Feijie
Ge
a,
Yi
Dan
a,
Yahya
Al-Khafaji
b,
Timothy J.
Prior
b,
Long
Jiang
*a,
Mark R. J.
Elsegood
c and
Carl
Redshaw
*b
aState Key Laboratory of Polymer Materials Engineering of China (Sichuan University), Polymer Research Institute of Sichuan University, Chengdu 610065, Sichuan, China. E-mail: jianglong@scu.edu.cn; Fax: +86-28-85402465; Tel: +86-28-85407286
bDepartment of Chemistry, University of Hull, Hull, HU6 7RX, UK. E-mail: C.Redshaw@Hull.ac.uk; Fax: +44 (0)1482 466410; Tel: +44 (0)1482 465219
cChemistry Department, Loughborough University, Leicestershire LE11 3TU, UK
First published on 22nd December 2015
The vanadyl complexes [VO(OtBu)L1] (1) and {[VO(OiPr)]2(μ-p-L2p)} (2) {[VO(OR)]2(μ-p-L2m)} (R = iPr 3, tBu 4) have been prepared from [VO(OR)3] (R = nPr, iPr or tBu) and the respective phenol, namely 2,2′-ethylidenebis(4,6-di-tert-butylphenol) (L1H2) or α,α,α′,α′-tetra(3,5-di-tert-butyl-2-hydroxyphenyl–p/m-)xylene-para-tetraphenol (L2p/mH4). For comparative studies, the known complexes [VO(μ-OnPr)L1]2 (I), [VOL3]2 (II) (L3H3 = 2,6-bis(3,5-di-tert-butyl-2-hydroxybenzyl)-4-tert-butylphenol) were prepared. An imido complex {[VCl(Np-tolyl)(NCMe)]2(μ-p-L2p)} (5) has been prepared following work-up from [V(Np-tolyl)Cl3], L2pH4 and Et3N. The molecular structures of complexes 1–5 are reported. Complexes 1–5 and I and II have been screened for their ability to ring open polymerise ε-caprolactone, L-lactide or rac-lactide with and without solvent present. The co-polymerization of ε-caprolactone with L-lactide or rac-lactide afforded co-polymers with low lactide content; the reverse addition was ineffective.
One metal attracting attention in polymerisation catalysis is vanadium.4 Given the toxicity associated with this metal is relatively low,5 we have embarked upon a program to screen various vanadium systems for their ability to deliver, via ROP, biodegradable polymers with desirable properties. We note that reports on the use of group 5 complexes for the ROP of cyclic esters are scant.6 Herein, we investigate the potential of vanadyl complexes with ligands derived from di-(L1H2) or tetra-phenols (L2p/mH4) and for comparative studies the tri-phenol (L3H3) (see Scheme 1) for the ROP of both L-lactide and ε-caprolactone and the co-polymerisation thereof, and report the effects of the structures of the complexes on the properties of the final polymeric products. Such chelating phenoxide ligation has proved useful in olefin polymerisation,4 but their use in the ROP of lactides and lactones, particularly that of the tri- and tetra-phenolic ligand sets, is rather limited.6d,7,8 Use of a tetra-phenolate ligand set, both meta and para forms, also allowed us to probe for possible cooperative effects, viz.1versus2–4. With this in mind, the crystal structures of the monomeric vanadyl complex [VO(OtBu)L1] (1) and the dinuclear vanadyl complexes {[VO(OiPr)]2(μ-p-L2p)} (2), {[VO(OR)]2(μ-p-L2m)} (R = iPr 3, tBu 4) are also presented; the molecular structures of I and II have been reported elsewhere.9
Bond lengths (Å)/angles (°) | 3 | 4·2CH2Cl2 | 4·3CH2Cl2 |
---|---|---|---|
V1–O(phenolate) | 1.790(2) | 1.7908(15) | 1.790(6) |
1.790(2) | 1.7963(15) | 1.806(5) | |
V1![]() |
1.581(3) | 1.5890(16) | 1.577(6) |
V1–O(alkoxide) | 1.733(3) | 1.7298(16) | 1.736(6) |
V2–O(phenolate) | 1.791(2) | 1.7891(16) | 1.792(6) |
1.793(2) | 1.7904(15) | 1.796(5) | |
V2![]() |
1.584(3) | 1.5908(17) | 1.572(7) |
V2–O(alkoxide) | 1.734(3) | 1.7323(16) | 1.735(6) |
O1–V1–O2 | 111.31(7) | ||
O2–V1–O5 | 106.92(8) | ||
O5–V1–O6 | 112.95(8) | ||
V1–O1–C1 | 126.91(13) | ||
V1–O16–C1 | 126.91(13) | ||
V1–O6–C65 | 143.63(15) |
Extension of this synthetic methodology to the tetra-phenol α,α,α′,α′-tetra(3,5-di-tert-butyl-2-hydroxyphenyl–p-)xylene-para-tetra-phenol (L2pH4) afforded the dinuclear complex {[VO(OiPr)]2(μ-p-L2p)} (2) in good yield.
