Vanadium( V ) phenolate complexes for ring opening homo- and co-polymerisation of 3 -caprolactone, L lactide and rac -lactide †

The vanadyl complexes [VO(O t Bu)L 1 ] ( 1 ) and {[VO(O i Pr)] 2 ( m - p -L 2p )} ( 2 ) {[VO(OR)] 2 ( m - p -L 2m )} (R ¼ i Pr 3 , t Bu 4 ) have been prepared from [VO(OR) 3 ] (R ¼ n Pr, i Pr or t Bu) and the respective phenol, namely 2,2 0 -ethylidenebis(4,6-di- tert -butylphenol) (L 1 H 2 ) or a , a , a 0 , a 0 -tetra(3,5-di- tert -butyl-2-hydroxyphenyl – p / m -) xylene- para -tetraphenol (L 2p/m H 4 ). For comparative studies, the known complexes [VO( m -O n Pr)L 1 ] 2 ( I ), [VOL 3 ] 2 ( II ) (L 3 H 3 ¼ 2,6-bis(3,5-di- tert -butyl-2-hydroxybenzyl)-4- tert -butylphenol) were prepared. An imido complex {[VCl(N p -tolyl)(NCMe)] 2 ( m - p -L 2p )} ( 5 ) has been prepared following work-up from [V(Np-tolyl)Cl 3 ], L 2p H 4 and Et 3 N. 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 3 -caprolactone, L -lactide or rac -lactide with and without solvent present. The co-polymerization of 3 -caprolactone with L -lactide or rac -lactide a ﬀ orded co-polymers with low lactide content; the reverse addition was ine ﬀ ective.


Introduction
As a biodegradable polyester that has the potential for replacing traditional polymers, polylactide (PLA) has attracted much attention in recent years. 1 3-Caprolactone too is attracting interest as a precursor to biodegradable polycaprolactone (PCL) polymers. 2 Both types of polymer are readily formed via ring opening polymerization (ROP) of the respective monomer catalyzed by metal complexes, which can be a living process thereby allowing for good control. Given the bio-applications of PLA and PCL type polymers, there is a drive to develop syntheses using new catalysts that offer advantages over the established stannous octoate, 3 yet which possess low toxicity. Furthermore, the properties associated with PLA and PCL can be quite different, as typied by their differences in elasticity, and so there is a drive to produce 3-Cl/LA co-polymers of varying composition (and properties).
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-(L 1 H 2 ) or tetra-phenols (L 2p/m H 4 ) and for comparative studies the tri-phenol (L 3 H 3 ) (see Scheme 1) for the ROP of both L-lactide and 3-caprolactone and the co-polymerisation thereof, and report the effects of the structures of the complexes on the properties of the nal polymeric products. Such chelating phenoxide ligation has proved useful in olen polymerisation, 4 but their use in the ROP of lactides and lactones, particularly that of the tri-and tetraphenolic ligand sets, is rather limited. 6d,7,8 Use of a tetraphenolate ligand set, both meta and para forms, also allowed us to probe for possible cooperative effects, viz. 1 versus 2-4. With this in mind, the crystal structures of the monomeric vanadyl complex [VO(OtBu)L 1 ] (1) and the dinuclear vanadyl complexes {[VO(OiPr)] 2 (m-p-L 2p )} (2), {[VO(OR)] 2 (m-p-L 2m )} (R ¼ iPr 3, tBu 4) are also presented; the molecular structures of I and II have been reported elsewhere. 9

Results and discussion
