1 : 1 integration ratio. This may be rationalized by considering the structure of these molecules wherein there are two N centers coordinated and two free. In all the other compounds, the imine protons appear as a set of three signals in 2 : 1 : 1 integration ratio.

The ¹³C NMR of 1–9 shows the presence of moieties corresponding to the different carbon environments in these complexes. For 1, 2, 8 and 9, the CH=N appears as a single signal whereas for the others, two signals, one for the coordinated CH=N and other for the non coordinated moiety are observed. The overall conclusions drawn from ¹³C NMR studies are in agreement with the conclusions drawn from the ¹H NMR spectra of 1–10.

Electrospray ionization mass spectrometric (ESI–MS) studies on 1–10 reveals clearly the presence of the molecular ion peak and suggests that these compounds appear in the monomeric state. Elemental analyses of 1–10 shows that the experimentally determined values are in good agreement with the calculated ones, suggesting acceptable purity of these isolated compounds.

Zirconium and hafnium alkoxides retain their dimeric structure even in solution.¹⁶ However, in this work it is observed that the dimeric core of these alkoxides is disturbed. This is contrary to our observation with salens where the cavity provided by the ligand is sufficient to accommodate the dimeric core of the metal alkoxide.^8e

Variable temperature ¹H NMR studies were performed using compound 1 and 7 (Fig. 1) (See ESI for 1).† It was observed that as the temperature is lowered gradually, the signals for CH=N group become gradually broad. This signifies that there is scope for ligand scrambling due to the presence of coordinatively unsaturated metal center. In 1, the signal corresponding to the OiPr group remains as a single broad signal at all temperatures. This proves that the preferred structure is monomeric and the equilibrium with oligomeric species is completely absent.

Fig. 1 Variable temperature ¹H NMR of 7 in CDCl₃.

Single-crystal X-ray diffraction studies

Crystals of 3, 5 and 10 suitable for X-ray diffraction studies were grown from saturated toluene solution of the respective compounds at −24 °C, over a period of two weeks. These compounds are monomeric in the solid state containing a single metal center that adopts a distorted octahedral geometry (Fig. 2, Fig. 3 and Fig. 4). Out of the four nitrogen centers, two are coordinated to the metal center where as the other two remain free. Such a product configuration is thermodynamically preferred as compared to the situation where both the nitrogen centers remain simultaneously coordinated to the metal. All the bond lengths and angles match well with the literature data.^6e,6k,6w The crystal data and structure refinement details for 3, 5 and 10 are enumerated in Table 1.

Fig. 2 Molecular structure of 3: thermal ellipsoids drawn at 30% probability level, hydrogen atoms have been omitted for clarity. Selected bond lengths (A°) and angles (°): Ti(1)–O(1) 1.7887(17), Ti(1)–O(2) 1.8062(18), Ti(1)–O(3) 1.9946(17), Ti(1)–O(4) 1.9773(16), Ti(1)–N(1) 2.2252(19), Ti(1)–N(2) 2.2285(19), N(1)–Ti(1)–N(2) 175.05(7), O(3)–Ti(1)–N(1) 92.71(7).

Fig. 3 Molecular structure of 5: thermal ellipsoids drawn at 30% probability level, hydrogen atoms have been omitted for clarity. Selected bond lengths (A°) and angles (°): Zr(1)–O(1) 2.110(4), Zr(1)–O(2) 1.920(4), Zr(1)–O(3) 2.093(4), Zr(1)–O(4) 1.933(4), Zr(1)–N(1) 2.376(5), Zr(1)–N(2) 2.382(5), N(1)–Zr(1)–N(2) 175.01(16), O(3)–Zr(1)–N(1) 97.78(16).

Fig. 4 Molecular structure of 10: thermal ellipsoids drawn at 30% probability level, hydrogen atoms have been omitted for clarity. Selected bond lengths (A°) and angles (°): Hf(1)–O(1) 2.0284(17), Hf(1)–O(2) 1.9227(16), Hf(1)–N(1) 2.4206(19), N(1)–Hf(1)–N(1) 78.38(9), O(2)–Hf(1)–N(1) 165.99(7).

Table 1 Crystal data and structure refinement details for 3, 5 and 10

Compounds	3	5	10
Molecular formula	C79H104N4O6Ti	C70H90Cl2N4O4Zr	C54H60N4O8Hf
Formula weight	1253.54	1213.58	1071.55
T/K	173(2)	173(2)	173(2)
Wavelength (Å)	0.71073	0.71073	0.71703
Crystal system,	Monoclinic	Monoclinic	Monoclinic
Space group	P21/c	P21/n	C2/c
a/Å	12.0283(3)	12.070(2)	19.017(3)
b/Å	28.2315(9)	22.740(5)	19.507(3)
c/Å	21.6400(8)	27.720(6)	15.973(2)
α (°)	90.00	90.00	90.00
β (°)	98.2510(10)	100.89(3)	117.708(6)
γ (°)	90.00	90.00	90.00
V/Å^3	7272.4(4)	7471(3)	5245.9(13)
Z, Calculated density (g cm^−3)	4, 1.145	4, 1.079	4, 1.357
Absorption coefficient (mm^−1)	0.171	0.262	2.043
F(000)	2704	2576	2192
Crystal size/mm	0.35 × 0.28 × 0.25	0.38 × 0.35 × 0.22	0.23 × 0.20 × 0.15
Limiting indices	−12 ≤ h ≤ 16	−10 ≤ h ≤ 15	−31 ≤ h ≤ 29
	−38 ≤ k ≤ 38	−30 ≤ k ≤ 29	−31 ≤ k ≤ 31
	−22 ≤ l ≤ 27	−32 ≤ l ≤ 36	−24 ≤ l ≤ 25
Reflections collected/unique	56295	52626	36623
Independent reflections	17605	17045	11509
Data/restraints/parameters	17605/0/834	17045/0/750	11509/0/309
Goodness-of-fit on F^2	0.968	0.988	1.009
Final R indices [I >2σ(I)]	R = 0.0608	R = 0.0988	R = 0.0411
	wR = 0.1471	wR = 0.2589	wR = 0.0807
R indices (all data)	R = 0.1316	R = 0.2163	R = 0.0719
	wR = 0.1847	wR = 0.3318	wR = 0.0917

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Ring-opening polymerization

These compounds have proved to be powerful catalysts for the bulk ring-opening polymerization of different cyclic esters and lactide. We did not perform any polymerization studies with 10 since it was synthesized for structural characterization alone. The results are depicted in Table 2.

Table 2 Polymerization data for CL, VL, rac-BL, L-LA and rac-LA using 1–9 in 200 : 1 ratio

