Di‐Zinc–Aryl Complexes: CO2 Insertions and Applications in Polymerisation Catalysis

Abstract Two new di‐zinc–aryl complexes, [LZn2Ph2] and [LZn2(C6F5)2], coordinated by a diphenol tetraamine macrocyclic ligand have been prepared and fully characterised, including by single‐crystal X‐ray diffraction experiments. The complexes’ reactivities with monomers including carbon dioxide, cyclohexene oxide, phthalic anhydride, isopropanol and phenol were investigated using both experimental studies and density functional theory calculations. In particular, [LZn2Ph2] readily inserts carbon dioxide to form a carboxylate, at 1 bar pressure, whereas [LZn2(C6F5)2] does not react. Under these conditions [LZn2Ph2] shows moderate activity in the ring‐opening copolymerisation of cyclohexene oxide/carbon dioxide (TOF=20 h−1), cyclohexene oxide/phthalic anhydride (TOF=33 h−1) and the ring‐opening polymerisations of rac‐lactide (TOF=99 h−1) and ϵ‐caprolactone (TOF=5280 h−1).


Introduction
Since their originald iscovery by Frankland in 1848, [1] organometallicz inc compounds have become aw ell-established component of the synthetic chemists' toolbox. They have been successfully applieda ss toichiometric reagents in Negishi cross-coupling reactions, [2] metal-halogene xchange, [3] the alkylation of trifluoromethyl ketones [4] and the epoxidation of enones. [5] Zinc is an attractive choice of metal for catalysis, [6] due to its low toxicity,l ow cost and lack of colour and redox chemistry.H omogeneous zinc catalysts show promise in reactions including the ring-opening polymerisation (ROP) of cyclic esters, [7] the formationo fc yclic carbonates, [8] aldol reactions [9] and hydroamination reactions. [10] They have been particularly effective as catalysts for CO 2 /epoxide ring-opening copolymerisation (ROCOP), which provides au seful method of adding value to captured CO 2 . [11] Some of the most active and selective catalysts are zinc complexes coordinated by b-diiminate or phenoxy-amine ligand scaffolds. [12] With some of these different catalysts ystems, short-chain telechelic polycarbonates have been observed, [13] which are potentially useful for chain extension reactions to form block copolymers, [14] polyurethanes, [15] or nanomaterials. [16] The presence of such a,w-dihydroxyl end-capped polymers is generally attributed to the presence of diols, formed through the reaction of epoxides with trace water,w hich act as chain-transfera gentsd uring polymerisation. Darensbourg and co-workers recently gleaned furtheri nsight into the nature of this reaction, and established that this hydrolysis is catalysed by the polymerisation catalyst [(salen)Co(O 2 CCF 3 )],i nb othC O 2 /cyclohexene oxide (CHO) and CO 2 /propylene oxide (PO) ROCOP systems. Carefuls pectroscopics tudies demonstrated that these [(salen)Co(O 2 CCF 3 )]-catalysed hydrolysis reactions occur prior to any initiation of CO 2 / epoxide ROCOP,a st he catalyst is initially occupied in the conversion of epoxidest od iols. [14a] Fundamental reactivities of polymerisation catalysts towards oxygenateds mall-molecules in ROCOPs ystems, including alcohols, carbon dioxide and other monomers, are of particularr elevance to furtheru nderstand the reactions occurring with chain-transfer agents, and for the preparation of new catalysts for CO 2 /epoxide ROCOP, and so we studied the reactivity of zinc catalyst systems with ar ange of small molecules. Controlling the nature of the bond between them etal and the initiatingg roup or growing polymer chain end is of key interesti np olycarbonate synthesis, [17] and has led to the development of "switchable" zinc catalysts, which can catalyseb oth the ROP of lactones and the ROCOP of epoxides with CO 2 or anhydrides, thus enabling the controlled synthesis of block copolymers from am ixture of monomers. [18] Considering