Unexpected side reactions and chain transfer for zinc-catalyzed copolymerization of cyclohexene oxide and carbon dioxide

Unexpected side reactions and chain transfer for zinc-catalyzed copolymerization of cyclohexene oxide and carbon dioxide
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  Unexpected Side Reactions and Chain Transfer for Zinc-CatalyzedCopolymerization of Cyclohexene Oxide and Carbon Dioxide Wouter J. van Meerendonk, Robbert Duchateau,* Cor E. Koning, andGert-Jan M. Gruter  Laboratory of Polymer Chemistry, Eindhoven University of Technology and Dutch Polymer Institute, P.O. Box 513, 5600 MB Eindhoven, The Netherlands Received April 14, 2005; Revised Manuscript Received June 24, 2005  ABSTRACT: A detailed investigation was conducted into the chain microstructure and reactionmechanisms of polycyclohexene carbonate (PCHC) synthesized with   -diketiminato zinc catalysts (EtBDI)-ZnOEt, (EtBDI)ZnOMe (EtBDI ) 2-(2,6-diethylphenyl)amido-4-(2,6-diethylphenyl)imino-2-pentene), anda bis(phenoxy)zinc catalyst [(2,6-F 2 C 6 H 3 O) 2 Zn ‚ THF] 2 . For these complexes several different initiation,propagation, and termination reactions were identified by analyzing the MALDI-TOF-MS spectra of theresulting polymers. The bis(phenoxy)zinc system showed some ether linkages while the   -diketiminatozinc catalyzed system produces perfectly alternating cyclohexene oxide - CO 2  copolymers. The presenceof a mixture of cyclopentyl, cyclohexyl, and cyclohexenyl end groups for all three catalyst systems pointsto an unexpected side reaction that is initiated by traces of water. This study also led to the discoverythat, under conditions where traces of water or alcohols are present, the   -diketiminato zinc catalyst isnot a truly living system since a very rapid reversible chain transfer mechanism is operating, wherealcohols and/or hydroxyl-terminated polymer chains act as the chain transfer agents. Introduction The aliphatic polycarbonates synthesized from carbondioxide and oxiranes (Scheme 1) are receiving anincreasing interest from research groups all over theworld. 1 The potential of carbon dioxide as a comonomerin aliphatic polycarbonate production was first recog-nized by Inoue et al., who copolymerized propylene oxideand carbon dioxide with a ZnEt 2  /H 2 O mixture. 2  Abouta decade later the same group discovered that activecatalytic systems were not limited to zinc-based systemsas porphyrinato aluminum compounds in combinationwith suitable cocatalysts also proved to be effective inthe copolymerization of oxiranes and carbon dioxide. 3  Afteraperiodoflittleprogressinthisfield,Darensbourgand co-workers introduced zinc phenoxide catalysts(Figure 1) for the copolymerization of cyclohexene oxide(CHO) and CO 2 , in the early 1990s. 4 Molecular weightsobtained with these catalysts are generally high, butthe polydispersity is not well-controlled (up to 18) whileactivities are generally low. More recently, Coates et al. 5 developed highly active, single-site   -diketiminato zinccatalysts (Figure 1). This breakthrough has led to arenewed interest in this field which resulted in thedevelopment of highly selective and active chromium, 6 cobalt, 7 and manganese 8 porphyrinato and salen sys-tems. 9  Although various catalysts have been developed forthe oxirane - CO 2  copolymerization, the complete mech-anism of the polymerization reaction is still not fullyunderstood. 5e,10 For example, there is still some debateabout whether   -diketiminato zinc complexes have tobe dimeric to be an active catalytic system. Further-more, termination or chain transfer reactions havenever seriously been addressed in the past. In the caseof the   -diketiminato zinc catalysts, the absence of anytermination or chain transfer mechanism seemed justi-fied as the low polydispersities reported for this systemand the linear relationship found between  M  h n  vs con- version are indicative for a living nature. 