Vanadyl phenolate complexes Interaction of [VO(OtBu) 3 ] with the ethylidene-bridged diphenol 2,2 0 -ethylidenebis(4,6-di-tert-butylphenol), 2,2 0 -CH 3 CH [4,6-(tBu) 2 C 6 H 2 OH] 2 (LH 2 ) in reuxing toluene afforded, aer workup, the monomeric complex [VO(OtBu)L] (1) in good isolated yield (ca. 76%). Complex 1 is presumed to form via displacement of two molecules of tert-butanol in a similar fashion reported for related n-propoxide complexes. 9 In the IR of 1, there is a strong stretch at 1003 cm À1 assigned to the n(V] O) group. Crystals of 1 suitable for X-ray diffraction were readily grown from a saturated acetonitrile solution at 0 C. The structure of 1 is shown in Fig. 1 (for ORTEP, see Fig. S1 †), with selected bond lengths and angles given in the caption; crystal structure data are given in Table S2. † The vanadium centre adopts a distorted tetrahedral environment with angles varying from ideal in the range 107.7(2) to 112.1(2) . The chelating ligand forms an eight membered metallocycle adopting a attened chair conformation, with a bite angle of 110.1(2) , which is somewhat larger than that found in the monomeric vanadyl complex {VOCl[2,2 0 -CH 2 (4-Me,6-tBuC 6 H 2 O) 2 ]} [106.9(2) ] and the dimeric complex [VO(OnPr)L 1 ] 2 [94.49 (10) ]. 9a,10 The V-O bond lengths to the bisphenolate ligand [1.789(5) and 1.783 (5) A] are typical of those previously observed for vanadium aryloxides, 9,10 whilst the V-O alkoxide distance [1.739(5) A] is shorter than those typically observed in alkoxy vanadium complexes, but similar to that reported in the monomeric imido vanadium complex [V(NAr)(OtBu)L] [1.738(2) A] (Ar ¼ p-ClC 6 H 4 ). 11 The alkoxide ligand is best described as bent with a V(1)-O(4)-C(40) angle of 145.9(5) , which is slightly smaller than the analogous angle in the imido complexes [V(NAr)(OtBu) L] [146.77(12)-151.9(2) ] (Ar ¼ p-ClC 6 H 4 , p-tolyl). 11 Extension of this synthetic methodology to the tetra-phenol a,a,a 0 ,a 0 -tetra (3,5- 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 A. 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    (3) 1.5908 (17) 1.572 (7) (7) ; tBu, 112.0(2) ]; and the metallocycle adopts a attened 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 a,a,a 0 ,a 0 -tetra (3,5- Crystals of 3 and of 4 suitable for an X-ray diffraction studies were obtained on cooling (to À20 C) their respective saturated dichloromethane solutions. The molecular structure of 3$2CH 2 Cl 2 is shown in Fig. 3 (for ORTEP, see  Table 1 where they are compared with those of 4$2CH 2 Cl 2 and 4$3CH 2 Cl 2 . In 3, the vanadium centres can be described as adopting a pseudo tetrahedral geometry. The bite angle formed by the tetraphenolate at each vanadium is 110.55 (11) , which is slightly larger than that observed for 1; again the metallocycle adopts a chairboat conformation. The iso-propoxide ligand can be described as bent with a V2-O8-C68 angle of 140.5(3) ; the V-O isopropoxide bond lengths are similar to those observed elsewhere. 12 For 4$2CH 2 Cl 2 , there is one molecule of the complex and two molecules of CH 2 Cl 2 (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 tertbutoxide ligand is somewhat bent [V1-O6-C65 ¼ 143.63(15) ], with a slightly larger angle than that observed in the isopropoxide 3 and presumably reects the greater steric bulk of the tert-butoxide. The molecules pack in layers, however there is no signicant interaction between the layers.
From a repeated synthesis of 4, a different solvate was obtained, namely 4$3CH 2 Cl 2 . In the molecular structure of 4$3CH 2 Cl 2 , determined using synchrotron radiation, 14 there is one well-dened dichloromethane which is involved in intramolecular interactions (see  Table S1 in ESI † for geometry) and a C-H/p interaction with the aromatic ring C51 > C56 {H(73B)/ring centroid ¼ 2.56(2) A}.