Crystals of 2 suitable for X-ray diffraction were readily grown from a saturated dichloromethane solution at 0 °C. The structure of 2 is shown in Fig. 2 (for ORTEP, see Fig. S2†), with selected bond lengths and angles given in the caption; crystal structure data are given in Table 5. The tetra-phenolate ligand is centrosymmetric with one vanadyl cation bound above the plane of the central aromatic ring and one beneath. The separation of these two identical metal centres is 11.756 Å. Each vanadium centre can be described as adopting a pseudo tetrahedral geometry. The bite angle formed by the tetra-phenolate at each vanadium is 109.4(2)°, which is slightly smaller than that observed for 1 (110.1(2)°) and for the recently reported alkoxide complexes {[VO(OR)]2(μ-p-L2p)} [R = nPr, 111.73(7)°; tBu, 112.0(2)°]; and the metallocycle adopts a flattened boat conformation. The isopropoxide ligand can be described as bent with a V2–O8–C68 angle of 130.1(5)°.
Similar use of the meta tetra-phenol α,α,α′,α′-tetra(3,5-di-tert-butyl-2-hydroxyphenyl–m-)xylene-meta-tetra-phenol (L2mH4) with [VO(OR)3] (R = nPr or tBu) afforded the dinuclear complexes {[VO(OR)]2(μ-m-L2m)} (R = iPr 3, tBu 4) in good yield. Crystals of 3 and of 4 suitable for an X-ray diffraction studies were obtained on cooling (to −20 °C) their respective saturated dichloromethane solutions. The molecular structure of 3·2CH2Cl2 is shown in Fig. 3 (for ORTEP, see Fig. S3†), with selected bond lengths (Å) and angles (°) given in Table 1 where they are compared with those of 4·2CH2Cl2 and 4·3CH2Cl2.
![]() | ||
Fig. 3 Molecular structure of complex 3·2CH2Cl2, indicating the atom numbering scheme. Hydrogen atoms and solvent molecules of crystallisation have been removed for clarity. |
In 3, the vanadium centres can be described as adopting a pseudo tetrahedral geometry. The bite angle formed by the tetra-phenolate at each vanadium is 110.55(11)°, which is slightly larger than that observed for 1; again the metallocycle adopts a chair-boat conformation. The iso-propoxide ligand can be described as bent with a V2–O8–C68 angle of 140.5(3)°; the V–O iso-propoxide bond lengths are similar to those observed elsewhere.12
For 4·2CH2Cl2, there is one molecule of the complex and two molecules of CH2Cl2 (modelled by the Platon SQUEEZE procedure) in the asymmetric unit.13 Each vanadium center adopts a pseudo-tetrahedral geometry (see Fig. 4; for ORTEP, see Fig. S4†), with bond angles in the range 106.92(8)–112.95(8)°; the bite angle of the chelate is 111.31(7)°. The tert-butoxide ligand is somewhat bent [V1–O6–C65 = 143.63(15)°], with a slightly larger angle than that observed in the iso-propoxide 3 and presumably reflects the greater steric bulk of the tert-butoxide. The molecules pack in layers, however there is no significant interaction between the layers.