11 The bis-(phenoxy)zinc catalysts, on the other hand, do givepoly(cyclohexene carbonate) (PCHC) with extremelyhigh polydispersities. The lack of conformity betweenthese seemingly similar polymerizations triggered us totake a closer look at these systems. In our previousresearch on the catalytic copolymerization of cyclohex-ene oxide and CO 2  some anomalies were found whichcould not be explained by the mechanism proposedpreviously, 5e and a more detailed investigation of thepolymer microstructure was undertaken with triple-SEC and MALDI-TOF-MS. In this contribution wepresent our results on a mechanistic study of thecyclohexene oxide - CO 2  copolymerization catalyzed by   -diketiminato zinc catalysts and a bis(phenoxy)zinccatalyst (Figure 1) and compared the poly(cyclohexenecarbonate) products obtained using the two catalystsystems. * Corresponding author: R.Duchateau@tue.nl. Figure 1.  Examples of copolymerization catalysts developedby Darensbourg et al. (left) and Coates and co-workers (right). Scheme 1. Copolymerization of Carbon Dioxide andCyclohexene Oxide 7306  Macromolecules  2005,  38,  7306 - 7313 10.1021/ma050797k CCC: $30.25 © 2005 American Chemical SocietyPublished on Web 07/30/2005  Experimental Section Materials. Cyclohexene oxide (Aldrich) was dried on CaH 2 ,distilled, and stored under argon on molsieves (4 Å) prior touse. Carbon dioxide ( > 99.9993% pure) was purchased fromHoekLoos and used without any further purification. Toluenewas dried over an alumina column and stored on molsieves (4 Å). The (EtBDI)ZnOR catalysts (R ) Me, Et; EtBDI ) 2-(2,6-diethylphenyl)amido-4-(2,6-diethylphenyl)imino-2-pentene) andthe bis(phenoxy)zinc catalyst [(2,6-F 2 C 6 H 3 O) 2 Zn ‚ THF] 2  (Figure1) were synthesized according to literature procedures. 4e,5b  Analytical Techniques.  1 H NMR spectra were recordedon a Varian Gemini 2000 (300 MHz) and a Varian Mercury Vx (400 MHz) spectrometer. Size exclusion chromatography(SEC) traces were recorded on a Waters GPC equipped witha Waters model 510 pump and a model 410 differentialrefractometer (40 °C). THF was used as the eluent at a flowrate of 1.0 mL min - 1 . A set of two linear columns (Mixed C.Polymer Laboratories, 30 cm, 40 °C) was used. Molecularweights were calculated relative to polystyrene standards.Data acquisition and processing was performed using WatersMillennium32 software. Triple-SEC measurements were per-formed on a system consisting of a three-column set (two PLgelMixed-C 5  µ  columns and one PLgel Mixed-D 5  µ  column fromPolymer Laboratories), with a guard column (PLgel 5  µ Polymer Laboratories), a gradient pump (Waters Alliance2695, flow rate of 1.0 mL min - 1 isocratic), a photodiode arraydetector (Waters 2996), and a differential refractive indexdetector (Waters 2414) as concentration detectors, a lightscattering detector (Viscotek), a viscosity detector (Viscotek,dual detector 250), and THF as a solvent. THF was filteredtwice (0.2  µ m filter) and stabilized with BHT (4-Me-2,6-( t -Bu) 2 C 6 H 2 OH). Data acquisition and processing were performedwith Viscotek TriSec GPC Software (version 3.0 Rev. B.03.04).MALDI-TOF-MS analysis was carried out on a Voyager DE-STR from Applied Biosystems. The matrix, DCTB ( trans -2-[3-(4- tert -butylphenyl)-2-methyl-2-propenylidene]malononi-trile), was synthesized according to literature procedures. 12 Potassium trifluoroacetate (Aldrich, > 99%) was added to thepolymer samples as cationization agent. The matrix wasdissolved in THF at a concentration of 40 mg mL - 1 . Thepotassium trifluoroacetate was added to THF at a typicalconcentration of 1 mg mL - 1 . Polymer was dissolved in THFat  ∼ 1 mg mL - 1 . In a typical MALDI-TOF-MS analysis thematrix, potassium trifluoroacetate and the polymer solutionwere premixed in a ratio of 10:1:5. The premixed solutionswere hand-spotted on the target well and left to dry. Spectrawere recorded in both the linear and reflector mode. ExampleofaTypicalPolymerizationUsing    -Diketim-inatoZincCatalysts.