Entry	Catalyst	Monomer	T/°C	Yield/%	Time^a/min	M n ^b/kg mol^−1	M w / M n	P r ^c
a Time of polymerization measured by quenching the polymerization reaction when all monomer was found consumed. b Measured by GPC at 27 °C in THF relative to polystyrene standards. c Calculated using homonuclear-decoupled ^1H NMR spectrum.^17
1	1	CL	80	99	5	70.8	1.32
2	1	VL	80	97	3	61.3	1.22
3	1	rac-BL	80	96	9	40.1	1.33
4	1	L-LA	130	98	9	52.9	1.11
5	1	rac-LA	130	99	6	46.6	1.13	0.72
6	2	CL	80	97	7	66.9	1.35
7	2	VL	80	95	5	58.7	1.25
8	2	rac-BL	80	97	12	37.5	1.34	0.73
9	2	L-LA	130	99	10.5	50.2	1.12
10	2	rac-LA	130	98	8	45.2	1.14
11	3	CL	80	98	10	64.8	1.38
12	3	VL	80	96	8	57.7	1.29
13	3	rac-BL	80	98	17	35.2	1.30
14	3	L-LA	130	99	13	48.4	1.14
15	3	rac-LA	130	99	11	44.6	1.15	0.75
16	5	CL	80	99	9	96.6	1.32
17	5	VL	80	96	6	65.1	1.21
18	5	rac-BL	80	97	15	38.2	1.34
19	5	L-LA	130	99	18	92.6	1.13
20	5	rac-LA	130	98	8	88.0	1.15	0.74
21	6	CL	80	97	12	95.0	1.31
22	6	VL	80	98	8	63.2	1.23
23	6	rac-BL	80	97	18	36.9	1.34
24	6	L-LA	130	99	21	90.2	1.16
25	6	rac-LA	130	99	11	87.4	1.14	0.73
26	7	CL	80	97	12	52.4	1.34
27	7	VL	80	98	11	49.7	1.27
28	7	rac-BL	80	97	20	36.8	1.31
29	7	L-LA	130	99	19	47.2	1.19
30	7	rac-LA	130	99	10	45.6	1.20	0.75
31	8	CL	80	97	15	50.3	1.38
32	8	VL	80	96	14	47.4	1.29
33	8	rac-BL	80	98	23	34.4	1.34
34	8	L-LA	130	99	25	46.5	1.11
35	8	rac-LA	130	98	13	44.7	1.12	0.77
36	9	CL	80	99	19	49.4	1.39
37	9	VL	80	98	18	45.6	1.21
38	9	rac-BL	80	98	30	34.0	1.55
39	9	L-LA	130	97	28	44.7	1.14
40	9	rac-LA	130	99	17	43.8	1.13	0.76

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Analysis of the data depicted in Table 2 shows that there is a reasonable degree of control in these polymerizations. In general, the reactivity decreases down the group from titanium to hafnium with the exception of rac-LA polymerization, where the reactivity of titanium and zirconium catalysts coincidentally remain almost the same. The ring-opening polymerizations are anticipated to proceed via a coordination-insertion mechanism for which Lewis acidity of metal center is important. If we compare the electron-releasing effects of three substituents in phenolic moiety of ligand, the order will be OMe > Me > Cl. As the electron donating tendency of substituent on the phenyl ring increases, the Lewis acidity of metal will decrease. Hence, it leads to loss of reactivity towards polymerization. So the reactivity order will be Cl > Me > OMe for a given monomer, although the differences in M_n are marginal. Another important observation for each monomer is that the zirconium complexes yield the respective polymers with maximum M_n, followed by titanium, with hafnium yielding the least. Hence, we conclude that the presence of different para substituents on the central aryl ring in these ligands do not influence the polymerization to a significant degree. Also, we have concluded previously that the presence of different alkyl substituents at different positions of the aniline ring does not produce notable effects during these polymerizations.^8b Comparison with the literature shows that our system is far better in terms of time taken for polymerization, M_n and MWDs.^{6b,6d,6e,6j,6p,6q,6w} We have done a closer comparison with the results reported for the imino(phenoxide) complexes.^6v We find here that our results are better even for tacticity in addition to time taken for polymerization, M_n and MWDs.

We have concluded from homonuclear decoupling ¹H NMR that the polymerization of L-LA using 1–9 yields heterotactic enriched polymer (Table 2). The polymerization of rac-BL is highly stereoselective leading to the formation of syndiotactic polymer. In the ¹³C NMR, the r-centered diad (δ = 169.38 ppm) shows a very strong contribution, indicating syndiotacticity. This conclusion is supported by the presence of a prominent signal (δ = 40.85 ppm) in the methylene region which corresponds to the rr triad.¹⁸

Since polymerization of L-LA yielded best results, we studied the variations of M_n and MWDs with [L-LA]_o/[Cat]_o for L-LA using 2, 5 and 8. The results are depicted in Table 3 and Fig. 5. The plot suggests that there is a continuous rise in the in the M_n with an increase in the [L-LA]_o/[Cat]_o for the catalysts indicated. Again the MWDs are found to vary linearly with the increase in [L-LA]_o/[Cat]_o ratio.

Fig. 5 M _n vs. [L-LA]_o/[Cat]_ovs. M_w/M_n plot for 2, 5 and 8.

Table 3 Polymerization data for L-LA using 2, 5 and 8 in various [L-LA]_o/[Cat]_o ratios at 130 °C

Entry	Catalyst	[L-LA]o/[Cat]o	Time^a/min	Yield/%	M n ^b/kgmol^−1	M w/Mn
a Time of polymerization measured by quenching the polymerization reaction when all monomer was found consumed. b Measured by GPC at 27 °C in THF relative to polystyrene standards.
1	2	200	10.5	99	50.2	1.12
2	2	400	15	99	92.9	1.15
3	2	600	18	98	149.7	1.16
4	2	800	22	97	211.3	1.17
5	2	1000	27	96	254.1	1.19
6	2	1200	33	99	307.3	1.22
7	5	200	18	98	92.6	1.13
8	5	400	22	99	184.3	1.15
9	5	600	27	99	295.0	1.16
10	5	800	32	97	374.3	1.18
11	5	1000	36	96	451.2	1.21
12	5	1200	41	98	552.8	1.24
13	8	200	25	99	44.6	1.21
14	8	400	29	99	88.4	1.24
15	8	600	35	97	143.7	1.26
16	8	800	39	98	185.6	1.29
17	8	1000	43	99	225.2	1.30
18	8	1200	49	97	259.0	1.32

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Again, the plot of M_n vs % conversion for 2, 5 and 8 (Fig. 6) was found linear, suggesting a good degree of control in these polymerizations. Fig. 5 and 6 seem to suggest that the polymerization of L-LA is highly controlled and propagates in a living manner.

Fig. 6 M _n vs % conversion for 2, 5 and 8.

Next, we proceeded with the kinetic studies for the polymerization of L-LA using 3, 6 and 9 in ratio [L-LA]_o/[Cat]_o = 200 at 130 °C. We decided to choose these catalysts since they yielded polymer with highest M_n values (Table 2). The results are presented in Fig. 7. There is a first-order dependence of the rate of polymerization on L-LA concentration with complete absence of induction period. The plot of [L-LA]_o/[L-LA]_tvs. time was found to be linear. The values of the apparent rate constant (k_app) for L-LA polymerization catalyzed by 3, 6 and 9 were evaluated from the slope of these lines and were found to be 0.61395 min⁻¹, 0.44329 min⁻¹ and 0.22971 min⁻¹ respectively. This implies that the rate of polymerization follows the order Ti > Zr > Hf.

Fig. 7 Semi-logarithmic plots of L-LA conversion in time initiated by 3, 6 and 9: [L-LA]_o/[Cat]_o = 200 at 130 °C.

In order to have a complete idea on the polymerization mechanism, we synthesized a low molecular weight oligomer of L-LA. Compound 5 was reacted with L-LA in 1 : 10 stoichiometric ratio at 130 °C for 30 mins. The residue was dissolved in minimum amount of CH₂Cl₂ and poured into cold methanol. The oligomer was isolated and was subjected to MALDI-TOF and ¹H NMR studies. The results (Fig. 8 and Fig. 9) indicate that the polymerization proceeds with the coordination-insertion mechanism as shown in Scheme 2 and the phenolate ligand (L²) is incorporated as one of the end terminal groups. The possibility of activated monomer mechanism does not exist since no alcoholic initiators have been added externally to the polymerization reactions.

Fig. 8 MALDI-TOF spectrum of the crude product obtained from a reaction between 5 and L-LA in 1 : 10 ratio at 130 °C.

Fig. 9 ¹H NMR spectrum of the crude product obtained from a reaction between 5 and L-LA in 1 : 10 ratio at 130 °C (x = peaks corresponding to L-LA).

