Dijkstra, Pieter J.

Feijen, Jan

Zhong, Zhiyuan

English

The ring-opening polymerization of ω-pentadecalactone (PDL), a 16-membered lactone, in the bulk and in solution using yttrium isopropoxide as an initiator was investigated. All isopropoxide groups of yttrium isopropoxide participated in the initiation and the polymerization took place via acyl-oxygen cleavage of the monomer. The molecular weights of the resulting polymers could be controlled effectively by varying the monomer/initiator ratio. An induction period was observed for the bulk polymerization conducted at 60°C. DSC analyses indicated that poly(ω-pentadecalactone) is a highly crystalline polymer, having a high crystallization rate

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Macromol.Chem.Phys.201,1329–133 (2000) 1329Controlledring-openingpolymerizationofx-pentadecalactonewith yttrium isopropoxideasaninitiatorZhiyuanZhong,PieterJ. Dijkstra,JanFeijen*Departmentof ChemicalTechnologyandInstitutefor BiomedicalTechnology, Universityof Twente,P.O.Box 217,7500AE Enschede,TheNetherlandsFax:+31-53-4893823,Email: j.feijen@ct.utwente.nl(Received:August18,1999;revised:October4, 1999)Intr oductionInterestin the design,synthesisand properties of biode-gradablepolymers is increasing steadily owing to thepotential applicationsin biology, medicine,pharmacyandasenvironmentally friendly materials. Glycolide, lactidesand e-caprolactoneare among the most usedmonomersfor thesynthesisof biodegradable polymersby ring-open-ing polymerization,andseveral efficient catalystsystemsfor their preparation haverecentlybeendeveloped1–7,21).In contrastto the ring-opening polymerization of smalland medium-membered lactones, polymerization oflarge-memberedlactoneshasreceived lessattention. It iswell documentedthat large-memberedlactoneshavelittlering-strain and consequently are extremely difficult topolymerize by non-enzymatic methods. Severalinvesti-gatorshavereported on polyestersderived from macro-lactones8–11), but in all cases,no detailed description ofthe polymerization was given. Just recently, it wasdemonstrated clearly that 12 and 13-membered lactonescould undergo ring-opening polymerization using analkali metal methoxideas initiator12). Nevertheless,rela-tively high temperaturesof 90–1508C for bulk polymeri-zation,and long polymerizationtimes werenecessary toreach good yields, and only relatively low molecular-weightpolymersaccompaniedby smallamountsof cyclicandlinear oligomerswere obtained.Anionic polymeriza-tion of the 16-membered lactone,x-pentadecalactone,initiated with potassium alkoxides under mild conditionswas reportedby Jedliński, et al.13). No direct correlationbetweenthe molecular weight of the resultant polymersand the monomer/initiator ratio was observed. In addi-tion, the formation of cyclic oligomers via backbitingwas confirmed by GPC and MS analyses.Hori, et al.,gave an examplefor the ring-opening homopolymeriza-tion of a 16-membered dilactone,ethylenedodecanedio-ate, catalyzed by a distannoxanecomplex14). After 96 hpolymerization in the bulk at 958C, the polymer wasobtainedin goodyield.Enzymatic ring-opening polymerization of macrolac-tones has recently beenreported15–17). In contrast to thelow polymerizability of macrolactonesby non-enzymaticmethods,it wasfoundthatmonomers could beenzymati-cally polymerizedevenfasterthan e-caprolactone, prob-ably due to a better recognition of the macrolactonebylipase17). However, large quantitiesof enzymesand longreaction timeswere required. For the ring-openingpoly-merization of x-pentadecalactone, enzyme/monomerratios of 4/1 by weight and 24–72 h reaction at 808Cwere applied. The molecular weight of the polymerscould not becontrolled15).Previous researchrevealed that lanthanide catalystshave a high activity for the ring-opening polymerizationof e-caprolactone and lactides7,20–23). In this paper, thering-openingpolymerizationof x-pentadecalactone bothin solutionand in bulk initiated by yttrium isopropoxideis described. x-Pentadecalactone,a commercialy avail-able compound,was selectedas a model compound tostudy thering-opening polymerizationof macrolactones.Full Paper: Thering-openingpolymerizationof x-penta-decalactone(PDL), a 16-memberedlactone,in the bulkand in solutionusingyttrium isopropoxideasan initiatorwasinvestigated.All isopropoxidegroupsof yttrium iso-propoxideparticipatedin the initiation andthe polymeri-zation took placevia acyl-oxygencleavageof the mono-mer. The molecular weights of the resulting polymerscould be controlledeffectively by varying the monomer/initiator ratio. An induction period was observedfor thebulk polymerization conductedat 608C. DSC analysesindicatedthat poly(x-pentadecalactone)is a highly crys-talline polymer, havinga highcrystallizationrate.Macromol. Chem.Phys.201, No. 12 i WILEY-VCH VerlagGmbH, D-69451Weinheim2000 1022-1352/2000/1208–1329$17.50+.50/01330 Z. Zhong,P. J.Dijkstra,J.FeijenExperimental partMaterialsx-Pentadecalactone(Aldrich, Brussels,Belgium) was dis-tilled undervacuumandstoredin a glove box. Yttrium iso-propoxide (Strem, Amsterdam,the Netherlands)was usedwithout further purification. Toluenewasdried by refluxingoversodiumanddistilled underargonprior to use.All glass-ware for the polymerizationwasdried overnightin an ovenat 1308C beforeuse.PolymerizationPolymerizationreactionswere carried out in a previouslydried and Argon-purged ampoule.(1) Bulk polymerization:A certainamountof initiator andmonomerwereintroducedinto the ampouleequippedwith a Teflon-coatedmagneticstirrer in a glovebox.Theampoulewassealedwith a stopperandtransferredto a Schlenkline. Thepolymerizationstartedafterplacingthereactionvesselinto anoil baththermostatedat a prescribedtemperature.After a predeterminedtime, 0.1N aqueousHCl wasquickly addedto thereactionmixture toterminatethe polymerization.The mixture wasdissolvedinCHCl3, anda samplefor 1H NMR analysisto determinetheconversionwas taken.The remainingsolution was washedwith distilled water for threetimes,andthenprecipitatedinan excessof methanol.The polymer was isolatedby filtra-tion, washedwith methanol, and then dried at 408C invacuo.(2) Solution polymerization:Since the ring-openingpolymerizationof x-pentadecalactonein solution at roomtemperatureis very slow, solutionsof monomerandinitiatorin toluenewere introducedinto an ampoulein a glove box.The ampoulewasthensealedwith a stopperandtransferredto a Schlenkline. The polymerization startedafter placingtheampouleinto anoil baththermostatedat 808C. After 2 or4 h polymerization,completeconversionwas yielded. Thepost-treatmentwasasdescribedabove.Measurements1H NMR spectrausingCDCl3 asa solventwererecordedona 300MHz Varian Inova and standardizedby the CDCl3peakat d = 7.25.Conversionsandthedegreeof polymeriza-tion (DP) weredeterminedby integrationof therelevantsig-nals. Thermal analysisof polymerswas carried out with aPerkin-ElmerDSC-7 differential scanningcalorimeter. Thesampleswereheatedfrom –100to 1308C at a rateof 108C/min, keptat 1308C for 2 min, cooledto –1008C at a certainrate (10, 20 or 408C/min), and then a secondheatingscan(108C/min) was recorded.Gel permeationchromatography(GPC)wasperformedwith a Waters6000A GPCapparatusequippedwith four WatersStyragelHR columns(105, 104,500, 100 Å, pore diameter),combinedwith a H502 vis-cometer detector (Viscotek Corp.) for absolutemolecularweight determinations.Polymerswere dissolvedin chloro-form (1.0 wt.-%) and elution was performedat 258C at aflow rateof 1.5mL/min usingchloroformaseluent.Resultsand discussionIn recent publications, it has been demonstrated thatyttrium isopropoxide is a highly effective initiator for thepolymerization of isocyanates18), ethyleneoxide19), lac-tones and lactides20–23). In this paper, the ring-openingpolymerization of x-pentadecalactone, a monomercon-sideredto have a low polymerizability when non-enzy-maticmethodsareused,hasbeenexplored.The results of the ring-opening polymerization of x-pentadecalactoneperformed in the bulk using differentreactionconditionsare summarized in Tab.1. From thedata obtained, it was observedthat all polymerizationsproceededvery fast.Within thetemperaturerangestudied(60l1008C), the polymerization rate increasedwithincreasing temperature. On the other hand, when theTab.1. Ring-openingbulk polymerizationof x-pentadecalactoneby