( EtBDI)ZnOEt (194 mg, 295  µ mol, 0.06mol %) was dissolved in a mixture of CHO (50 mL, 495 mmol)and toluene (16.7 mL). After complete dissolution of thecatalyst, the mixture was injected into a preheated (50 °C) 200mL autoclave that was previously dried under vacuum at 100°C for 12 h. The autoclave was pressurized to 9 bar with carbondioxide, and the polymerization commenced. After a setpolymerization time, a sample was taken for 1 H NMR analyses(300 MHz, CDCl 3 ) to determine the conversion by integrationof the methine peaks in the  1 H NMR spectra:  1 H NMR (300MHz, CDCl 3 ):  δ  4.65 (br, CH (PCHC), 2H), 3.11 (s, C  H   (CHO),2H). The samples for SEC analyses were prepared as follows: About 0.5 mL of each of the reaction mixtures was addeddropwise to a 10-fold excess of petroleum ether (40 - 70), uponwhich the poly(cyclohexene carbonate) (PCHC) precipitated. After separation, the polymer was redissolved in the SECeluent THF. Polymers needed for MALDI-TOF-MS analyseswere prepared in a similar manner. Example of a Typical Polymerization Using the Bis-(phenoxy)zinc Catalyst.  After a mixture of cyclohexeneoxide, toluene, and the catalyst was injected into the 200 mLsteel autoclave, the reactor was heated to 30 °C and pressur-ized to 50 bar with carbon dioxide. All valves were closed, andthe reactor was further heated to 80 °C, resulting in a pressureof about 80 bar. Analyses and work-up procedures were similaras described above. Results and Discussion In a recent study on high throughput experimenta-tion, we reported the linear behavior of   M  h n  vs time forthe ( i PrBDI)ZnN(SiMe 3 ) 2  ( i PrBDI  )  2-(2,6-diisopropy-lphenyl)amido-4-(2,6-diisopropylphenyl)imino-2-pen-tene)) catalyst, pointing to a living character of thesystem. 11  A closer look, however, showed the  M  h n  valuesobtained by SEC (  M  h n (experimental)) to be substantiallylower than the values calculated using the  1 H NMRmonomer conversion values and the catalyst concentra-tion (  M  h n (theory)). The SEC measurements were pro-cessed using a polystyrene calibration line, but due tothe absence of Mark - Houwink parameters for PCHC,the  M  h n  values were validated by means of triple-SECand MALDI-TOF-MS measurements, which all gave very similar molecular weights. To rule out the occur-rence of fortuitous events causing this deviation, similarpolymerization experiments with (EtBDI)ZnOR (R  ) Me, Et) as the catalysts were carried out, which showeda similar (R  )  Et) or even larger (R  )  Me) deviationbetween  M  h n (theory) and  M  h n (experimental). The behav-ior of   M  h n  vs time for both the ( i PrBDI)ZnN(SiMe 3 ) 2  andthe (EtBDI)ZnOMe catalyst is shown in Figures 2 and3, respectively. The possibility of formation of cycliccarbonate as the srcin of the deviation of theoreticalfrom experimental values of the molecular weights couldalso be excluded.No cyclohexene carbonate was detectedby NMR. Furthermore, this would lead to a lowermolecular weight but not to an increase in the numberof polymer chains. Catalyst deactivation can also beexcluded as a possible reason for the deviation betweenthe experimental and theoretical  M  h n  values. Fast cata-lyst deactivation at the beginning of the polymerizationwould lead to a lower concentration of active catalysts.For a living system, the expected  M  h n  would thereforebe higher instead of lower than the theoretical  M  h n  forthe same conversion and yield. Slow deactivation duringthe polymerization process will not affect the numberof chains much but is expected to broaden the molecularweight distribution, which was not observed either. Figure 2.  Deviation between  M  h n (experimental) and  M  h n- (theory) for ( i PrBDI)ZnN(SiMe 3 ) 2  catalyst. Figure 3.  Deviation between  M  h n (experimental) and  M  h n -(theory) for (EtBDI)ZnOMe catalyst.  Macromolecules, Vol. 38, No. 17, 2005  Copolymerization of Cyclohexene Oxide and CO 2  7307  Hence, the found deviation is most probably the resultof a nonliving behavior of the catalyst.The next questions that arise are what kind of chaintermination or chain transfer mechanism is playing arole and how for example temperature and catalystconcentration affect these processes. The influence of temperature and catalyst concentration on the numberof chains and the  M  h n  was investigated, and the resultsfor the   -diketiminato zinc system (EtBDI)ZnOEt aredisplayed in Table 2.Changing the temperature does not have a significanteffect on the number of chains per active site. However,the temperature does have a considerable effect on themolecular weight and the selectivity of the reaction.Lowering the temperature from 50 to 30 °C resulted ina considerable drop in molecular weight, most probablydue to the slow initiation and low activity of the catalystat that temperature. 