Scheme 2 Mechanism of ring-opening polymerization.

Ethylene polymerization

The catalytic activity of compounds 1–9 activated by MAO were tested towards the polymerization of ethylene. The results are depicted in Table 4.

Table 4 Polymerization data for ethylene using 1–9 and MAO in toluene at 80 °C

Entry	Catalyst ^a	Activity^b	Yield^c (g)	M w/ kgmol^−1	M n/ kgmol^−1	M w/Mn
a All experiments were performed in toluene at MAO:catalyst ratio = 1000, 80 °C for 30 min, catalyst = 8 mg, solvent = 7 mL. b Activity in (g PE/mol cat ×h) × 10^5. c gm of polymer obtained after 30 min.
1	1	4.28	1.5	132.8	75.0	1.77
2	2	4.05	1.4	131.5	73.4	1.79
3	3	3.75	1.3	130.1	71.9	1.81
4	4	5.22	1.8	149.3	82.0	1.82
5	5	4.70	1.6	149.0	80.5	1.85
6	6	4.36	1.5	149.2	79.8	1.87
7	7	4.25	1.3	128.4	66.9	1.92
8	8	4.17	1.3	126.3	64.8	1.95
9	9	3.82	1.2	125.0	62.8	1.99

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The trials were performed in toluene at 80 °C using MAO as the activator. Analysis of the data reveals that the zirconium compounds yield the best results in terms of activity, M_n and M_w. This is followed by the results using titanium catalysts and finally for hafnium catalysts.

Conclusions

In summary, we have synthesized a library of new alkoxides containing group 4 metals and bis(imino)phenoxide backbone as the ancillary ligand. These compounds are extremely active towards the polymerization of different cyclic esters, lactide and ethylene. The zirconium catalysts were found to yield better polymerization results in comparison to the titanium and hafnium analogues. Presence of different substituents either on the central aryl ring or on different anilines present in the various ligands do not affect the polymerizations in a significant manner.

Experimental

General experimental details

All reactions were performed under a dry argon atmosphere employing standard Schlenk techniques or in a glove box where the oxygen and moisture levels were maintained normally below 1 ppm. The reactions to synthesize 1–10 were done in toluene which was rigorously dried over sodium benzophenone ketyl and distilled fresh prior to use. The deuterated solvents used for NMR studies were dried using standard procedures, distilled and stored in the glove box throughout the program of study. A Bruker Avance 400 instrument was used to record all ¹H and ¹³C NMR with the chemical shifts referenced to residual solvent resonances and are reported as parts per million relative to SiMe₄. A Waters Q-Tof micro mass spectrometer was utilized to record the ESI-MS spectra and MALDI-TOF measurements were performed on a Bruker Daltonics instrument in dihydroxy benzoic acid matrix. Elemental analyses reports were obtained using a Perkin Elmer Series 11 analyzer. The starting materials namely Ti(OiPr)₄, Zr(OiPr)₄·iPrOH and Hf(OtBu)₄ were purchased from Aldrich and used as received. The various monomers used for this study were purchased from aldrich, dried/purified and stored in the glove box. The ligands L¹, L² and L⁴,¹⁹ were synthesized using reported literature methods. MAO was purchased from Aldrich.

Synthesis of L³

To a stirred methanolic solution of 2,6-diformyl-4-methoxy phenol (1g, 5.5 mmol), 2,6-diisopropyl aniline (1.97 g, 11.1 mmol) was added drop wise and after completion of addition the orange solution was set to reflux for 12 h. The reaction mixture was cooled to ambient temperature during which the orange product precipitated. The orange color solid was filtered off and washed with cold methanol and dried. Yield = 76%. Anal. Calc. for C₃₃H₄₂N₂O₂: C, 79.48; H, 8.49; N, 5.62. Found: C, 79.81; H, 8.52; N, 5.58. ¹H NMR (400 MHz, CDCl3): δ = 1.24–1.31 (d, CH(CH₃)₃, 24H), 3.03–3.09 (m, CH(CH₃)₃, 4H), 3.96 (s, Ar–O–CH₃, 3H), 7.22–7.30 (m, Ar–H, 8H), 8.62 (s, CH=N, 2H), 13.15 (s, O–H, 1H). ¹³C NMR (100 MHz, CDCl3): δ = 23.64 (CH(CH₃)₃), 27.92 (CH(CH₃)₃), 55.84 (Ar–O–CH₃), 118.85 (Ar–C), 118.92 (Ar–C), 122.86 (Ar–C), 123.39 (Ar–C), 137.65 (Ar–C), 149.32 (Ar–C), 155.21 (Ar–O–CH₃), 157.15 (Ar–O), 162.72 (CH=N). m/z calculated for [M]⁺ calculated 498.698 found 498.

Synthesis of catalysts

A general procedure describing the synthesis of 1–10 is outlined:

In an argon filled glove box, to a stirred solution of metal alkoxide (0.13 mmol) in 5 mL toluene at −24 °C was added a solution of L¹–L⁴ (0.26 mmol) in 5 mL toluene. The reaction mixture was allowed to warm up to room temperature and stirred additionally for 24 h. The solvent was removed under reduced pressure and the residue obtained was crystallized from concentrated toluene solution at −24 °C.

Compound 1. Yield = 86%. Mp: 151 °C. Anal. Calcd for C₇₉H₁₀₄N₄O₄Ti: C 77.67, H 8.58, N 4.59. Found: C 77.72, H 8.32, N 4.65. ¹H NMR (400 MHz, CDCl₃): δ = 0.93–1.29 (m, CH(CH₃)₂, 48H), 1.30–1.58 (m, O–CH(CH₃)₂, 12H), 2.29 (s, O–Ar–CH₃, 6H), 2.39 (s, Ar–CH₃, 3H), 3.08–3.10 (m, CH(CH₃)₂, 8H), 4.48–4.52 (m, O–CH(CH₃)₂, 2H), 7.05–7.78 (m, Ar–H, 21H), 8.55 (s, CH=N, 4H). ¹³C NMR (100 MHz, CDCl₃): δ = 21.29 (Ar–CH₃), 23.38 (O–CH(CH₃)₂), 23.69 (CH(CH₃)₂), 25.36 (CH(CH₃)₂), 25.53 (CH(CH₃)₂), 26.27 (CH(CH₃)₂), 27.36 (CH(CH₃)₂), 28.06 (CH(CH₃)₂), 28.22 (CH(CH₃)₂), 28.42 (CH(CH₃)₂), 76.78 (O–CH(CH₃)₂), 121.71 (Ar–C), 123.16 (Ar–CH₃), 123.48 (Ar–C), 123.93 (Ar–C), 124.10 (Ar–C), 125.32 (Ar–C), 125.40 (Ar–C), 126.59 (Ar–C), 138.53 (Ar–CH₃), 139.45 (Ar–C), 141.11 (Ar–C), 150.45 (Ar–C), 158.44 (Ar–O), 164.39 (CH=N). ESI m/z calculated for [M]⁺. C₇₂H₉₆N₄O₄Ti: 1129.424 found 1129.520.