the general reactivity of zinc-alkyl complexes, there are an umber of reportso fr eactions with alcohols or carboxylic acids. [19] The insertion of CO 2 into Zn-alkoxide bonds has also been studied in depth. [11b] Some examples relevant to catalysis include the reversible insertion of CO 2 into di-zinc-alkoxide complexesb ased on am acrocyclic bis(anilido)tetraimine ligand,t of orm di-zinc carbonate and mixed carbonate/alkoxide products. [20] Considering BDI-Zn (BDI = b-diiminate)c omplexes, which are well-studied catalysts for CO 2 /epoxide ROCOP,zinc-alkoxides rapidly insert CO 2 ,whilst the epoxide coordination and ring-opening is an equilibrium process. [11b, 19c] Despite these studies, the reaction of Zn-alkyl complexes with carbon dioxide remainsm uch less explored, [21] andt he initial reactivity of such complexes in the presence of CO 2 ,e poxide and diols is still not well understood. Kinetic studies have shown that CO 2 insertiono ccurs rapidly for as eries of zinc hydride complexes, to form the corresponding zinc formate complexes, in which the reactionk inetics were limited by the rate of CO 2 dissolution in toluene solvent (k obs = 0.033 m min À1 ). [22] Recently,s ome of us reported ad iphenol tetraamine-based macrocyclic ligand that was used to prepare as eries of dinuclear catalysts, [13d, 23] including di-zincc arboxylate compounds. [12b, 24] These complexes showeda ctivitiesf or both the ROCOP of CO 2 /epoxide and of epoxide/anhydride, and were notable in being able to selectively polymerise at just 1bar pressure of CO 2 . [25] Here, we apply the same ligand and investigate the potential to prepare di-zinc-bis(aryl) precatalysts.T o gain insight into the reactions that may occur between such precatalysts and the key monomers or chain-transfer agents presentd uring polymerisation (Scheme 1), the reactivity of the complexes towards stoichiometric epoxide (CHO),p hthalic anhydride (PA), CO 2 and alcohols was explored. The effectofelectron-withdrawing substituents on the aryl co-ligand was also compared, through experimental andc omputational comparisons, between di-zinc-bis(phenyl) and di-zinc-bis(pentafluorophenyl) complexes.
Structural elucidation by X-ray diffractionr evealed that the two complexes are very similar,a nd sit across ac entre of symmetry at the middle of the Zn 2 O 2 rings ( Figure 1). In contrastto other relatedd i-Zn complexes based on LH 2 ,i nw hich the ligand adopts ab owl shape, here the ligand adopts an "S" shape. [24b] The pentacoordinate Zn atoms, which are bound within the ligand, each share two phenol oxygen atoms. For both 1 and 2,t here is as ignificant difference between the two different ArOÀZn bond lengths, of 0.13 in 1,a nd 0.09 in 2.
Completing the pentacoordinate geometry,e ach Zn also bonds to two amine nitrogen atoms ando ne aryl carbon atom. The aryl-CÀZn bond lengths lie within the expected range, [26] althought he bond is 0.03 shorteri n1 than in 2. One curious feature is the presence of HÀFi nteractions in 2, observed betweent he amine NH and the fluoryl substituents (F21ÀH8;2 .56(1) ). Nevertheless,t he nature of the co-ligand does not appear to affect the phenol CÀOb ond length, which is almost identicalw ithin 1 and 2 (O1ÀC1, 1.341(2) and 1.338(2) ,r espectively).

Reactivity studies
It was of interestt oi nvestigate the reactivity of 1 and 2 towards CO 2 ,t op robe their potentialu se as polymerisation catalysts. [12b, 24] It waso bserved that 1 reactedw ith CO 2 at 1bar of CO 2 pressure, in C 6 D 6 at 25 8C, to afford the corresponding dibenzoate complex [LZn 2 (OCO-Ph) 2 ]( 3) with complete conversion occurring after two hours,a so bservedb y 1 HNMR analysis (Scheme 2, Figure S6). The rate of CO 2 insertion was significantly enhanced by heating the solutiont o8 08C, affording complete conversion of 1 to 3,w ithin 5min.