The known vanadyl complexes [VO(m-OnPr)L 1 ] 2 (I), [VOL 3 ] 2 (II) were prepared as described previously (L 3 H 3 ¼ 2,6-bis(3,5-ditert-butyl-2-hydroxybenzyl)-4-tert-butylphenol) via the reaction of [VO(OnPr) 3 ] and the respective chelating phenol. 9 Vanadium imido phenolate complex Given the oxo group is isoelectronic with the imido group, we extended the studies to the reaction of [V(Np-MeC 6 H 4 )Cl 3 ], 15 with p-L 2p H 4 in the presence of Et 3 N. Following work-up (extraction into MeCN), the red/brown imido complex {[V(Np-MeC 6 H 4 )(NCMe)Cl] 2 (m-p-L 2p )}$2MeCN (5$2MeCN) was isolated in good yield. Single crystals of 5, obtained on prolonged standing at ambient temperature, were subjected to an X-ray diffraction study. The structure of 5 is shown in Fig. 6 (for ORTEP, see    geometrical parameters associated with a linear imido function [V1-N1 1.670(3) A; V1-N1-C40 170. 5(3) ]. The structure of 5 closely resembles that of the recently reported complexes {[V(NAr)(THF)Cl] 2 (m-p-L 2p )} (Ar ¼ p-MeC 6 H 4 , p-CF 3 C 6 H 4 ), in which THF occupies one of the axial position at the metal as opposed to MeCN in 5. 4d 51 V NMR data for 1-5 and I and II are presented in Table 2, and all vanadyl complexes appear in the range d À410 to À498 ppm, with the 5-coordinate VO 4 centre in II slightly upeld of the other 4-coordinate VO 3 containing species. The imido complex 5 appears somewhat downeld, a position which also reects the presence of the chloride ligand; line widths are also increased in the presence of imido groups. 15 Ring opening polymerisation (ROP) studies 3-Caprolactone (3-CL). Given its ease of preparation on a multi-gramme scale, complex 4 was used to determine the optimum conditions (temperature, time and concentration) needed for the ROP of 3-caprolactone. It was observed that the ratio 200 : 1 for [CL] : [cat] was best both in the presence or absence of BnOH, over a period of 24 h at 80 C. For all catalyst systems, runs conducted at temperatures of #45 C or for #12 h led to either no polymer or low yields (see Table 3). All systems were relatively well behaved with only one run (run 3) affording a PDI of over 1.80, whilst the majority of runs were below 1.40. The presence of BnOH was also examined for 4 (runs 25-27), and under the optimized conditions, the conversion was about 10% lower whilst the observed M n was about 30% lower. For 5, bearing a terminal chloride ligand, the use of BnOH was benecial in terms of % conversion, and the observed molecular weights (M n ) were also higher. In terms of pro-catalyst structure, there appeared to be no advantage in having two metals present given % conversion for 1 z I and II under optimized conditions (runs 3, 6 and 9). In the case of the tetraphenolate systems, it appears that use of systems (3 and 4) derived from the meta pro-ligand set m-L 2m H 4 are more effective than those (2) derived from the para pro-ligand p-L 2p H 4 (runs 13-15 and 16-26 versus 10-12) Table 3. This suggests in 3 and 4   present, signals were assignable to a hydroxyl end group (CH 2 OH) 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 alkoxidevanadium bond. For runs conducted in the presence of BnOH, 1 H 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 (M n ) polymers (see Table 4). The monomeric tert-butoxide complex 1 was found to afford the best yield (90%) and highest molecular weight (M n ) PCL ($16 300). 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 (M n ). 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 (M n ) $14 000 g mol À1 .
In general for the CL runs, despite the narrow polydispersity, the polymer molecular weights (M n ) were much lower than expected, indicating in all cases that signicant trans-esterication 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 (M n ) 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 51 V NMR signal (Table 2)  L-Lactide (L-LA). Complexes 1 to 5 and II and III have been screened for their potential to act as catalysts for the ROP of L-lactide. In this case, complex 3 was chosen to establish the optimized conditions for the ROP of L-lactide (L-LA). Using a ratio of 200 : 1 for L-LA to pro-catalyst, it was found that at temperatures below 80 C, there was no catalytic activity even aer 35 h. At 80 C, there was no activity aer 6 h, and polymer was only isolated at 24 h affording a yield of 20%. Prolonging the reaction time increased the yield to 50%. Further increasing the temperature to 110 C afforded only a slight improvement in yield, with a slight increase in PDI. Varying the ratio of L-LA to pro-catalyst led to a slight improvement in the yield (55%), together with an increase in the molecular weight (M n ) and a slight broadening of the PDI. Given these results, the other complexes were screened using a ratio of 200 : 1 for L-LA to procatalyst, and in the case of 5, screening was conducted both in the absence and presence of BnOH (see Table 5).   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 (M n ) were similar. In the case of complex 2, which differs from 3 in the nature of the tetraphenolate employed (i.e. para versus meta), activity was observed at 80 C on prolonging the reaction time. Yields using 2 aer 24 h were typically higher than for 3, but then aer 36 h, the yields were approximately the same (slightly higher using 3); molecular weights (M n ) followed the same trend. PDIs for runs employing 2 were higher than those for 3. However, these results, unlike those for 3-caprolactone, did not suggest that use of the meta ligand had any benecial 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 aer 24 h, with yields similar to the vanadyl complex 2, but with higher molecular weight (M n ) polymers formed.