Compound 2. Yield = 91%. Mp: 155 °C. Anal. Calcd for C₇₇H₉₈Cl₂N₄O₄Ti: C 73.26, H 7.82, N 4.44. Found: C 73.31, H 7.71, N 4.39. ¹H NMR (400 MHz, CDCl₃): δ = 0.92–1.27 (m, CH(CH₃)₂, 48H), 1.28–1.35 (m, O–CH(CH₃)₂, 12H), 2.35 (s, Ar–CH₃, 3H), 2.96–3.03 (m, CH(CH₃)₂, 8H), 3.96–4.11 (m, O–CH(CH₃)₂, 2H), 7.09–7.28 (m, Ar–H, 21H), 8.48 (s, CH=N, 4H). ¹³C NMR (100 MHz, CDCl₃): δ = 23.51 (O–CH(CH₃)₂), 24.68 (CH(CH₃)₂), 24.87 (CH(CH₃)₂), 25.27 (CH(CH₃)₂), 25.44 (CH(CH₃)₂), 25.84 (CH(CH₃)₂), 27.85 (CH(CH₃)₂), 28.07 (CH(CH₃)₂), 28.16 (CH(CH₃)₂), 76.96 (O–CH(CH₃)₂), 119.90 (Ar–C), 123.19 (Ar–C), 124.64 (Ar–C), 127.63 (Ar–Cl), 128.27 (Ar–C), 138.18 (Ar–C), 138.76 (Ar–C), 139.58 (Ar–C), 149.62 (Ar–C), 158.01 (Ar–O), 162.29 (CH=N). ESI m/z calculated for [M + Na]⁺. C₇₀H₉₀Cl₂N₄O₄Ti: 1170.261 found 1193.055.

Compound 3. Yield = 92%. Mp: 149 °C. Anal. Calcd for C₇₉H₁₀₄N₄O₆Ti: C 75.69, H 8.36, N 4.47. Found: C 75.80, H 8.13, N 4.61. ¹H NMR (400 MHz, CDCl₃): δ = 1.00–1.34 (m, CH(CH₃)₂, 48H), 1.36–1.43 (m, O–CH(CH₃)₂, 12H), 2.41 (s, Ar–CH₃, 3H), 3.07–3.16 (m, CH(CH₃)₂, 8H), 3.88 (s, O–Ar–OCH₃, 6H), 4.51–4.60 (m, O–CH(CH₃)₂, 2H), 7.04–7.59 (m, Ar–H, 21H), 8.19 (s, CH=N, 2H), 8.62 (s, CH=N, 2H). ¹³C NMR (100 MHz, CDCl₃): δ = 23.55 (O–CH(CH₃)₂), 24.60 (CH(CH₃)₂), 24.86 (CH(CH₃)₂), 25.27 (CH(CH₃)₂), 25.46 (CH(CH₃)₂), 25.74 (CH(CH₃)₂), 27.81 (CH(CH₃)₂), 28.00 (CH(CH₃)₂), 28.13 (CH(CH₃)₂), 56.16 (Ar–OCH₃), 76.64 (O–CH(CH₃)₂), 119.75 (Ar–C), 123.09 (Ar–C), 124.68 (Ar–C), 128.17 (Ar–C), 138.18 (Ar–CH₃), 138.86 (Ar–C), 139.48 (Ar–C), 149.97 (Ar–C), 150.67 (Ar–OCH₃), 150.81 (Ar–OCH₃), 157.94 (Ar–O), 161.69 (CH=N), 164.38 (CH=N). ESI m/z calculated for [M]⁺. C₇₂H₉₆N₄O₆Ti: 1161.423 found 1161.464.

Compound 4. Prepared from literature procedure.^8b

Compound 5. Yield = 82%. Mp: 168 °C. Anal. Calcd for C₇₇H₉₈Cl₂N₄O₄Zr: C 70.83, H 7.56, N 4.29. Found: C 71.63, H 7.48, N 4.32. ¹H NMR (400 MHz, CDCl₃): δ = 0.49–1.18 (m, CH(CH₃)₂, 48H), 1.29–1.30 (d, O–CH(CH₃)₂, 12H), 2.44 (s, Ar–CH₃, 3H), 2.80–3.10 (m, CH(CH₃)₂, 8H), 3.71–3.83 (m, O–CH(CH₃)₂, 2H), 7.10–7.35 (m, Ar–H, 21H), 8.06 (s, CH=N, 2H), 8.45 (s, CH=N, 1H), 8.60 (s, CH=N, 1H). ¹³C NMR (100 MHz, CDCl₃): δ = 22.14 (Ar–CH₃), 22.75 (O–CH(CH₃)₂), 23.30 (CH(CH₃)₂), 24.02 (CH(CH₃)₂), 25.32 (CH(CH₃)₂), 25.50 (CH(CH₃)₂), 26.23 (CH(CH₃)₂), 27.00 (CH(CH₃)₂), 27.42 (CH(CH₃)₂), 28.27 (CH(CH₃)₂), 70.63 (O–CH(CH₃)₂), 121.33 (Ar–C), 122.73 (Ar–C), 123.33 (Ar–CH₃), 123.61 (Ar–C), 124.36 (Ar–C), 127.02 (Ar–Cl), 128.37 (Ar–C), 129.18 (Ar–CH₃), 133.81 (Ar–C), 137.56 (Ar–C), 138.26 (Ar–CH₃), 140.68 (Ar–C), 140.94 (Ar–C), 149.89 (Ar–C), 150.40 (Ar–C), 157.84 (Ar–O), 164.32 (CH=N), 171.09 (CH=N). ESI m/z calculated for [M]⁺. C₇₀H₉₀Cl₂N₄O₄Zr: 1213.618 found 1213.622.

Compound 6. Yield = 87%. Mp: 159 °C. Anal. Calcd for C₇₉H₁₀₄N₄O₆Zr: C 73.16, H 8.08, N 4.32. Found: C 73.41, H 7.92, N 4.62. ¹H NMR (400 MHz, CDCl₃): δ = 0.39–0.66 (m, CH(CH₃)₂, 48H), 0.98–1.29 (m, O–CH(CH₃)₂, 12H), 2.36 (s, Ar–CH₃, 3H), 2.73–3.13 (m, CH(CH₃)₂, 8H), 3.74–3.82 (m, O–CH(CH₃)₂, 2H), 3.87 (s, Ar–O–CH₃, 6H), 6.87–7.19 (m, Ar–H, 21H), 8.00 (s, CH=N, 2H), 8.08 (s, CH=N, 1H), 8.59 (s, CH=N, 1H). ¹³C NMR (100 MHz, CDCl₃): δ = 22.82 (O–CH(CH₃)₂), 23.74 (CH(CH₃)₂), 25.32 (CH(CH₃)₂), 25.48 (CH(CH₃)₂), 25.65 (CH(CH₃)₂), 26.31 (CH(CH₃)₂), 27.29 (CH(CH₃)₂), 27.35 (CH(CH₃)₂), 28.26 (CH(CH₃)₂), 56.42 (Ar–OCH₃), 70.16 (O–CH(CH₃)₂), 121.78 (Ar–C), 123.26 (Ar–CH₃), 123.78 (Ar–C), 124.06 (Ar–C), 125.45 (Ar–C), 126.67 (Ar–C), 128.38 (Ar–C), 129.18 (Ar–CH₃), 133.81 (Ar–C), 137.56 (Ar–C), 138.42 (Ar–CH₃), 140.82 (Ar–C), 140.94 (Ar–C), 149.70 (Ar–C), 150.02 (Ar–OCH₃), 150.28 (Ar–O), 158.98 (Ar–O), 161.34 (CH=N), 171.64 (CH=N). ESI m/z calculated for [M]⁺. C₇₂H₉₆N₄O₆Zr: 1204.780 found 1204.790.