![]() | ||
Fig. 4 Molecular structure of complex 4·2CH2Cl2, indicating the atom numbering scheme. Hydrogen atoms and solvent molecules of crystallisation have been removed for clarity. |
From a repeated synthesis of 4, a different solvate was obtained, namely 4·3CH2Cl2. In the molecular structure of 4·3CH2Cl2, determined using synchrotron radiation,14 there is one well-defined dichloromethane which is involved in intramolecular interactions (see Fig. 5, for ORTEP, see Fig. S5†). In particular, there is a C–H⋯O H-bond to the oxo group O7 (see Table S1 in ESI† for geometry) and a C–H⋯π interaction with the aromatic ring C51 > C56 {H(73B)⋯ring centroid = 2.56(2) Å}.
![]() | ||
Fig. 5 Molecular structure of complex 4·3CH2Cl2, indicating the atom numbering scheme. Hydrogen atoms except those on the CH2Cl2 have been removed for clarity. |
The known vanadyl complexes [VO(μ-OnPr)L1]2 (I), [VOL3]2 (II) were prepared as described previously (L3H3 = 2,6-bis(3,5-di-tert-butyl-2-hydroxybenzyl)-4-tert-butylphenol) via the reaction of [VO(OnPr)3] and the respective chelating phenol.9
51V NMR data for 1–5 and I and II are presented in Table 2, and all vanadyl complexes appear in the range δ −410 to −498 ppm, with the 5-coordinate VO4 centre in II slightly upfield of the other 4-coordinate VO3 containing species. The imido complex 5 appears somewhat downfield, a position which also reflects the presence of the chloride ligand; line widths are also increased in the presence of imido groups.15
Runa | Cat | Temp/°C | Time/h | [CL]0![]() ![]() ![]() ![]() |
Conv.b (%) | M n,GPC c | M n,cal d | PDIe |
---|---|---|---|---|---|---|---|---|
a All reactions were carried out in toluene under nitrogen. b Determined by 1H NMR. c M n values were determined by GPC in THF vs. PS standards and were corrected with a Mark–Houwink factor of 0.56. d Calculated by (F.W. monomer × [monomer]/[cat]) × conversion + F.W. BnOH. e (Mw/Mn) were determined by GPC. | ||||||||
1 | 1 | 45 | 24 | 200![]() ![]() ![]() ![]() |
84 | 2440 | 19![]() |
1.11 |
2 | 1 | 60 | 24 | 200![]() ![]() ![]() ![]() |
99 | 4450 | 22![]() |
1.43 |
3 | 1 | 80 | 24 | 200![]() ![]() ![]() ![]() |
99 | 5430 | 22![]() |
2.01 |
4 | I | 45 | 24 | 200![]() ![]() ![]() ![]() |
98 | 4600 | 22![]() |
1.15 |
5 | I | 60 | 24 | 200![]() ![]() ![]() ![]() |
99 | 5760 | 22![]() |
1.80 |
6 | I | 80 | 24 | 200![]() ![]() ![]() ![]() |
98 | 5110 | 25![]() |
1.66 |
7 | II | 45 | 24 | 200![]() ![]() ![]() ![]() |
98 | 2860 | 22![]() |
1.20 |
8 | II | 60 | 24 | 200![]() ![]() ![]() ![]() |
98 | 3040 | 22![]() |
1.14 |
9 | II | 80 | 24 | 200![]() ![]() ![]() ![]() |
99 | 5950 | 22![]() |
1.32 |
10 | 2 | 45 | 24 | 200![]() ![]() ![]() ![]() |
55 | 1620 | 12![]() |
1.17 |
11 | 2 | 60 | 24 | 200![]() ![]() ![]() ![]() |
95 | 2850 | 21![]() |
1.20 |
12 | 2 | 80 | 24 | 200![]() ![]() ![]() ![]() |
99 | 4110 | 22![]() |
1.14 |
13 | 3 | 45 | 24 | 200![]() ![]() ![]() ![]() |
— | — | — | — |