5e,10 On the other hand, increasingthe temperature affects both the molecular weight andthe selectivity of the catalyst toward PCHC formation. At 80 °C, the selectivity drops to around 50%, and aconsiderable amount of   trans -cyclohexene carbonate isformed by backbiting of the PCHC chain. This severelyreduces the molecular weight, as can be seen from entry5. A similar change in selectivity was observed by Allenand co-workers during CO 2 - propylene oxide copolym-erization, which selectively afforded poly(propylenecarbonate) at 25 °C, while at 50 °C exclusively propylenecarbonate was formed. 5c Kinetics studies performed byCoates et al revealed a near second-order dependencein zinc for the copolymerization, which indicates thepresence of dimeric active species. 5e  At higher temper-ature, the monomer dimer equilibrium of the catalystshifts toward the monomer form. 5,10 Regardless of whether the monomeric species is an active catalyst forthe copolymerization, the monomeric zinc species canstill undergo intramolecular backbiting, yielding cyclo-hexene carbonate and lower molecular weight polycar-bonates. Increasing the catalyst concentration leads toa reduction of the number of chains per catalyst whilea small increase in molecular weight is also observed. Although influence of the aforementioned shift in themonomer dimer equilibrium of the catalyst toward thedimeric species at higher concentrations cannot beexcluded, 5e,10 the observed effect is most likely causedby the lower effect of poisoning impurities, present inthe reactor or in the monomer, at higher catalystconcentrations (vide infra). As was reported earlier, 4 the bis(phenoxy)zinc system[(2,6-F 2 C 6 H 3 O) 2 Zn ‚ THF] 2  shows very high polydisper-sities. This broad molecular weight distribution srci-nates from multimodal Schulz - Flory distributions,which are hardly affected by temperature or catalystconcentration. Figure 4 shows characteristic SEC plotsof polycarbonate samples taken at set time intervals. Already at the beginning of the polymerization thepolydispersity is higher than 6 and increases during thepolymerization to a value of 18. Although the polydis-persity drops somewhat toward the end of the polym-erization, it is clear that the shape of the SEC plots donot change significantly. Similar distributions were alsofound with the bis(salicylaldiminato)zinc complexesmade by Darensbourg and co-workers. 13 Such a distri-bution pattern is characteristic for multiple nonlivingactive sites present in the system. Koning et al. alreadycommented upon these broad polydispersities and pos-tulated an influence of different phases during thepolymerization. 4b Slow or incomplete initiation in com-bination with the formation of aggregates is anotherprobable cause of the high polydispersities for thiscatalyst system.To investigate the possible occurrence of transesteri-fication reactions, both catalytic systems were tested ina prolonged polymerization, for which the reactionmixture was kept at polymerization conditions for a longtime (10 days) after full conversion had already beenreached. Although intermolecular transesterificationdoes not affect the number of polymer chains butrandomization should lead to a molecular weight dis-tribution of 2. However, for the bis(phenoxy)zinc systemthe PDI remained very high ( ∼ 9), while for the   -diketim-inato zinc catalysts, even after the prolonged polymer-ization, the polydispersities were as low as 1.12. In-tramolecular transesterification, on the other hand,would lead to cyclohexene carbonate or ring structures Table 1. Details of Experimental Conditions and Results for Polymers Analyzed with MALDI-TOF-MS a catalyst monomer temp (°C)  P  (bar) time (h)  M  h n  (g/mol - 1 )  M  h w  /   M  h n  conv (%) chains per catalyst b MALDIspectrum(EtBDI)ZnOMe CHO 50 9 2 3700 1.11 9 5.6 Figure 5(EtBDI)ZnOEt CHO 50 9 2 11600 1.18 40 2.4[(2,6-F 2 C 6 H 3 O) 2 Zn ‚ THF] 2  CHO 80 80 28 25800 18.4 19 1.7 Figure 6 a  All polymerizations were performed with 300  µ mol of catalyst, 15 mL of CHO, and 35 mL of toluene.  