Compound 7. Yield = 85%. Mp: 168 °C. Anal. Calcd for C₈₁H₁₀₈N₄O₄Hf: C 70.49, H 7.89, N 4.06. Found: C 70.99, H 8.01, N 4.06. ¹H NMR (400 MHz, CDCl₃): δ = 1.21–1.36 (m, CH(CH₃)₂, 48H), 1.44 (s, O–C(CH₃)₃, 9H), 1.67 (s, O–C(CH₃)₃, 9H), 2.37 (s, O–Ar–CH₃, 6H), 2.44 (s, Ar–CH₃, 3H), 2.90–3.12 (m, CH(CH₃)₂, 8H), 6.82–7.27 (m, Ar–H, 21H), 8.02 (s, CH=N, 2H), 8.27 (s, CH=N, 1H). 8.58 (s, CH=N, 1H). ¹³C NMR (100 MHz, CDCl₃): δ = 22.05(O–Ar–CH₃), 22.75 (CH(CH₃)₂), 23.53 (CH(CH₃)₂), 24.89 (CH(CH₃)₂), 27.82 (CH(CH₃)₂), 28.07 (CH(CH₃)₂),32.10 (CH(CH₃)₂), 32.38 (CH(CH₃)₂), 33.09 (CH(CH₃)₂), 33.37 (O–C(CH₃)₃), 33.94 (O–C(CH₃)₃), 76.32 (O–C(CH₃)₃), 118.74 (Ar–C), 122.05 (Ar–C), 122.65 (Ar–C), 123.12 (Ar–CH₃), 124.38 (Ar–C), 125.30 (Ar–C), 128.19 (Ar–C), 128.95 (Ar–C), 137.52 (Ar–C), 149.45 (Ar–C), 158.02 (Ar–O), 162.19 (CH=N), 163.42 (CH=N). ESI m/z calculated for [M]⁺. C₇₄H₁₀₀N₄O₄Hf: 1288.100 found 1288.353.

Compound 8. Yield = 81%. Mp: 175 °C. Anal. Calcd for C₇₉H₁₀₂Cl₂N₄O₄Hf: C 66.77, H 7.23, N 3.94. Found: C 66.90, H 7.39, N 4.12. ¹H NMR (400 MHz, CDCl₃): δ = 1.21–1.46 (m, CH(CH₃)₂, 48H), 1.63 (s, O–C(CH₃)₃, 9H), 1.66 (s, O–C(CH₃)₃, 9H), 2.88–3.12 (m, CH(CH₃)₂, 8H), 6.86–7.28 (m, Ar–H, 16H), 8.04 (s, CH=N, 2H), 8.27 (s, CH=N, 2H). ¹³C NMR (100 MHz, CDCl₃): δ = 21.40 (Ar–CH₃), 22.38 (CH(CH₃)₂), 22.78 (CH(CH₃)₂), 24.99 (CH(CH₃)₂), 27.73 (CH(CH₃)₂), 27.86 (CH(CH₃)₂), 28.01 (CH(CH₃)₂), 31.64 (CH(CH₃)₂), 32.16 (CH(CH₃)₂), 32.33 (O–C(CH₃)₃), 33.68 (O–C(CH₃)₃), 76.31 (O–C(CH₃)₃), 118.65 (Ar–C), 122.03 (Ar–C), 122.69 (Ar–C), 123.15 (Ar–CH₃), 124.36 (Ar–C), 125.24 (Ar–C), 127.70 (Ar–Cl), 128.16 (Ar–C), 128.97 (Ar–C), 137.67 (Ar–C), 149.65 (Ar–C), 157.92 (Ar–O), 162.27 (CH=N). ESI m/z calculated for [M+H]⁺. C₇₂H₉₄Cl₂N₄O₄Hf: 1328.937 found 1329.931.

Compound 9. Yield = 89%. Mp: 168 °C. Anal. Calc for C₈₁H₁₀₈N₄O₆Hf: C 68.89, H 7.71, N 3.97. Found: C 67.79, H 7.91, N 4.06. ¹H NMR (400 MHz, CDCl₃): δ = 1.22–1.35 (m, CH(CH₃)₂, 48H), 1.36–1.37 (s, O–C(CH₃)₃, 18H), 2.43 (s, Ar–CH₃, 3H), 3.05–3.12 (m, CH(CH₃)₂, 8H), 3.99 (s, O–Ar–OCH₃, 6H), 7.11–7.32 (m, Ar–H, 21H), 8.61 (s, CH=N, 4H). ¹³C NMR (100 MHz, CDCl₃): δ = 21.38 (Ar–CH₃), 22.37 (CH(CH₃)₂), 22.89 (CH(CH₃)₂), 23.53 (CH(CH₃)₂), 27.68 (CH(CH₃)₂), 27.84 (CH(CH₃)₂), 27.96 (CH(CH₃)₂), 31.17 (CH(CH₃)₂), 31.17 (CH(CH₃)₂), 32.22 (O–C(CH₃)₃), 33.08 (O–C(CH₃)₃), 56.11 (Ar–OCH₃), 69.85 (O–C(CH₃)₃), 118.43 (Ar–C), 122.49 (Ar–C), 122.68 (Ar–C), 123.06 (Ar–CH₃), 125.22 (Ar–C), 128.15 (Ar–C), 128.95 (Ar–C), 137.77 (Ar–C), 138.08 (Ar–CH₃), 139.48 (Ar–C), 148.52 (Ar–C), 152.15 (Ar–OCH₃), 155.87 (Ar–O), 162.68 (CH=N). ESI m/z calculated for [M]⁺. C₇₄H₁₀₀N₄O₆Hf: 1320.099 found 1320.368.

Compound 10. Yield = 95%. Mp: 175 °C. Anal. Calc for C₅₅H₆₃N₄O₈Hf: C 60.79, H, 5.84, N 5.16. Found: C 61.02, H, 5.92, N 4.98. ¹H NMR (400 MHz, CDCl₃): δ =1.27 (s, O–C(CH₃)₃, 18H), 2.25 (s, O–Ar–CH₃, 6H), 3.58 (s, Ar–O–CH₃, 6H), 3.86 (s, Ar–O–CH₃, 6H), 6.45–7.35 (m, Ar–H, 20H), 7.92 (s, CH=N, 2H), 8.96 (s, CH=N, 2H). ¹³C NMR (100 MHz, CDCl₃): δ = 20.06 (O–Ar–CH₃), 32.95 (Ar–O–CH₃), 55.36 (Ar–O–CH₃), 55.44 (Ar–O–CH₃), 75.52 (O–C(CH₃)₃), 113.22 (Ar–C), 114.28 (Ar–C), 122.27 (Ar–C), 123.17 (Ar–C), 123.61 (Ar–C), 125.85 (Ar–C), 126.03 (Ar–C), 132.62 (Ar–C), 137.52 (Ar–C), 145.14 (Ar–C), 145.84 (Ar–C), 155.84 (Ar–O), 157.63 (Ar–O–CH₃), 157.85 (Ar–O–CH₃), 161.02 (CH=N), 166.44 (CH=N).

Crystallographic data

Crystals of appropriate dimensions of 3, 5 and 10, suitable for X-ray diffraction studies were grown from toluene at −24 °C. A Bruker AXS (Kappa Apex 2) CCD diffractometer equipped with graphite monochromated Mo (Kα) (λ = 0.7107 Å) radiation source was employed for the collection of crystal data. ω and ϕ scans was employed to collect the data. The frame width for ω was set to 0.5° for all data collection. The data were collected with 100% completeness for θ up to 25°. The frames were integrated and the data collected were reduced for Lorentz and polarization corrections using SAINT-NT. The data set was subjected to the multi-scan absorption correction. All structures were solved using SIR-92 and refined using SHELXL-97.²⁰ The non hydrogen atoms were refined with anisotropic displacement parameter. All the hydrogen atoms could be located in the difference Fourier map. The hydrogen atoms bonded to carbon atoms were fixed at chemically meaningful positions and were allowed to ride with the parent atom during refinement. These data were deposited with CCDC.