Comparing results for 1 versus I suggests that there is no benet 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 (M n ) are far lower than calculated values. In contrast to the 3-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 (M n ) 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 transesterication processes occurring.
For 3, a plot (Fig. 8, runs 23-26, Table 5) of the average molecular weight (M n ) 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.
rac-Lactide (rac-LA). Complexes 1-5 and I and II were also screened for their ability to ROP rac-lactide and the results are given in Table 6. Temperatures of at least 80 C were found necessary to achieve activity and yields were found to be at best  moderate #50% as found for L-lactide. The ROP appeared to be well controlled in terms of PDI with values in the range 1.09-1.30 observed (Fig. 9). There was no obvious advantage in the use of meta vs. para ligation in 2 and 3 at either 80 or 110 C. Looking at 3 vs. 4 (OiPr vs. OtBu), at 80 C, the isopropoxide 3 afforded higher molecular polymer (M n ) in higher yield, whereas at 110 C the trend was reversed. Observed molecular weights (M n ) 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 1 H 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 gures. 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 CH 2 Cl 2 resulted in little or no polymer.
The presence of cyclic PLA was ruled out by comparison with literature MALDI-ToF and 1 H NMR spectra. 18 Co-polymerisation of 3-caprolactone and L-lactide. Complexes 1 to 5 and I and II have also been screened for their potential to act as catalysts for the co-polymerisation of CL with L-lactide under the optimized conditions found for the homopolymerisations in toluene, i.e. 80 C, 200 : 200 : 1 for CL : LA : pro-cat over 24 h (Â2). In all cases (Table 7), good yields (65-83%) of co-polymer were formed, but with low lactide content (2.5-9.0%) as observed from 1 H NMR spectra (ESI, Fig. S23 †); the highest % incorporation of LA (9%) was found for 7 in the presence of BnOH. In the 1 H NMR, the end groups for alkoxide and hydroxyl were also evident. Observed molecular weights were in general higher than those observed for the homo-polymerisations conducted in toluene. Thermal analysis of the co-polymer by DSC revealed two melting points at 55.1 C (PCL) and 170.5 C (PLA), see ESI Fig. S24. † If the addition of the monomers was reversed, i.e. L-LA added rst, no co-polymer was isolated aer work-up. This suggested that a PLA chain end was not capable of PCL chain growth.
Co-polymerisation of 3-caprolactone and rac-lactide. Copolymerisations involving CL and rac-LA gave similar results to the co-polymerisation with L-LAsee Table 8. Yields were in the range 60-88% and the incorporation of rac-lactide was 4.5-14.3%, and again the highest incorporation was noted for 5. In the 1 H NMR spectrum, the end groups for alkoxide and hydroxyl were also evident, see ESI Fig. S25. † We also recorded the DSC (see Fig. S26, ESI †) which, as for the co-polymerization of CL and L-LA revealed two peaks: a large peak at 55.09 and a much smaller peak at 170.49.
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

Conclusions
In conclusion, we have examined the ROP behaviour of a series of vanadyl complexes bearing chelating di-, tri-and tetraphenolate ligands towards 3-caprolactone, L-lactide or rac-lactide with and without solvent present, and the co-polymerisation of 3-CL with lactide. For the homo-polymerisation of 3-CL, under   This journal is © The Royal Society of Chemistry 2016 the optimized conditions in toluene, yields were typically of the order of 70%. It was observed that there was no advantage in having two metals present (cf. to one), whilst for the tetraphenolates use of the meta ligand set appeared benecial (cf. the para ligand set), which perhaps reects the closer proximity of the metal centres in the former. Conducting the runs in the absence of solvent led to higher conversions, typically >95%. For the ROP of L-lactide, the performances of 1-5 (and I and II) were not so good, affording PLA in only low to moderate (#55%) yields at higher temperatures and over prolonged reaction periods. There was no evidence of any benecial cooperative effects and conducting the runs in the absence of solvent afforded no improvement. Results for rac-lacide were similar to L-lactide; 2D J-resolved 1 H NMR spectroscopy indicated the formation of moderately isotactic PLA. Co-polymerisation of CL with L-lactide afforded good yields (65-83%) of co-polymer, but with low lactide content (2.5-9.0%); reversing the order of monomer addition resulted in no product. Similar results were observed for co-polymerisation of CL with rac-lactide.