yttrium isopropoxideEntry [M]/[I]in mol/molTemp.in 8CTimein minConv.a)in %M—n 610–4 M—w/M—nd)Calcb) 1H NMRc) GPCd)1 13 60 10 100 0.12 0.16 n.d.e) n.d.e)2 26 60 10 100 0.24 0.25 n.d.e) n.d.e)3 39 60 10 100 0.36 0.39 n.d.e) n.d.e)4 52 60 10 100 0.48 0.49 n.d.e) n.d.e)5 65 60 10 100 0.60 0.64 n.d.e) n.d.e)6 150 60 30 100 1.4 1.3 1.6 1.67 200 60 30 93.5 1.7 1.7 1.7 1.88 200 80 30 100 1.8 1.7 1.5 2.19 200 100 5 94.5 1.7 1.3 1.6 1.610 400 60 60 61.6 2.3 2.1 2.9 1.611 400 80 30 94.0 3.4 2.6 3.2 1.612 400 100 5 70.0 2.6 1.7 2.5 1.5a) Calculated from 1H NMR spectraof thecrudepolymerizationmixtures.b) Calculated basedon thefollowing formula: M—n, calc = (2406 [M]/[I] 6conversion)/2.6.c) Experimentalvaluesobtained from 1H NMR endgroupanalysis.d) Determined by GPCanalysis.e) Not determined. Loseaccuracyin this molecular weightrange.Controlledring-openingpolymerization of x-pentadecalactone... 1331polymerizationtemperaturewasfixed, a longerpolymeri-zationtime wasnecessary to reachgood conversionwithincreasingmonomer/initiator ratios. Absolute molecularweightsdetermined from GPC were in good agreementwith experimental values calculated from 1H NMR end-groupanalysiswhen themolecular weight is not too high,and are close to the theoretical values (entry 6–10,Tab.1). At high DP, no accurate valuescould be deter-mined from 1H NMR becausethe signals of the end-groupbecometoo small. However, the calculatedvaluescorrelatedwell with thosedeterminedby GPC.Themole-cular weight distributions were around 1.6 (with theexception of entry8).Thedependenciesof thedegreeof polymerization(DP)of x-pentadecalactone on monomer/initiator ratiosinitiated by yttrium isopropoxide conducted both in thebulk at 608C andin solution at 808C areshown in Fig. 1.The DPsof the obtainedpolymerswere calculatedfromintegral ratios of signals at d = 4.05 (1CH21OC(O)1)and3.63(terminalmethylene(1CH21OH) protons).Theresultingdegreesof polymerization (DP) of thepolymerscorrelatedvery well with the monomer/initiator ratios.The lines throughthe origin had slopesof 0.38 for bulkpolymerization and 0.39 for solution polymerization,respectively, both very closeto the theoretical value (5/13). It could be concluded that all isopropoxide groupswere active and broughtaboutthe polymerization of x-pentadecalactone. The polymerization proceededin acontrolled manner, analogous to the ring-opening poly-merizationof e-caprolactoneandlactides23).The conversion of x-pentadecalactoneasa function ofpolymerization time at a reaction temperatureof 608Cwas determined by manual sampling followed by 1HNMR analyses.The monomerhasa characteristicsignalat d = 4.15(1(O)CO1CH21), while theseprotons in thepolymer chains are found at d = 4.05. The conversionswere calculated from the peak integrals (Fig. 2). Aninduction period wasobserved, which wasalso reportedfor the ring-opening polymerization of e-caprolactone byaluminum alkoxide24) or yttrium isopropoxide21). Thisinduction period hasbeenrelatedto structural rearrange-ment processes of the initiator to form the actual activesites.The ratherslow initiation comparedto propagationmight explain the slightly broadmolecularweight distri-butionsof theresulting polymers.Thepolymerswere analyzedby protonNMR andtypi-cal spectraaregivenin Fig. 3. Thesignals at d = 5.06and3.63 wereassignedto the methine protonsof the isopro-poxycarbonyl endgroup anda-methylene protonsof thealcohol endgroupandhaveintegral ratioscloseto 1 : 2within the limits of the NMR experimental errors, whichagreeswith their theoreticalratio. In orderto confirm thechemical shift of the terminal hydroxyl end group,a 1HNMR spectrumwas recordedafter reacting the polymerwith trifluoroacetic anhydride (Fig. 3B). Comparingthesetwo spectra,it wasfound(Fig. 3B) that theoriginalsignal at d = 3.63 (triplet) completely disappeared,andanew peak with equivalent intensity at d = 4.30 (triplet)appeared.The downfield shift of the a-methylene pro-tons, owing to the formation of the trifluoroacetic esterendgroup,fully demonstratedthe presenceof the hydro-xyl endgroups.Theisopropoxycarbonyl groupat oneendand the hydroxyl group at the other end of the polymerchains revealed that the polymerization took place viaacyl-oxygen bond cleavage of x-pentadecalactone(Scheme1).The molecular weightsandmolecular weight distribu-tionsof theresulting polymersweremeasuredby gel per-meation chromatography(GPC).A typical GPCchroma-togramis shown in Fig. 4. Unlike theresults reportedpre-viously13), the GPC chromatogramof poly(x-pentadeca-lactone)displayeda unimodalandsymmetric peak,withFig. 