Typical procedure for the bulk polymerization of CL, VL, rac-BL, L-LA and rac-LA

The procedure for the bulk polymerizations in 200 : 1 ratio between respective monomers and 1–9 are outlined below:

For CL polymerization, in an ampoule sealed under argon atmosphere, 11.28 μmol of 1–9 was added to 0.25 mL of monomer. The contents were stirred and immersed in a bath preheated to 80 °C. A rise in viscosity of the polymerization reaction was observed and finally the stirring ceased. The contents were dissolved into minimum quantity of CH₂Cl₂ and poured into cold methanol. The polymer precipitated immediately and was isolated by filtration. The filtered product was dried in vacuum in a vacuum oven until a constant weight was attained. Similarly for, VL polymerization, 13.47 μmol of 1–9 was used for 0.25 mL of monomer. The polymerization was performed at 80 °C and same procedure for work up was followed.

Again, for rac-BL polymerization, 15.33 μmol of 1–9 was used for 0.25 mL of monomer. The polymerization was performed at 80 °C and same procedure for work up was followed.

For L-LA or rac-LA, polymerization, 8.67 μmol of 1–9 and 0.25 g L-LA or rac-LA were taken in an ampoule under an argon atmosphere. The contents were rapidly stirred at 130 °C. Once the monomer melted completely, rise in viscosity of the polymerization reaction was observed and finally the stirring ceased. The contents were dissolved into minimum quantity of CH₂Cl₂ and poured into cold methanol. The polymer precipitated immediately and was isolated by filtration. The filtered product was dried in vacuum until a constant weight was attained.

Typical procedure for ethylene polymerization

The polymerizations were done in a 50 mL flask equipped with a magnetic stirrer. The flask was charged in an argon filled glove box with 8 mg of 1–9, 7 mL toluene along with the required amount of MAO. The flask was connected to a Schlenk line and the contents were subjected to repeated freeze thaw cycles. Finally, the flask was immersed in an oil bath preheated to 80 °C and ethylene gas was bubbled. Polymerization was noticed immediately. The ethylene feed was passed for 30 mins and subsequently the polymerization was quenched with acidic methanol. The polymer produced was collected by filtration and dried till constant weight was achieved.

Characterization of polymers

Data concerning molecular weights and the polydispersity indices of the polymer samples obtained by the ring-opening polymerization of various cyclic ester monomers and lactide were determined by using a GPC instrument with Waters 510 pump and Waters 410 differential refractometer as the detector. Three columns namely WATERS STRYGEL-HR5, STRYGEL-HR4 and STRYGEL-HR3 each of dimensions (7.8 × 300 mm) were connected in series. Measurements were done in THF at 27 °C. Number average molecular weights (M_n) and polydispersity (M_w/M_n) (MWDs) of polymers were measured relative to polystyrene standards.

Molecular weights (M_n and M_w) and the polydispersity indices (M_w/M_n) of ethylene samples were determined by GPC instrument with Waters 510 pump and Waters 2414 differential refractometer as the detector. The column namely WATERS STRYGEL-HR4 of dimensions (4.6 × 300 mm) was connected during the experiment. Measurements were done in tri-chloro benzene (TCB). Number average molecular weights (M_n) and molecular weight distributions (MWDs) of polymers were measured relative to polystyrene standards.

Polymerization kinetics

Bulk polymerization of different cyclic ester monomers and lactide using 3, 6 and 9 were carried out at 130 °C under an argon atmosphere. At different appropriate intervals of time, 0.2 mL aliquots were removed from the reaction mixture and poured directly into CDCl₃ containing 250 ppm BHT. The contents of the quenched aliquots obtained at various time intervals were analyzed by ¹H NMR for the determination of conversion. The [L-LA]_o/[L-LA]_t ratio was calculated by integration of the peak corresponding to the methine proton for the polymer and monomer. Apparent rate constant were obtained from the slopes of the best fit lines.

Acknowledgements

This work was supported by the research grant received from Department of Science and Technology, New Delhi, Government of India. TKS and BR thank the Council of Scientific and Industrial Research and the University Grants Commission, New Delhi for a research fellowship. The authors thank Mr. V. Ramkumar, Department of Chemistry, Indian Institute of Technology Madras, for X-ray diffraction studies. The authors express their gratitude to the referees for their comments and suggestions.