b Calculation based on both SECdata and conversion data from  1 H NMR, assuming that 100% of the catalyst is active. Table 2. Effect of Catalyst (EtBDI)ZnOEt Concentration and Temperature on Chain Transfer a entry catalyst (  µ mol) CHO (mL) toluene (mL) temp (°C)conversion toPCHC (%)  M  h n  (g mol - 1 )  M  h w  /   M  h n chains percatalyst1 200 15 35 50 26.4 9729 1.08 2.92 300 15 35 50 39.4 11561 1.18 2.43 400 15 35 50 45.3 12645 1.09 1.94 300 15 35 30 14.4 4058 1.76 2.55 300 15 35 80 9.5 b 2853 1.23 2.3 a  All polymerizations were performed at 9 bar CO 2  pressure for 2 h.  b  As a result of backbiting around 11% of the CHO was convertedto the cyclic carbonate (trans-CHC). Figure 4.  Characteristic SEC plots of polycarbonate samplessynthesized with [(2,6-F 2 C 6 H 3 O) 2 Zn ‚ THF] 2 , taken at set timeintervals. 7308  van Meerendonk et al.  Macromolecules, Vol. 38, No. 17, 2005  in combination with a smaller chain fragment and thusindeed to an increase of the number of polymer chainsper active site. However, the molecular weight of thepolymers did not decline, and MALDI-TOF-MS analysisof polymer samples did not show any sign of ringstructures or randomization (head-to-head, tail-to-tailstructures) of the polymer chains, ruling out anyrandom chain scission/recombination reactions. How-ever, MALDI-TOF-MS did show some unexpected re-sults. MALDI-TOF-MS Analysis.  MALDI-TOF-MS analy-ses of the polymers produced by the bis(phenoxy)zincand   -diketiminato zinc systems revealed some interest-ing features. In Figure 5 a Poisson type molecularweight distribution is found for the PCHC producedwith the (EtBDI)ZnOMe zinc catalyst in the experimentdescribed in Table 1. On the other hand, the PCHCformed with the [(2,6-F 2 C 6 H 3 O) 2 Zn ‚ THF] 2  catalyst alsodescribed in Table 1 shows a Schulz - Flory type molec-ular weight distribution (Figure 6).The repeating unit is 142 Da, which is the mass of acyclohexene carbonate unit. The various peaks (eachpeak, in turn, is split in its isotope pattern) within arepeating unit are the result of different end groups thatare present in the polymer. Since the formation of different end groups  after  polymerization could not beruled out completely a priori, various work-up proce-dures were used. 14 However, no changes were observedin either SEC or MALDI-TOF-MS spectra, indicatingthat the various end groups were formed during thepolymerization. In all polymers prepared with a   -diket-iminato zinc catalyst CO 2  is the first monomer to insertin the zinc - initiator bond. This is not surprising sincehomopolymerization of CHO is not possible with thiscatalyst under typical polymerization conditions. 5 Cor-respondingly, perfectly alternating cyclohexene oxide - CO 2  copolymers are formed. On the other hand, theMALDI-TOF-MS spectrum of the PCHC made with theDarensbourg catalyst [(2,6-F 2 C 6 H 3 O) 2 Zn ‚ THF] 2  (Figure6) also showed the presence of polymeric chains where Figure 5.  MALDI-TOF-MS spectrum of the K  + adduct of PCHC obtained with (EtBDI)ZnOMe and an enlargement of part of thespectrum. Figure 6.  MALDI-TOF-MS spectrum of the K  + adduct of PCHC obtained with [(2,6-F 2 C 6 H 3 O) 2 Zn ‚ THF] 2  and an enlargement of part of the spectrum.  Macromolecules, Vol. 38, No. 17, 2005  Copolymerization of Cyclohexene Oxide and CO 2  7309  one or more CO 2  units are missing. Interestingly, thephenoxides first react selectively with cyclohexene oxidebefore inserting a carbon dioxide monomer. This is inagreement with the capability, albeit poor, of [(2,6-F 2 C 6 H 3 O) 2 Zn ‚ THF] 2  to homopolymerize cyclohexeneoxide to the corresponding polyether. 