Complex 3 reproducibly gave rather complex NMR spectra, at high and low temperatures, in ar ange of different solvents including CDCl 3 ,C 6 D 6 and [D 8 ]THF.H owever,i n[ D 4 ]methanol, am uch better resolved 1 HNMR spectrum was obtained (Figures S7 and S8). The spectrum confirmed the formation of the di-zinc complex-there are diastereotopic benzylic (4.23 and 3.34 ppm) and methylene (2.91-2.83 ppm) resonances,a nd the NH resonance is observed at 3.15 ppm. It possesses C 2 symmetry in methanol. The benzoate ligandsa re clearly presenta s evidencedb yt he deshielded ortho-phenyl resonance at 7.87 ppm. In the 13 CNMR spectrum, quaternary carbon resonances were too weak to be observed (including by HMBC experiments) and so a 13 C-carbonyl-labelled sample of 3 was prepared, by the reaction of 1 with two equivalents of 13 C-labelled benzoica cid. The carbonyl resonance of 3 is clearly observed at 174.6 ppm, shiftedf rom free benzoicacid (170.1 ppm).
In contrast, the fluoryl analogue, complex 2,d id not react with CO 2 under identical reactionc onditions. It is proposed that the decrease in nucleophilicity of the aryl group, due to the electron-withdrawing fluoryl substituents, disfavours CO 2 insertion. This is supported by the observation of al onger, weaker ZnÀCb ond in 2 (2.049(1) )c ompared to 1 (2.016 (1) ) in the solid-state crystal structure. [27] At heoretical study was carried out in order to gain ab etter understanding of the CO 2 insertioninto the Zn-aryl bonds. DFTwas used to calculate the potentiale nergy surface for the stepwise CO 2 insertion into the Zn-aryl bond for complexes 1 and 2 ( Figure 3), to provide insight into the activation energy barriers and the relative stability of the intermediates and products. The calculations were carried out using DFT protocol wb97xd/6-31G(d)/srcf(cpcm = dichloromethane) at 353 K, whichh as previously shown ag ood agreement with experimentsf or relatedr eactions tudies of similard inuclearz inc complexes (see the Supporting Information for further details). [24b] This study focussed on the previously unreported barrier of CO 2 insertion into the Zn-aryl bond and the calculations revealt he energy barrier to be 9.0 kcal mol À1 higherf or I' (overall barrier DG =+28.7 kcal mol À1 )t han for I (overall barrier DG =+19.7 kcal mol À1 ). The carbonate products derived from complex I are more stable than the corresponding fluoryl analogues obtained from I' (DDG up to 20.4 kcal mol À1 between VI CO 2 and VI CO 2 0 ), giving furthers upport to the experimental observation that CO 2 inserts more readily into the ZnÀPh bond than the ZnÀC 6 F 5 analogue.
The calculated mechanism shows CO 2 insertion occurring at only one metal centre, withoutp articipation of the second metal or second aryl co-ligand( Figure 3). NBO analysisw as car- ried out for III-TS CO 2 ,w hich shows as ignificant interaction between the ZnÀCb ond and incoming C CO 2 atom (see Supporting Information, Figure S9). This contrasts with what was previously observed in the case of ab ridginga cetate co-ligand, in which CO 2 insertion into aZ n-alkoxide bond occurs through ab imetallic mechanism, along with "shuttling" of the electron density of the acetate co-ligand to balance the charge. [24b] To allow ac omparison between these two systems, the potential energy surface for the second CO 2 insertion wasi nvestigated ( Figure 3). Considering the most stable conformation VI CO 2 ,t he second CO 2 insertion into I was found to occur by means of ab imetallic mechanism,w ith nucleophilic attack of the aryl to the CO 2 ,a nd forming ac omplex with the carboxylate coordinated to one metal centre;c oncomitantly,t he bridgingc oligand balancest he charges. The energy barrier for this second insertionw as found to be 18.6 kcal mol À1 (between VI CO 2 and VIII CO 2 ), which lies close to that determined for the first CO 2 insertion (19.7 kcal mol À1 ). Overall, the formationo ft he bis-carboxylate complex, IX CO 2 ,i sh ighly thermodynamically favoured, with DG = À49.5 kcal mol À1 .