References

1. R. E. Drumright, P. R. Gruber and D. E. Henton, Adv. Mater., 2000, 12, 1841–1846.
2. (a) J. Lunt, Polym. Degrad. Stab., 1998, 59, 145–152; (b) S. K. Ritter, Chem. Eng. News, 2002, 80, 26–27; (c) E. T. H. Vink, K. R. Rábago, D. A. Glassner and P. R. Gruber, Polym. Degrad. Stab., 2003, 80, 403–419.
3. (a) A. –C. Albertsson and I. K. Varma, Adv. Polym. Sci., 2000, 157, 1–40; (b) R. A. Gross and B. Kalra, Science, 2002, 297, 803–807; (c) M. Okada, Prog. Polym. Sci., 2002, 27, 87–133.
4. (a) H. R. Kricheldorf, Chemosphere, 2000, 43, 49–54; (b) Y. Ikada and H. Tsuji, Macromol. Rapid Commun., 2000, 212, 117–132; (c) O. Coulembier, P. Degée, J. L. Hedrick and P. Dubois, Prog. Polym. Sci., 2006, 31, 723–747.
5. (a) B. J. O'Keefe, M. A. Hillmyer and W. B. Tolman, J. Chem. Soc., Dalton Trans., 2001, 2215–2224; (b) G. W. Coates, J. Chem. Soc., Dalton Trans., 2002, 467–475; (c) O. Dechy-Cabaret, B. Martin-Vaca and D. Bourissou, Chem. Rev., 2004, 104, 6147–6176; (d) J. Wu, T. –L. Yu, C. –T. Chen and C. –C. Lin, Coord. Chem. Rev., 2006, 250, 602–626; (e) S. Penczek, M. Cypryk, A. Duda, P. Kubisa and S. Slomkowski, Prog. Polym. Sci., 2007, 32, 247–282; (f) R. H. Platel, L. M. Hodgson and C. K. Williams, Polym. Rev., 2008, 48, 11–63; (g) C. A. Wheaton, P. G. Hayes and B. J. Ireland, Dalton Trans., 2009, 4832–4846; (h) M. Labet and W. Thielemans, Chem. Soc. Rev., 2009, 38, 3484–3504; (i) C. M. Thomas, Chem. Soc. Rev., 2010, 39, 165–173; (j) M. J. Stanford and A. P. Dove, Chem. Soc. Rev., 2010, 39, 486–494.
6. (a) H. R. Kricheldorf, M. Berl and N. Scharnagl, Macromolecules, 1988, 21, 286–293; (b) D. Takeuchi, T. Nakamura and T. Aida, Macromolecules, 2000, 33, 725–729; (c) Y. Kim and J. G. Verkade, Macromol. Rapid Commun., 2002, 23, 917–921; (d) Y. Kim and J. G. Verkade, Organometallics, 2002, 21, 2395–2399; (e) Y. Kim, G. K. Jnaneshwara and J. G. Verkade, Inorg. Chem., 2003, 42, 1437–1447; (f) Y. Takashima, Y. Nakayama, T. Hirao, H. Yasuda and A. Harada, J. Organomet. Chem., 2004, 689, 612–619; (g) Y. Kim and J. G. Verkade, Macromol. Symp., 2005, 224, 105–118; (h) S. K. Russell, C. L. Gamble, K. J. Gibbins, K. C. S. Juhl, W. S. Mitchell, A. J. Tumas and G. E. Hofmeister, Macromolecules, 2005, 38, 10336–10340; (i) C. K. A. Gregson, I. J. Blackmore, V. C. Gibson, N. J. Long, E. L. Marshall and A. J. P. White, Dalton Trans., 2006, 3134–3140; (j) D. Patel, S. T. Liddle, S. A. Mungur, M. Rodden, A. Blake and P. L. Arnold, Chem. Commun., 2006, 1124–1126; (k) S. Gendler, S. Segal, I. Goldberg, Z. Goldschmidt and M. Kol, Inorg. Chem., 2006, 45, 4783–4790; (l) A. J. Chmura, M. G. Davidson, M. D. Jones, M. D. Lunn, M. F. Mahon, A. F. Johnson, P. Khunkamchoo, S. L. Roberts and S. S. F. Wong, Macromolecules, 2006, 39, 7250–7257; (m) A. J. Chmura, M. G. Davidson, M. D. Jones, M. D. Lunn and M. F. Mahon, Dalton Trans., 2006, 887–889; (n) K. –C. Hsieh, W. –Y. Lee, L. –F. Hsueh, H. M. Lee and J. –H. Huang, Eur. J. Inorg. Chem., 2006, 2306–2312; (o) J. Cayuela, V. Bounor-Legaré, P. Cassagnau and A. Michel, Macromolecules, 2006, 39, 1338–1346; (p) C. J. Chuck, M. G. Davidson, M. D. Jones, G. Kociok-Köhn, M. D. Lunn and S. Wu, Inorg. Chem., 2006, 45, 6595–6597; (q) Y. Sarazin, R. H. Howard, D. L. Hughes, S. M. Humphrey and M. Bochmann, Dalton Trans., 2006, 340–350; (r) F. Gornshtein, M. Kapon, M. Botoshansky and M. Eisen, Organometallics, 2007, 26, 497–507; (s) R. C. J. Atkinson, K. Gerry, V. C. Gibson, N. J. Long, E. L. Marshall and L. J. West, Organometallics, 2007, 26, 316–320; (t) J. Lee, Y. Kim and Y. Do, Inorg. Chem., 2007, 46, 7701–7703; (u) S. Mun, J. Lee, S. H. Kim, Y. Hong, Y. –H. Ko, Y. K. Shin, J. H. Lim, C. S. Hong, Y. Do and Y. Kim, J. Organomet. Chem., 2007, 692, 3519–3525; (v) A. J. Chmura, D. M. Cousins, M. G. Davidson, M. D. Jones, M. D. Lunn and M. F. Mahon, Dalton Trans., 2008, 1437–1443; (w) A. J. Chmura, M. G. Davidson, C. J. Frankis, M. D. Jones and M. D. Lunn, Chem. Commun., 2008, 1293–1295; (x) Y. Ning, Y. Zhang, A. R. Delgado and E. Y. –X. Chen, Organometallics, 2008, 27, 5632–5640; (y) R. F. Munha, L. G. Alves, N. Maulide, M. T. Duarte, I. E. Marko, M. D. Fryzuk and A. M. Martins, Inorg. Chem. Commun., 2008, 11, 1174–1176; (z) K. Krauzy-Dziedzic, J. Ejfler, S. Szafert and P. Sobota, Dalton Trans., 2008, 2620–2626.
7. (a) P. Dobrzynski and J. Polym, J. Polym. Sci., Part A: Polym. Chem., 2004, 42, 1886–1900; (b) J. Cheng, J. Sun, K. Wu and X. Yan, Huaxue Gongye Yu Gongcheng Jishu, 2006, 27, 5–7; (c) A. J. Chmura, D. M. Cousins, M. G. Davidson, M. D. Jones, M. D. Lunn and M. F. Mahon, Dalton Trans., 2008, 1437–1443; (d) A. Grafov, S. Vuorinen, T. Repo, M. Kemell, M. Nieger and M. Leskelä, Eur. Polym. J., 2008, 44, 3797–3805; (e) G. Zi, Q. Wang, L. Xiang and H. Song, Dalton Trans., 2008, 5930–5944; (f) A. L. Zelikoff, J. Kopilov, I. Goldberg, G. W. Coates and M. Kol, Chem. Commun., 2009, 6804–6806; (g) A. D. Schwarz, A. L. Thompson and P. Mountford, Inorg. Chem., 2009, 48, 10442–10454; (h) R. H. Howard, C. Alonso-Moreno, L. M. Broomfield, D. L. Hughes, J. A. Wright and M. Bochmann, Dalton Trans., 2009, 8667–8682; (i) E. Kim, E. W. Shin, I. –K. Yoo and J. S. Chung, J. Mol. Catal. A: Chem., 2009, 298, 36–39; (j) E. L. Whitelaw, M. D. Jones, M. F. Mahon and G. Kociok-Kohn, Dalton Trans., 2009, 9020–9025; (k) M. D. Jones, M. G. Davidson and G. Kociok-Kohn, Polyhedron, 2010, 29, 697–700; (l) F. Zhang, H. Song and G. Zi, J. Organomet. Chem., 2010, 695, 1993–1999; (m) E. Sergeeva, J. Kopilov, I. Goldberg and M. Kol, Inorg. Chem., 2010, 49, 3977–3979; (n) A. D. Schwarz, K. R. Herbert, C. Paniagua and P. Mountford, Organometallics, 2010, 29, 4171–4188; (o) M. Hu, M. Wang, H. Zhu, L. Zhang, H. Zhang and L. Sun, Dalton Trans., 2010, 39, 4440–4446; (p) M. G. Davidson, S. D. Bull, T. R. Forder, C. J. Frankis and M. D. Jones, Polymer Preprints, 2010, 51, 390–391; (q) A. Stopper, I. Goldberg and M. Kol, Inorg. Chem. Commun., 2011, 14, 715–718; (r) S. L. Hancock, M. F. Mahon and M. D. Jones, Dalton Trans., 2011, 40, 2033–2037.
8. (a) R. R. Gowda, D. Chakraborty and V. Ramkumar, Eur. J. Inorg. Chem., 2009, 2981–2993; (b) T. K. Saha, B. Rajashekhar, R. R. Gowda, V. Ramkumar and D. Chakraborty, Dalton Trans., 2010, 39, 5091–5093; (c) R. R. Gowda, D. Chakraborty and V. Ramkumar, Polymer, 2010, 51, 4750–4759; (d) R. R. Gowda, D. Chakraborty and V. Ramkumar, J. Organomet. Chem., 2011, 696, 572–580; (e) T. K. Saha, V. Ramkumar and D. Chakraborty, Inorg. Chem., 2011, 50, 2720–2722.