General
All manipulations were carried out under an atmosphere of dry nitrogen using conventional Schlenk and cannula techniques or in a nitrogen-lled glove box. Diethyl ether and tetrahydrofuran were reuxed over sodium and benzophenone. Toluene was reuxed over sodium. Dichloromethane and acetonitrile were reuxed over calcium hydride. All solvents were distilled and degassed prior to use. IR spectra (nujol mulls, KBr or NaCl windows) were recorded on a Nicolet Avatar 360 FT IR spectrometer; 1 H NMR spectra were recorded at room temperature on a Varian VXR 400 S spectrometer at 400 MHz or a Gemini 300 NMR spectrometer or a Bruker Advance DPX-300 spectrometer at 300 MHz. The 1 H NMR spectra were calibrated against the residual protio impurity of the deuterated solvent. Elemental analyses were performed by the elemental analysis service at the London Metropolitan University or the University of Hull. Matrix Assisted Laser Desorption/Ionization-Time of Flight (MALDI-ToF) mass spectrometry was performed on Bruker autoex III smart beam in linear mode. MALDI-ToF mass spectra were acquired by averaging at least 100 laser shots. 2,5-Dihydroxylbenzoic acid was used as matrix and tetrahydrofuran as solvent. Sodium chloride was dissolved in methanol and used as the ionizing agent. Samples were prepared by mixing 20 ml of polymer solution in tetrahydrofuran (2 mg ml À1 ) with 20 ml of matrix solution (10 mg ml À1 ) and 1 ml of a solution of ionizing agent (1 mg ml À1 ). Then 1 ml of these mixtures was deposited on a target plate and allowed to dry in air at room temperature.
L-Lactide (99.7%, with D-lactide content less than 1%) was provided by Shenzhen Brightchina Industrial Co. Ltd (Shenzhen, China) as monomer. rac-Lactide and 3-caprolactone were purchased from Sigma Aldrich and were used as received. Benzyl alcohol was distilled over CaH 2 under vacuum. Other solvents were all of analytical grade and provided by Kelong Chemical Reagent Factory of Chengdu (Chengdu, China), and Polymerisation reactions were performed in a Schlenk tube equipped with a magnetic stirrer. For solution polymerisation, a mixture of monomer (14 mmol of 3-caprolactone, L-lactide or rac-lactide), vanadyl complexes 1-5 or I and II (0.14 mmol) and BnOH (0.14 mmol, if needed) were added into a Schlenk tube at room temperature under nitrogen protection. Aer 3 cycles of gassing and degassing, pre-degassed toluene (7 ml) was added to the reaction mixture under nitrogen protection. The polymerisation was started by placing the reaction mixture into an oil bath pre-heated to the polymerisation temperature (T p ). For bulk polymerisation, all reaction conditions were the same except that no solvent was used. In all cases, the polymerisation was stopped by addition of 1 ml methanol aer the prescribed reaction time. Products with different polymerisation times were taken out with a syringe and precipitated into cold methanol with magnetic stirring to eliminate residual catalyst. Precipitates were ltrated, washed with methanol and dried before GPC determination. 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.

Crystallography
Single crystal diffraction data were collected by the UK National Crystallography Service using a Rigaku FR-E+ diffractometer. This operates with a SuperBright rotating anode X-ray generator and high ux optics. This is designed to deal with the most challenging samples sent to the service.
Despite the high ux, the crystal of 1 examined was found to scatter X-rays very poorly and little appreciable diffraction was observed beyond $1.1 A. It was possible to solve the structure using this data and routine renements of a structural model were possible. It was possible to use anisotropic displacement parameters for all non-hydrogen atoms and the renement 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 rened within SHELXL-2014. 20 Further details are provided in Table S2. †