14 | 3 | 60 | 24 | 200![]() ![]() ![]() ![]() |
49 | 1610 | 11![]() |
1.15 |
15 | 3 | 80 | 24 | 200![]() ![]() ![]() ![]() |
99 | 2880 | 22![]() |
1.35 |
16 | 4 | 80 | 1 | 200![]() ![]() ![]() ![]() |
— | — | — | — |
17 | 4 | 80 | 6 | 200![]() ![]() ![]() ![]() |
49 | 1120 | 11![]() |
1.12 |
18 | 4 | 80 | 12 | 200![]() ![]() ![]() ![]() |
89 | 2500 | 20![]() |
1.23 |
19 | 4 | 45 | 24 | 200![]() ![]() ![]() ![]() |
— | — | — | — |
20 | 4 | 60 | 24 | 200![]() ![]() ![]() ![]() |
50 | 1620 | 11![]() |
1.12 |
21 | 4 | 80 | 24 | 100![]() ![]() ![]() ![]() |
89 | 2140 | 10![]() |
1.16 |
22 | 4 | 80 | 24 | 200![]() ![]() ![]() ![]() |
99 | 3520 | 22![]() |
1.33 |
23 | 4 | 80 | 24 | 400![]() ![]() ![]() ![]() |
99 | 4920 | 45![]() |
1.42 |
24 | 4 | 80 | 24 | 600![]() ![]() ![]() ![]() |
99 | 5720 | 67![]() |
1.47 |
25 | 4 | 45 | 24 | 200![]() ![]() ![]() ![]() |
10 | 800 | 2280 | 1.13 |
26 | 4 | 60 | 24 | 200![]() ![]() ![]() ![]() |
97 | 1200 | 22![]() |
1.29 |
27 | 4 | 80 | 24 | 200![]() ![]() ![]() ![]() |
99 | 2510 | 22![]() |
1.30 |
28 | 5 | 45 | 24 | 200![]() ![]() ![]() ![]() |
— | — | — | — |
29 | 5 | 60 | 24 | 200![]() ![]() ![]() ![]() |
97 | 2170 | 22![]() |
1.16 |
30 | 5 | 80 | 24 | 200![]() ![]() ![]() ![]() |
99 | 3150 | 22![]() |
1.21 |
31 | 5 | 45 | 24 | 200![]() ![]() ![]() ![]() |
— | 440 | — | 1.16 |
32 | 5 | 60 | 24 | 200![]() ![]() ![]() ![]() |
99 | 3260 | 22![]() |
1.14 |
33 | 5 | 80 | 24 | 200![]() ![]() ![]() ![]() |
99 | 5780 | 22![]() |
1.43 |
In the 1H NMR spectra of the resulting PCL (e.g. see ESI, Fig. S7†), for runs involving pro-catalysts with a V-OR moiety present, signals were assignable to a hydroxyl end group (CH2OH) and an alkyl ester (e.g., isopropyl ester for 3). This indicated that the polymerization procedure involved rupture of the monomer acyl–oxygen bond and insertion in the alkoxide–vanadium bond. For runs conducted in the presence of BnOH, 1H NMR spectra were more complicated in terms of end group, with both OBn and OR (e.g. t-Bu for 4) present (see ESI, Fig. S8†).
Interestingly, in the absence of solvent, these systems performed far better at 80 °C, with conversions ≥ 95%, polydispersities ≤ 1.62 (see Table 4) and in general afforded higher observed molecular weight (Mn) polymers (see Table 4). The monomeric tert-butoxide complex 1 was found to afford the best yield (90%) and highest molecular weight (Mn) PCL (∼16300). By contrast, III afforded lowest conversion and highest PDI, which we assume is due to the lack of a readily accessible alkoxide bond (a phenoxide linkage of the tri-phenolate would need to be broken). Interestingly, the isopropoxides 2 and 3 gave very similar results, whilst the tert-butoxide 4 afforded a polymer of much lower molecular weight (Mn). For complex 5, it was necessary to add an equivalent of BnOH to achieve ROP activity (run 7 vs. 8, Table 4), and the resulting polymer was of higher molecular weight (Mn) ∼14
000 g mol−1.