Unfortunately,i tw as not possible to gain experimental evidence for the formation of any intermediates V CO 2 -VII CO 2 to confirm this step-wise model of CO 2 insertion (i.e.,m onitoring of the reaction by NMR spectroscopy detected only product 3). However,i ts eems reasonable to conclude that the insertion of CO 2 into the ZnÀPh bond is accessible under the reaction conditions, while the CO 2 insertion with the fluoryl analogueh as as ignificantly highere nergy barrier and is thermodynamically less favoured overall.
Ac atalysts ystem,p repared from the in situ reaction of 1 with 1,2-cyclochexenediol, has previously been appliedt owards the controlled synthesis of block co-polymers, through selectivec atalysis combining the ROP of e-CL with the ROCOP of epoxides and anhydrides. [19e] However,t his catalysts ystem was prepared and used in situ without detailed characterisation. Thus, it was of interestt oi nvestigate the reactivity of 1 and 2 with alcohols (Scheme 1). In these studies, isopropanol was used as am odel for the chain-transfer agent 1,2-cyclohexenediol. It was selected as as econdary alcoholo fs imilars teric bulk but which simplified spectroscopic characterisation and computational studies comparedt o1 ,2-cyclohexenediol( vide infra). Although as olution of 1 in THF proved stable in the presenceo fi sopropanol (2 equivalents) at 25 8C, heating the reactionm ixture to 60 8Cf or 18 hl ed to complete consumption of 1 (Scheme 2). 1 HNMR analysis, in [D 8 ]THF,r evealed the formation of an ew species, [LZn 2 (OiPr) 2 ]( 4), along with the formationo fb enzene (singlet at 7.30 ppm) (Figures S10-S12 in the SupportingInformation). As the copolymerisations are typically performed at temperatures above 60 8C, this finding suggests that zinc-alkoxides pecies can form readily under polymerisation conditions. [12d] In contrastt ot he broad,c onvoluted 1 HN MR spectrum of 1 in [D 8 ]THF at 298 K, 4 has as harp, wellresolved 1 HN MR spectrum.C omplex 4 was most clearly characterised by the isopropoxide methyne (4.11ppm) and methyl signals (0.96 ppm),w hich weres hifted compared to free alcohol. Integration of the relevant resonances confirms the 2:1 isopropanol/ligand ratio, showingt hat complete conversion of both the ZnÀPh bonds to ZnÀOiPr groups has occurred. Catalyst 2 also reacted with isopropanol, under reflux conditions in [D 8 ]THF;however,amixture of specieswas observed, which included 4 and C 6 F 5 H, along with unreacted 2 and iPrOH. The reagents 2 and iPrOH were still observed after three days at reflux,m ostl ikely because the presence of the electron-withdrawing fluoryl substituents decreases the Brønsted basicity of the phenyl group. Despite several attempts,X -ray quality crystals of 4 could not be obtained.I nstead, the analogousr eaction of 1 with phenol( 2equiv) wasp erformed,w hich led to the formation of the corresponding di-zinc bis(phenolate) complex [LZn 2 (OPh) 2 ]( 5,F igure 4). Its 1 HNMR and HSQC experimentsr eveal the presence of diastereotopic benzylic and methylene protons (Figures S13 and S14).
Crystals of 5 suitable for X-ray diffractionw ere crystallised from am ixed THF/CH 2 Cl 2 solvents ystem. The molecular structure is centrosymmetric and very similar to 1,i nw hich the ligand adopts an "S" shaped conformationa nd both Zn centres are pentacoordinated by the macrocyclic ligand scaffold (two phenol Oa nd two amine N) and at erminal phenol group. There is as ignificant differencei nb ond lengths between the bridging and terminal phenols, in which the terminal C-O-Zn bond is significantly shorter (by 0.09 )t han the bridging phenolate bonds from the macrocycle.