9. (a) K. Ziegler, H. Kolzkamp, H. Breil and H. Martin, Angew. Chem., 1955, 67, 541–544; (b) G. Natta, P. Pino, P. Corradini, F. Danusso, E. Mantica, G. Mazzanti and G. Moraglio, J. Am. Chem. Soc., 1955, 77, 1708–1710; (c) G. Natta, Angew. Chem., 1956, 68, 393–396; (d) H. Sinn and W. Kaminsky, Adv. Organomet. Chem., 1980, 18, 99–149; (e) H. Sinn, W. Kaminsky, H. J. Vollmer and R. Woldt, Angew. Chem., Int. Ed. Engl., 1980, 19, 390–392.
10. (a) G. W. Coates and R. M. Waymouth, Science, 1995, 267, 217–219; (b) E. Hauptman, R. M. Waymouth and J. W. Ziller, J. Am. Chem. Soc., 1995, 117, 11586–11587; (c) R. Kravchenko, A. Masood and R. M. Waymouth, Organometallics, 1997, 16, 3635–3639; (d) J. L. Maciejewski-Petoff, M. D. Bruce, R. M. Waymouth, A. Masood, T. K. Lal, R. W. Quan and S. J. Behrend, Organometallics, 1997, 16, 5909–5916; (e) M. D. Bruce, G. W. Coates, E. Hauptman, R. M. Waymouth and J. W. Ziller, J. Am. Chem. Soc., 1997, 119, 11174–11182; (f) R. Kravchenko and R. M. Waymouth, Macromolecules, 1998, 31, 1–6; (g) R. Kravchenko, A. Masood, R. M. Waymouth and C. L. Myers, J. Am. Chem. Soc., 1998, 120, 2039–2046; (h) Y. Hu, M. T. Krejchi, C. D. Shah, C. L. Myers and R. M. Waymouth, Macromolecules, 1998, 31, 6908–6916; (i) E. D. Carlson, M. T. Krejchi, C. D. Shah, T. Terakawa, R. M. Waymouth and G. G. Fuller, Macromolecules, 1998, 31, 5343–5351; (j) S. Lin and R. M. Waymouth, Acc. Chem. Res., 2002, 35, 765–773.
11. (a) A. O. Patil and G. G. Haltky, ACS Symp. Ser., 2003, 857, 1–11; (b) V. C. Gibson and S. K. Spitzmesser, Chem. Rev., 2003, 103, 283–316.
12. (a) J. D. Scollard and D. H. McConville, J. Am. Chem. Soc., 1996, 118, 10008–10009; (b) F. G. N. Cloke, T. J. Geldbach, P. B. Hitchcock and J. B. Love, J. Organomet. Chem., 1996, 506, 343–345; (c) Z. Ziniuk, I. Goldberg and M. Kol, Inorg. Chem. Commun., 1999, 2, 549–551; (d) K. Nomura, K. Oya and Y. Imanishi, Polymer, 2000, 41, 2755–2764; (e) C. H. Lee, Y. –H. La and J. W. Park, Organometallics, 2000, 19, 344–351; (f) E. Y. Tshuva, I. Goldberg and M. Kol, J. Am. Chem. Soc., 2000, 122, 10706–10707; (g) E. Y. Tshuva, I. Goldberg, M. Kol and Z. Goldschmidt, Organometallics, 2001, 20, 3017–3028; (h) S. J. Kim, I. N. Jung, B. R.Yoo, S. H. Kim, J. J. Ko, D. J. Byun and S. O. Kang, Organometallics, 2001, 20, 2136–2144; (i) S. Danie`le, P. B. Hitchcock, M. F. Lappert and P. G. Merle, J. Chem. Soc., Dalton Trans., 2001, 13–19; (j) H. Hagimoto, T. Shiono and T. Ikeda, Macromol. Rapid Commun., 2002, 23, 73–76.
13. (a) H. Mack and M. S. Eisen, J. Organomet. Chem., 1996, 525, 81–87; (b) R. R. Schrock, R. Baumann, S. M. Reid, J. T. Goodman, R. Stumpf and W. M. Davis, Organometallics, 1999, 18, 3649–3670; (c) D. D. Graf, R. R. Schrock, W. M. Davis and R. Stumpf, Organometallics, 1999, 18, 843–852; (d) L. –C. Liang, R. R. Schrock, W. M. Davis and D. H. McConville, J. Am. Chem. Soc., 1999, 121, 5797–5798; (e) P. Mehrkhodavandi, P. J. Bonitatebus Jr. and R. R. Schrock, J. Am. Chem. Soc., 2000, 122, 7841–7842; (f) L. H. Gade, Chem. Commun., 2000, 173–181; (g) R. R. Schrock, P. J. Bonitatebus Jr. and Y. Schrodi, Organometallics, 2001, 20, 1056–1058; (h) R. R. Schrock, P. J. Bonitatebus Jr. and Y. Schrodi, Organometallics, 2001, 20, 3560–3573; (i) J. T. Goodman and R. R. Schrock, Organometallics, 2001, 20, 5205–5211; (j) E. Shaviv, M. Botoshansky and M. S. Eisen, J. Organomet. Chem., 2003, 683, 165–180.
14. (a) V. Busico, R. Cipullo, L. Caporaso, G. Angelini and A. L. Segre, J. Mol. Catal. A: Chem., 1998, 128, 53–64; (b) J. Richter, F. T. Edelmann, M. Noltemeyer, H.-G. Schmidt, M. Shmulinson and M. S. Eisen, J. Mol. Catal. A: Chem., 1998, 130, 149–162; (c) V. Volkis, M. Shmulinson, C. Averbuj, A. Lisovskii, F. T. Edelmann and M. S. Eisen, Organometallics, 1998, 17, 3155–3157; (d) K. C. Jayaratne and L. R. Sita, J. Am. Chem. Soc., 2000, 122, 958–959; (e) R. J. Keaton, K. C. Jayaratne, D. A. Henningsen, L. A. Koterwas and L. R. Sita, J. Am. Chem. Soc., 2001, 123, 6197–6198; (f) K. C. Jayaratne and L. R. Sita, J. Am. Chem. Soc., 2001, 123, 10754–10755; (g) V. Volkis, E. Nelkenbaum, A. Lisovskii, G. Hasson, R. Semiat, M. Kapon, M. Botoshansky, Y. Eishen and M. S. Eisen, J. Am. Chem. Soc., 2003, 125, 2179–2194.
15. (a) Y. Nakayama, K. Watanabe, N. Ueyama, A. Nakamura, A. Harada and J. Okuda, Organometallics, 2000, 19, 2498–2503; (b) R. M. Gauvin, J. A. Osborn and J. Kress, Organometallics, 2000, 19, 2944–2946; (c) P. Sobota, K. Przybylak, J. Utko, L. B. Jerzykiewicz, A. J. L. Pombeiro, M. da Silva and K. Szczegot, Chem.–Eur. J., 2001, 7, 951–958; (d) J. Saito, M. Mitani, J. Mohri, Y. Yoshida, S. Matsui, S. Ishii, S. Kojoh, N. Kashiwa and T. Fujita, Angew. Chem., Int. Ed., 2001, 40, 2918–2920; (e) N. Matsukawa, S. Matsui, M. Mitani, J. Saito, K. Tsuru and T. Fujita, J. Mol. Catal. A: Chem., 2001, 169, 99–104; (f) S. Matsui, M. Mitani, J. Saito, Y. Tohi, H. Makio, N. Matsukawa, Y. Takagi, K. Tsuru, M. Nitabaru, T. Nakano, H. Tanaka, N. Kashiwa and T. Fujita, J. Am. Chem. Soc., 2001, 123, 6847–6856.
16. M. Veith, S. Mathur, C. Mathur and V. Huch, J. Chem. Soc., Dalton Trans., 1997, 2101–2108.
17. L. F. Sánchez–Barba, A. Garcés, J. Fernández-Baeza, A. Otero, C. Alonso–Moreno, A. Lara–Sánchez and A. M. Rodríguez, Organometallics, 2011, 30, 2775–2789.
18. A. Amgoune, C. M. Thomas, S. Ilinca, T. Roisnel and J. –F. Carpentier, Angew. Chem., Int. Ed., 2006, 45, 2782–2784.
19. (a) L. Han, J. Du, H. Yang, H. Wang, X. Leng, A. Galstyan, S. Zariæ and W. –H. Sun, Inorg. Chem. Commun., 2003, 6, 5–9; (b) J. Du, L. Han, Y. Cui, J. Li, Y. Li and W. –H. Sun, Aust. J. Chem., 2003, 56, 703–706; (c) Al–Jeboori, M. Jaber, Al–Shihri and S. Ayed, J. Saudi Chem. Soc., 2001, 5, 341–352.
20. G. M. Sheldrick, SHELXL97. Program for crystal structure refinement, Göttingen, Germany.

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Footnotes

† Electronic supplementary information (ESI) available. CCDC reference numbers 836424–836426. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c1ra00543j

‡ This paper is dedicated to Prof. Herbert W. Roesky on the occasion of his 75th birthday.

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