Run | Cat | Time/min | [CL]0![]() ![]() ![]() ![]() |
M n,GPC a | M n,cal b | PDIc | Conv. (%) |
---|---|---|---|---|---|---|---|
a M n values were determined by GPC in THF vs. PS standards and were corrected with a Mark–Houwink factor of 0.56. b (F.W. monomer × [monomer]/[cat]) × conversion + F.W. BnOH. c M w/Mn were determined by GPC. | |||||||
1 | 1 | 20 | 200![]() ![]() ![]() ![]() |
16![]() |
22![]() |
1.45 | 99 |
2 | I | 20 | 200![]() ![]() ![]() ![]() |
4930 | 22![]() |
1.26 | 99 |
3 | II | 30 | 200![]() ![]() ![]() ![]() |
11![]() |
21![]() |
1.62 | 95 |
4 | 2 | 30 | 200![]() ![]() ![]() ![]() |
7200 | 22![]() |
1.34 | 97 |
5 | 3 | 30 | 200![]() ![]() ![]() ![]() |
7130 | 21![]() |
1.50 | 96 |
6 | 4 | 40 | 200![]() ![]() ![]() ![]() |
2660 | 22![]() |
1.20 | 98 |
7 | 5 | 30 | 200![]() ![]() ![]() ![]() |
14![]() |
22![]() |
1.49 | 97 |
8 | 5 | 60 | 200![]() ![]() ![]() ![]() |
— | — | — | — |
In general for the CL runs, despite the narrow polydispersity, the polymer molecular weights (Mn) were much lower than expected, indicating in all cases that significant trans-esterification reactions were occurring. Further evidence was provided by the MALDI-ToF mass spectra where, as well as the major population of peaks, there was evidence of a second, albeit minor, population (see ESI, Fig S9–S12† in toluene; Fig. S13 and S14,† no solvent).
For 6, a plot of average molecular weight (Mn) versus against conversion (Fig. 7, runs 21–24 Table 3) exhibited a linear relationship. Given the plot also shows that the PDI remained narrow, it suggests that under these conditions the ROP by 6 is proceeding in a living manner.
The production of only low molecular weight polymers using alkoxy vanadium systems has been noted previously.9 Herein, there was little correlation of 51V NMR signal (Table 2) versus catalytic activity (see ESI, Fig. S15†).
Runa | Cat | Temp/°C | Time/h | [LA]0![]() ![]() ![]() ![]() |
Conv.b (%) | M n,GPC c | M n,cal d | PDIe |
---|---|---|---|---|---|---|---|---|
a All reactions were carried out in toluene under nitrogen. b Determined by 1H NMR. c M n values were determined by GPC in THF vs. PS standards and were corrected with a Mark–Houwink factor of 0.58. d (F.W. monomer × [monomer]/[cat]) × conversion + F.W. BnOH. e (Mw/Mn) were determined by GPC. | ||||||||
1 | 1 | 45 | 31 | 200![]() ![]() ![]() ![]() |
— | — | — | — |
2 | 1 | 60 | 24 | 200![]() ![]() ![]() ![]() |
10 | 1480 | 2880 | 1.57 |
3 | 1 | 80 | 6 | 200![]() ![]() ![]() ![]() |
— | — | — | — |
4 | 1 | 80 | 24 | 200![]() ![]() ![]() ![]() |
54 | 2310 | 15![]() |
1.24 |
5 | 1 | 80 | 35 | 200![]() ![]() ![]() ![]() |
50 | 2100 | 14![]() |
1.26 |
6 | I | 45 | 34 | 200![]() ![]() ![]() ![]() |
— | — | — | — |
7 | I | 60 | 34 | 200![]() ![]() ![]() ![]() |
— | — | — | — |
8 | I | 80 | 6 | 200![]() ![]() ![]() ![]() |
— | — | — | — |
9 | I | 80 | 24 | 200![]() ![]() ![]() ![]() |
54 | 2930 | 15![]() |
1.73 |
10 | I | 80 | 31 | 200![]() ![]() ![]() ![]() |
54 | 2640 | 15![]() |
1.16 |
11 | II | 60 | 34 | 200![]() ![]() ![]() ![]() |
— | — | — | — |
12 | II | 80 | 6 | 200![]() ![]() ![]() ![]() |
— | — | — | — |
13 | II | 80 | 24 | 200![]() ![]() ![]() ![]() |
58 | 3010 | 16![]() |
1.46 |
14 | II | 80 | 34 | 200![]() ![]() ![]() ![]() |
60 | 3440 | 17![]() |
1.31 |
15 | 2 | 60 | 31 | 200![]() ![]() ![]() ![]() |
— | — | — | — |
16 | 2 | 80 | 6 | 200![]() ![]() ![]() ![]() |
— | — | — | — |
17 | 2 | 80 | 24 | 200![]() ![]() ![]() ![]() |