The reactivity of complex 1 with isopropanol was studied computationally,u sing CH 2 Cl 2 as solventa nd 353.15 Kt o mimic polymerisation conditions. The lowest energy pathway was found to have the incoming isopropanol molecule approaching the concavef ace of the "bowl"-shaped complex ( Figure 5). The energy barrier for the first protonolysiso f 1 with isopropanol is + 19.9 kcal mol À1 ,w hichi sa lmosti dentical to the calculated energy barrier forC O 2 insertion (+ 19.7 kcal mol À1 ). The product of the first protonolysis( V a HOR )i s thermodynamically favourable (À17.7 kcal mol À1 ). The intermediate can then reactw ith as econd equivalento fi sopropanol, with an energyb arrier of + 25.4 kcal mol À1 ,t oy ield complex 4 (VIII 2HOR ), which is calculated to have ar elative energy )h as al ower energy barrier of + 20.5 kcal mol À1 .The calculations show that the energy barriers for the protonolysisp athways are easily accessible,u nder polymerisation conditions, and that the products formed are highly stable relative to complex 1.Akeyf inding is that protonolysis, by reactionw ith chain-transfer agentsp resent during polymerisation, is likely to be ah ighly favourable reaction and that zinc-alkoxide complexes might be considered as the active sites for such catalytic systems.

Polymerisation studies
Following the successful reaction of 1 with CO 2 ,i ts catalytic activity within CHO/CO 2 copolymerisation was tested. Thep olymerisationsw ere runa t0 .1 mol %c atalystl oading( vs. the epoxide,C HO), using 1bar of CO 2 pressure (Table 1, entry 1), as analogousd i-Zn catalysts have previously shown acceptable activity under thesec onditions. [11j] The phenyl catalyst 1 is active (TOF = 20 h À1 )a nd exhibits good CO 2 uptake, giving > 99 %c arbonate linkages. The polymerisation is well-controlled, with am onomodald istribution and an arrow dispersity (1.06). Complex 1 displays similara ctivity to the previouslyr eported acetate analogue, [LZn 2 (OAc) 2 ]( TOF = 18 h À1 , entry 4), [12b] and significantly outperforms the bromide complex [LZn 2 Br 2 ], which is completely inactiveu nder identical reaction conditions. [25b] Notably,t he MALDI-ToF analysiss hows that the purified product is at elechelic polymer terminated by hydroxyl groups (Figure S15), af eature which has been observed with some different catalysts for this copolymerisation. [13] The formationo fd ihydroxyl end-capped polymers is consistentw ith reactions of [LZn 2 Ph 2 ]w ith alcohol (1,2-cyclohexenediol)t of orm the active site. [14a] The reactivity studies have also demonstrated the capability of 1 to react with CO 2 , within 5m inutes at 80 8C, suggesting that the product di-zincbis(benzoate) complex could initiate copolymerisation. However,b enzoate end groups were not observed in the NMRs pectroscopy or MALDI-ToF analysis. Thus it seems likely that the reaction of the zinc-aryl complexw ith diols, occurs even more rapidly than with CO 2 and is responsible for the true initiation under these conditions. In line with this observation, catalyst 2 is also active for CHO/CO 2 ROCOP, in spite of its complete lack of reactivity towards either CHO or CO 2 in model reactions. Rather 2 is proposed to reactw ith alcohols to generatea ctive alkoxide initiators (Scheme 1). Using catalyst 2,once again atelechelic polymer is formed, as confirmed by SEC and MALDI-To Fa nalysis( Figure S16). [14a] For both 1 and 2,t he theoretical M n values are approximately 12 times greater than the experimental values, which provides further support for the presence of ac hain-transfer agent.T he zinc-benzoate analogue, 3,w as also active for CO 2 /epoxide copolymerisation (entry 3) and the MALDI-ToF analysiso ft he resultantp olymer confirmed the presenceo fb oth a-benzoate, w-hydroxy and a,w-hydroxy end-capped polymers ( Figure S17). The presence of a-benzoate end-groups was confirmed by 1 HNMR spectroscopy (Figure S18).