4 For the PCHC prepared with (EtBDI)ZnOMe (Figure5), peak A is the expected chain end srcinating fromionization by K  + of a normal polymer of 13 cyclohexenecarbonate units, with a methoxide group on one side(resulting from the catalyst initiating moiety) and aproton on the other side (srcinating from hydrolysesof the polymer - catalyst bond after quenching withMeOH/HCl). Similarly, peak A in Figure 6 correspondsto the expected K  + ionized polymer that contains theinitiating phenoxide group 2,6-F 2 C 6 H 3 O of [(2,6-F 2 C 6 H 3 O) 2 Zn ‚ THF] 2  and is end-capped by a proton asa result of the hydrolysis after quenching. The incor-poration of the phenoxide ligand as an end group in thepolymer chain was previously observed by 19 F NMR andUV. 4e,15 Polymers with two hydroxyl end groups are alsoobserved for the bis(phenoxy)zinc system (peak D,Figure 6). The srcin of these end groups is probablywater that can act as a chain transfer agent. Peaks E,F, and G in Figure 6 correspond with polymers (peaks A  - D) in which a carbon dioxide unit is missing. Suchpeaks were absent in the   -diketiminato zinc system,which formed completely alternating copolymers.More difficult to assign are peaks B and C. Peaks Band C do not appear to be influenced by the initiatinggroup, as can be seen in Figure 7 where the spectra of PCHC prepared with the (EtBDI)ZnOEt and the (EtB-DI)ZnOMe are shown. The peak of the polymeric speciesinitiated with the alkoxide groups can clearly be seenas peaks A and A  ′  for the methoxide and ethoxide,respectively. However, peaks B and C do not shift.Moreover, both (EtBDI)ZnOR (R  )  Me, Et) and [(2,6-F 2 C 6 H 3 O) 2 Zn ‚ THF] 2  (Figures 5 and 6,respectively) showthe same peaks B and C at the same mass, whichindicates that the corresponding chain structures arecatalyst independent. In the case of [(2,6-F 2 C 6 H 3 O) 2 Zn ‚ THF] 2  peak B overlaps with peak A (Figure 6), but asimilar analysis on the analogous dimethyl-substitutedzinc phenoxide [(2,6-Me 2 C 6 H 3 O) 2 Zn ‚ THF] 2  clearly showsthe presence of peak B. Interestingly, peaks B and Cwere exclusively found when zinc-based catalysts wereused to catalyze the copolymerization. For example,when the porphyrinato chromium complex [TPP]CrClwas used as the catalyst, only polymers with twohydroxyl end groups (HO[C 6 H 10 OC( d O)O] n C 6 H 10 OH)were observed.The fact that the fragments B and C were formed,irrespective of the type of zinc catalysts, did point at acatalyst and initiator independent structure like ringsformed after transesterification. As already mentioned,no plausible ring structures could be modeled with themass of peak B or C, and SEC-DV measurementsshowed a linear Mark - Houwink plot indicating a linearpolymer. 4b Peak B corresponds to a polycarbonate witha C 5 H 9 O initiating group and a proton end group (HO-[C 6 H 10 OC( d O)O] n C 5 H 9 ). So far it is a complete mysterywhere the C 5 H 9 O fragment srcinates from. GC-MSanalysis of the cyclohexene oxide showed no impurities,and the fact that only the use of zinc catalysts gives riseto the formation of C 5 H 9 O end groups strongly suggestssome kind of zinc-based side reaction. A careful analysisof the isomer patterns revealed that peak C is acombination of two overlapping isotope patterns withan end group mass corresponding with a C 6 H 11 O and aC 6 H 9 O fragment, which indicate the presence of cyclo-hexyl and cyclohexenyl end groups in a 1:1 ratio (Figure8). Although their presence is evident, the srcin of the cyclohexyl and cyclohexenyl end groups is notobvious. 16 The possibility of chain scission during MALDI-TOF-MS experimentscan be excluded,sincethese peaks werenot observed in the MALDI-TOF-MS spectra of PCHCsamples prepared by porphyrinato chromium systems. 17 To exclude any zinc-based reactions during samplepreparation or MALDI-TOF-MS measurements, some   -diketiminato zinc alkoxide was added to PCHC samplesproduced with the porphyrinato chromium system.However, this did not lead to the appearance of cyclo-hexyl/cyclohexenyl (peak C) or the C 5 H 9 O - end groups(peak B). Hence, the cyclohexyl and cyclohexenyl endgroups as well as the cyclopentyl end group are formed Figure 7.  Comparison between MALDI-TOF-MS spectra of PCHC obtained with (EtBDI)ZnOMe and (EtBDI)ZnOEt. 7310  van Meerendonk et al.  Macromolecules, Vol. 38, No. 17, 2005
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