50 | 2120 | 14![]() |
1.34 |
18 | 2 | 80 | 36 | 200![]() ![]() ![]() ![]() |
58 | 2190 | 16![]() |
1.30 |
19 | 3 | 45 | 33 | 200![]() ![]() ![]() ![]() |
— | — | — | — |
20 | 3 | 60 | 35 | 200![]() ![]() ![]() ![]() |
— | — | — | — |
21 | 3 | 80 | 6 | 200![]() ![]() ![]() ![]() |
— | — | — | — |
22 | 3 | 80 | 24 | 200![]() ![]() ![]() ![]() |
15 | 1340 | 4320 | 1.11 |
23 | 3 | 80 | 36 | 100![]() ![]() ![]() ![]() |
52 | 2150 | 7490 | 1.11 |
24 | 3 | 80 | 36 | 200![]() ![]() ![]() ![]() |
52 | 2510 | 14![]() |
1.09 |
25 | 3 | 80 | 36 | 400![]() ![]() ![]() ![]() |
60 | 34![]() |
34![]() |
1.26 |
26 | 3 | 80 | 36 | 600![]() ![]() ![]() ![]() |
62 | 3760 | 53![]() |
1.29 |
27 | 3 | 110 | 31 | 200![]() ![]() ![]() ![]() |
51 | 2010 | 14![]() |
1.26 |
28 | 4 | 60 | 30 | 200![]() ![]() ![]() ![]() |
9 | 870 | 2590 | 1.23 |
29 | 4 | 80 | 6 | 200![]() ![]() ![]() ![]() |
— | — | — | — |
30 | 4 | 80 | 24 | 200![]() ![]() ![]() ![]() |
20 | 1930 | 5760 | 1.01 |
31 | 4 | 80 | 31 | 200![]() ![]() ![]() ![]() |
45 | 2460 | 12![]() |
1.10 |
32 | 5 | 80 | 31 | 200![]() ![]() ![]() ![]() |
— | — | — | — |
33 | 5 | 80 | 6 | 200![]() ![]() ![]() ![]() |
— | — | — | — |
34 | 5 | 80 | 24 | 200![]() ![]() ![]() ![]() |
53 | 3400 | 15![]() |
1.29 |
35 | 5 | 110 | 31 | 200![]() ![]() ![]() ![]() |
55 | 3640 | 15![]() |
1.35 |
In the case of 4, differing from 3 only in the nature of the alkoxide (tert-butoxide versus isopropoxide) there was some activity at 60 °C over 30 h, though the yield was low (10%). Reactions conducted at 80 °C afforded yields slightly lower than observed for 3; molecular weights (Mn) were similar. In the case of complex 2, which differs from 3 in the nature of the tetra-phenolate employed (i.e. para versus meta), activity was observed at 80 °C on prolonging the reaction time. Yields using 2 after 24 h were typically higher than for 3, but then after 36 h, the yields were approximately the same (slightly higher using 3); molecular weights (Mn) followed the same trend. PDIs for runs employing 2 were higher than those for 3. However, these results, unlike those for ε-caprolactone, did not suggest that use of the meta ligand had any beneficial effect in terms of the distance between the two vanadium centres and the resultant ROP activity. In the case of the imido complex 5 (a chloride complex), it proved necessary here to add BnOH to afford an active system. We note however that chlorides have previously been shown to be capable of the ROP of lactide.16 At 80 °C, activity was observed after 24 h, with yields similar to the vanadyl complex 2, but with higher molecular weight (Mn) polymers formed.
Comparing results for 1versusI suggests that there is no benefit in having two vanadyl centers present rather than one. Indeed, results for 1 suggest the opposite given that 1 can operate at 60 °C and also affords superior yields at 80 °C. Results for III are similar to those of II.
In all cases, observed molecular weights (Mn) are far lower than calculated values. In contrast to the ε-caprolactone screening, conducting the L-LA ROP runs in the absence of any solvent did not afford improved results and actually afforded little or no polymer.
As for PCL, the observed molecular weights (Mn) for the PLA are lower than the calculated values, and in the MALDI-ToF spectra (see ESI, Fig. S16–S18†) there was evidence of a second population consistent with some transesterification processes occurring.