It has previously been shown that analogues of 1 and 2 with acetate and halide co-ligands were effective catalysts for the ROCOP of epoxide (CHO)/anhydride (phthalic anhydride PA), [25] and that when am ixture of monomers is present, anhydride insertion occurs more rapidly than CO 2 insertion. [18a] In order to gain further understanding of the polymerisations, complex 1 was tested as ac atalyst for the ROCOP of PA/CHO, using a1mol %c atalyst loading at 100 8C, andn eate poxide as the solvent. After three hours, 100 %c onversion was achieved (Table 1, entry 5), with 98 %o fa lternating enchainment( % ester linkages). The polymerisation is well-controlled, giving am onomodalM Wd istribution and an arrow dispersity ( = 1.10). Here, 1 displays as lightly superior activity (TOF = 33 h À1 ) compared to its acetate (TOF = 24 h À1 ) [25a] andh alide (TOF = 17 h À1 ) [25b] analogues,u nder analogous conditions. Theoretical calculations suggest that the phenylc o-ligand could ring-open PA,a st he energy barrier is + 29.9 kcal mol À1 ,a nd the reaction gives an et energyg ain of 30.5 kcal mol À1 ( Figure S19). However,t he polyester analysis by MALDI-ToF again shows only as eries of a,w-dihydroxyl-terminated polymers. [25a, 28] In this case, the reactivity barrier for insertion of PA into the zincphenyl bond is significantly greater than the competing protonolysis pathway,by10.0 kcal mol À1 .This suggests that the reaction of complex 1 with alcohols is thermodynamically more favourablet han the reaction with PA.I ti ss upported by the absence of phenyl-capped polymers experimentally.A lthough 2 displays good activity for CHO/PA ROCOP (TOF = 24 h À1 , entry 6), it is less active than 1 and the polymerisation is poorlyc ontrolled,w ithabroad dispersity ( = 2.27) and low polyester selectivity (23 %p olyester vs. polyether). It is therefore observed that the C 6 F 5 co-ligandh as ad etrimental effect, although the exact nature of this influence is not completely clear.
Previously ac atalysts ystem formed in situb yr eaction between 1 and 1,2-cyclohexanediolw as investigated for the ROP of e-CL. [30a] This showed that the catalyst was highly effective, but that polymers with different topologies were formed: indeed, there was evidencef or chainsb oth end-capped by diol and chain extended from the diol. This is because the diol containst wo sterically hindered secondary alkoxide groups, which are relatively slower to initiate polymerisations. Given this previouss tudy,i tw as of interest to studyt he activation of catalyst 1 with am onofunctional alcohol, so as to ensure that there is only as ingle type of chain structure. In the presence of isopropanol, 1 was therefore appliedt ot he ROP of rac-lac-  [b] Polymer molecular weightsw ere determined usingS EC,c alibratedb yp olystyrene standards, and correction factors were applieda sr eported previously (1.85forP A/CHO, [19e] 0.58 for PLA [29] or 0.56 for PCL [30]  tide and e-caprolactone( Ta ble 1, entries 9-13). Under all conditions tested, 1 demonstrated good catalytic activity under immortalp olymerisation conditions (TOF = 5280 h À1 ,e ntry 9). The dispersities are broad,e specially for polycaprolactone (PCL). This is attributed to the ZnÀPh reaction with alcoholo ccurring relativelymore slowly at ambient temperature. Aseries of resonances assigned to a-isopropoxide, w-hydroxy end-capped polycaprolactone is observed in the MALDI-ToF spectrum (Figure S20, Ta ble1,e ntry 9).
The results show that di-zinc-aryl 1 can readily react with alcohols, either deliberately added or presenta sar esult of the reactiono fw ater and epoxide, to generate Zn-alkoxide active sites that can initiate polymerisations. In contrast, the di-zinc acetate complex, [LZn 2 OAc 2 ], does not react with alcohols and lactones and so is not as uitable catalyst for ROP (entry 11 and 13). It is known that the Zn-carboxylate can react with epoxides to generate Zn-alkoxide species in situ, which initiate the ROP of lactones,a sa pplied to prepare "switchable" catalysts. [18c] Alternatively, the Zn-alkoxide can readilyi nsert CO 2 ,whereas its acetate precursor cannot. Although 1 can insert CO 2 into the ZnÀCb ond to form ac arboxylate, the observation of a,wdihydroxyl end-capped polymers suggestst hat reaction of 1 with alcohols occurs more rapidly (Scheme 1). While the acetate catalysts can also undergo chain-transfer reactions with added alcohols or water, [15e, 23a] these reactions presumably occur after epoxide openingg enerates the Zn-alkoxide species.