For 3, a plot (Fig. 8, runs 23–26, Table 5) of the average molecular weight (Mn) of the poly(L-LA) as a function of the monomer conversion was linear, and with consistently low PDI values suggestive of a living process.
Runa | Cat | Temp/°C | Time/h | [LA]0![]() ![]() ![]() ![]() |
Conv.b (%) | M n,GPC c | M n,cal d | PDIe | Prf |
---|---|---|---|---|---|---|---|---|---|
a All reactions were carried out in toluene under nitrogen. b Determined by 1H NMR. c M n values were determined by GPC in THF vs. PS standards and were corrected with a Mark–Houwink factor of 0.58. d Calculated from (F.W. monomer × [monomer]/[cat]) × conversion + F.W. BnOH. e M w/Mn were determined by GPC. f Determined by analysis of the tetrad signal in the 1H NMR spectrum. | |||||||||
1 | 1 | 80 | 24 | 200![]() ![]() ![]() ![]() |
80 | 6050 | 23![]() |
1.30 | 0.61 |
2 | I | 60 | 24 | 200![]() ![]() ![]() ![]() |
— | — | — | — | — |
3 | I | 80 | 24 | 200![]() ![]() ![]() ![]() |
70 | 3650 | 20![]() |
1.15 | 0.60 |
4 | II | 60 | 24 | 200![]() ![]() ![]() ![]() |
— | — | — | — | — |
5 | II | 80 | 24 | 200![]() ![]() ![]() ![]() |
78 | 3860 | 22![]() |
1.09 | 0.63 |
6 | 2 | 60 | 24 | 200![]() ![]() ![]() ![]() |
— | — | — | — | — |
7 | 2 | 80 | 24 | 100![]() ![]() ![]() ![]() |
65 | 1460 | 9370 | 1.13 | — |
8 | 2 | 80 | 24 | 200![]() ![]() ![]() ![]() |
80 | 2720 | 23![]() |
1.19 | 0.58 |
9 | 2 | 80 | 24 | 400![]() ![]() ![]() ![]() |
75 | 3450 | 43![]() |
1.24 | 0.58 |
10 | 2 | 80 | 24 | 600![]() ![]() ![]() ![]() |
75 | 3570 | 64![]() |
1.23 | 0.58 |
11 | 2 | 110 | 24 | 200![]() ![]() ![]() ![]() |
75 | 2440 | 21![]() |
1.21 | 0.58 |
12 | 3 | 60 | 24 | 200![]() ![]() ![]() ![]() |
— | — | — | — | — |
13 | 3 | 80 | 12 | 200![]() ![]() ![]() ![]() |
44 | 790 | 12![]() |
1.17 | — |
14 | 3 | 80 | 24 | 200![]() ![]() ![]() ![]() |
83 | 2860 | 25![]() |
1.18 | 0.58 |
15 | 3 | 110 | 24 | 200![]() ![]() ![]() ![]() |
75 | 2480 | 21![]() |
1.27 | 0.61 |
16 | 4 | 80 | 24 | 200![]() ![]() ![]() ![]() |
79 | 2120 | 23![]() |
1.25 | 0.61 |
17 | 4 | 110 | 24 | 200![]() ![]() ![]() ![]() |
75 | 4120 | 21![]() |
1.23 | 0.61 |
18 | 5 | 80 | 24 | 200![]() ![]() ![]() ![]() |
— | — | — | — | — |
19 | 5 | 110 | 24 | 200![]() ![]() ![]() ![]() |
78 | 2210 | 22![]() |
1.10 | 0.58 |
20 | 5 | 130 | 24 | 200![]() ![]() ![]() ![]() |
— | — | — | — | — |
Observed molecular weights (Mn) were again lower than calculated values, and MALDI-Tof spectra (e.g. Fig. S19, ESI†) also revealed a number of minor populations.
As for L-lactide, use of no solvent afforded little or no observed catalytic activity.
To assign the stereochemistry of the PLA polymers we employed 2D J-resolved 1H NMR spectroscopy and assigned the peaks by reference to the literature.17 Representative spectra for runs 1, 5 and 17 are given in the ESI (Fig. S20–S22†), with the assignments given on the respective figures. These systems gave moderately isotactic PLA with a Pr value in the range 0.58–0.63.