Conclusion
In summary,t wo di-zinc-aryl complexes have been synthesised from the same macrocyclic ligand and characterised using Xray crystallographic and NMR spectroscopics tudies. Complex 1 cleanly insertsC O 2 under mild conditions, whilst 2 is inactive, highlighting differences caused by the electron-withdrawing fluoryl substituents. The complexes also react readily with alcohols, to generate the di-zinc-bis(alkoxide) complexes,w hich were fully characterised.B oth 1 and 2 efficiently initiate the alternating copolymerisations of cyclohexene oxide/carbon dioxide and cyclohexeneo xide/phthalic anhydride, demonstrating similar activities to the well-established acetate analogue. The reactivity and theoretical studies suggestt hat the competing reactions of 1 with CO 2 or diols are both viable initiation mechanismsf or CO 2 /epoxide ROCOP.H owever,t he polymerisation studies suggest that the protonolysiso f1 and 2,w ith added or generated alcohols, occurs more rapidly than CO 2 insertion, and is the predominant initiation mechanism. The in-situ-generated alkoxide complex is also an effective catalyst for the ROP of cyclic esters, including both rac-lactide and e-caprolactone, whereas the acetate analogue is completely inactive.
Overall, these studies have led to an improved understanding of the reactivity of di-zinc-bis(aryl) catalysts, and show how these versatile catalystsc an be applied to ar ange of ROP and ROCOP processes. We expect that the role alcohols can play in initiator formation will facilitate the development of im-provedf uture catalyst systems, whichw ill be the focus of our future studies.

Experimental Section
All metal complexes were synthesised under anhydrous conditions, using MBraun gloveboxes and standard Schlenk techniques. Solvents and reagents were obtained from Sigma Aldrich or Strem and were used as received unless stated otherwise. THF was dried by refluxing over sodium and benzophenone and stored under nitrogen. Isopropanol was dried over calcium hydride and distilled prior to use. Cyclohexene oxide (CHO) was dried over CaH 2 and fractionally distilled under nitrogen. Phthalic anhydride was purified by dissolving in benzene, filtering off impurities, recrystallising from chloroform and then subliming. All dry solvents and reagents were stored under nitrogen and degassed by several freeze-pumpthaw cycles. Research grade carbon dioxide was used for all copolymerisation studies. Macrocyclic ligand LH 2 was synthesised following literature procedures. [12b] NMR spectra spectra were recorded using aB ruker AV 400 MHz spectrometer.C orrelations between proton and carbon atoms were obtained by using COSY and HSQC NMR spectroscopic methods. Elemental analysis was determined by Stephen Boyer at London Metropolitan University.S EC was performed using two Mixed Bed PSS SDV linear Sc olumns in series, with THF as the eluent, at aflow rate of 1mLmin À1 ,onaShimadzu LC-20AD instrument at 40 8C. Polymer molecular weight (M n )w as determined by comparison against polystyrene standards, with ac orrection factor of 1.85 for PA/CHO, [19e] 0.58 for PLA, [29] and 0.56 for PCL. [30] The polymer samples were dissolved in SEC grade THF and filtered prior to analysis.
Crystal structure determination:Single crystal data were collected using Agilent Xcalibur PX Ultra A( 1 and 2), Agilent Xcalibur 3E ( 3) and Oxford Diffraction Xcalibur 3d iffractometers, and the structures were refined using the SHELXTL and SHELX-2013 program systems. [31,32] Selected parameters are given in the Supporting Information and full details are given in the deposited cif files. CCDC 1498754 (1), 1498755 (2), 1498756 (3)a nd 1498757 (5)c ontain the supplementary crystallographic data for this paper.T hese data are provided free of charge by The Cambridge Crystallographic Data Centre.