Runs conducted in different solvents, namely THF and CH2Cl2 resulted in little or no polymer.
The presence of cyclic PLA was ruled out by comparison with literature MALDI-ToF and 1H NMR spectra.18
Runa | Complex | CL![]() ![]() |
Yield% | M n c | M w/Mnd |
---|---|---|---|---|---|
a All reactions were carried out in toluene under nitrogen under optimum condition 24 h CL/24 h L-LA (80 °C). b Ratio of LA to CL observed in the co-polymer by 1H NMR. c M n values were determined by GPC in THF vs. PS standards and were corrected with a Mark–Houwink factor (Mn,GPC × 0.56 × % PCL + Mn,GPC × 0.58 × % PLLA). d PDI were determined by GPC. | |||||
1 | 1 | 390![]() ![]() |
83 | 15![]() |
1.87 |
2 | I | 383![]() ![]() |
77 | 3840 | 1.24 |
3 | II | 379![]() ![]() |
68 | 3600 | 1.16 |
4 | 2 | 385![]() ![]() |
73 | 4890 | 1.54 |
5 | 3 | 383![]() ![]() |
71 | 4560 | 1.66 |
6 | 4 | 380![]() ![]() |
66 | 5790 | 1.43 |
7 | 5 | 364![]() ![]() |
65 | 3400 | 1.26 |
Runa | Complex | CL![]() ![]() |
Yield% | M n c | M w/Mnd |
---|---|---|---|---|---|
a All reactions were carried out in toluene under nitrogen under optimum condition 24 h CL/24 h rac-LA (80 °C). b Ratio of LA to CL observed in the co-polymer by 1H NMR spectroscopy. c M n values were determined by GPC in THF vs. PS standards and were corrected with a Mark–Houwink factor (Mn,GPC × 0.56 × % PCL + Mn,GPC × 0.58 × % PLLA). d PDI were determined by GPC. | |||||
1 | 1 | 382![]() ![]() |
88 | 5850 | 1.42 |
2 | I | 375![]() ![]() |
73 | 3210 | 1.92 |
3 | II | 363![]() ![]() |
70 | 3910 | 1.21 |
4 | 2 | 360![]() ![]() |
75 | 3260 | 1.71 |
5 | 3 | 375![]() ![]() |
68 | 3840 | 1.60 |
6 | 4 | 367![]() ![]() |
60 | 3270 | 1.46 |
7 | 5 | 343![]() ![]() |
70 | 1770 | 1.18 |
Given the low molecular weight products isolated during these studies, the presence of impurities (e.g. lactic acid from non-recrystallized rac-lactide) acting as chain transfer agents (or co-initiator) cannot be ruled out. However, we note that there is interest in low molecular weight poly(lactide/caprolactone) polymers as bio adhesives.19
Co-polymerisations were conducted by adding monomer 1 (3.200 mmol) to the catalyst (0.016 mmol) in toluene pre-heated to 80 °C and stirring for 24 h, and then adding monomer 2 (3.320 mmol) and stirring for a further 24 h. The polymerisation mixture was quenched by adding methanol (1.0 ml), and the resultant solution was then poured into methanol (200 ml), the precipitate collected and dried in vacuo.
Despite the high flux, the crystal of 1 examined was found to scatter X-rays very poorly and little appreciable diffraction was observed beyond ∼1.1 Å. It was possible to solve the structure using this data and routine refinements of a structural model were possible. It was possible to use anisotropic displacement parameters for all non-hydrogen atoms and the refinement was stable with no unusual features. Although the crystal examined was weakly scattering the solution is sound and it gives extremely useful chemical information.
The data for 2, although weak, are more routine and standard procedures were applied in structure solution.
Structures were solved using Direct Methods implemented within SHELXS-2013 and refined within SHELXL-2014.20
Further details are provided in Table S2.†
Footnote |
† Electronic supplementary information (ESI) available: ORTEPs and X-ray crystallographic files CIF format for the structure determinations of compounds 1, 2, 3·2CH2Cl2, 4·2CH2Cl2, 4·3CH2Cl2 and 5·2CH3CN. CCDC 1040815, 1404533, 1404524–1404526 and 1404529. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c5ra24816g |
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