1. Degreeof polymerization (DP) of x-pentadecalactoneas a function of monomer/initiator ratio. Yttrium isopropoxidewasusedasthe initiator. (f) Bulk polymerizationat 608C, (6 )solutionpolymerization(in toluene)at 808CFig. 2. A plot of Ln ([M] 0/[M] t) versustime for the bulk poly-merizationof x-pentadecalactoneinitiated by yttrium isoprop-oxide at 608C with [M]/[I] = 150(mol/mol)1332 Z. Zhong,P. J.Dijkstra,J.Feijenno low molecularweight fraction detected. Theseresultsconfirmed thatall the isopropoxidegroupsof the initiatorhad a similar reactivity in the ring-openingpolymeriza-tion of x-pentadecalactone.The thermal properties of poly(x-pentadecalactone)werestudiedby differential scanningcalorimeter (DSC).For all samples,no obvious glasstransition wasdetectedwithin the temperature range studied (from –100 to1308C), even after rapid quenching from the melt. Inorderto getsome qualitative informationon thecrystalli-zationrate,DSCmeasurementswith variouscooling rateswere performed. The results (Fig. 5) displayeda largecrystallizationenthalpyin thecooling tracesandonly lit-tle differencebetweenthe melting temperature(Tm) andthe crystallization temperature (Tc) [(Tm–Tc) a 308C],indicatingthatpoly(x-pentadecalactone)hasa high crys-tallization rate.The equilibrium melting temperatureandenthalpyof non-defectivepoly(x-pentadecalactone)crys-talswerereportedas97.35 l 18C and232.95J/g,respec-Fig. 3. 300MHz 1H NMR spectraof poly(x-pentadecalactone) before (A)andafter(B) additionof excesstrifluoroaceticanhydrideFig. 4. Typical GPC profile of poly(x-pentadecalactone)(entry11 of Tab.1)Scheme1: Fig. 5. DSCcurvesof poly(x-pentadecalactone)Controlledring-openingpolymerization of x-pentadecalactone... 1333tively9). Here,anenthalpy between118.4and178.5 J/gata melting temperature rangeof 95 and1068C wasfound.Poly(x-pentadecalactone)s obtained in this study had adegreeof crystallinity in between50.8and76.6%.ConclusionsUsingx-pentadecalactoneasa modelmonomer, we stud-ied the ring-opening polymerization behavior of large-memberedlactoneswith yttrium isopropoxide conductedboth in bulk and in solution. Polymerization reactionsproceededquickly andin a controlledmanner. Themole-cularweightof theresulting polymerscould becontrolledby varying themonomer/initiator ratio.1) A. Hamitou, T. Ouhadi,R. Jérôme, Ph. Teyssie,J. Polym.Sci.Polym.Chem.Ed.15, 865 (1977)2) C. Song,X. Feng,Macromolecules17, 2764(1984)3) M. Endo,T. Aida, S. Inoue,Macromolecules20, 2982(1987)4) Z. Jedliński, W. Walach,P. Kurcok, G. Adamus,Makromol.Chem.192, 2051(1991)5) H. Yasuda, H. Yamamoto, K. Yokota, S. Miyake, A. Naka-mura,J. Am.Chem.Soc.114, 4908(1992)6) H. R. Kricheldorf,C. Boettcher, Makromol.Chem.194, 1665(1993)7) Y. Shen,Z. Shen,Y. Zhang, K. Yao, Macromolecules29,8289(1996)8) J. V. Koleske, R. D. Lundberg, J. Polym.Sci., Part A-2 10,323 (1972)9) B. Lebedev, A. Yevstropov, Makromol. Chem. 185, 1235(1984)10) P. Skoglund,Å. Fransson,Polymer39, 1899(1998)11) P. Skoglund,Å. Fransson,Polymer39, 3143(1998)12) R. Nomura, A. Ueno, T. Endo, Macromolecules 27, 620(1994)13) Z. Jedliński, M. Juzwa,G. Adamus,M. Kowalczuk, Macro-mol. Chem.Phys.197, 2923(1996)14) Y. Hori, H. Hongo, T. Hagiwara,Macromolecules32, 3537(1999)15) K. S. Bisht, L. A. Henderson,R. A. Gross,D. L. Kaplan, G.Swift, Macromolecules30, 2705(1997)16) H. Uyama,K. Takeya,N. Hoshi, S. Kobayashi, Macromole-cues28, 7046(1995)17) S. Kobayashi, H. Uyama, S. Namekawa, H. Hayakawa,Macromolecues31, 5655(1998)18) T. Ikeda,H. Sugimoto,S. Inoue,J. Macromol.Sci.Pure A34,1907(1997)19) Y. K. Choi,W. M. Stevels,M. J.K. Ankoné, P. J.Dijkstra, S.W. Kim, J.Feijen,Macromol.Chem.Phys.197, 3623(1996)20) V. Simic, N. Spassky, L. G. Hubert-Pfalzgraf, Macromole-cules30, 7338(1997)21) W. M. Stevels, M. J. K. Ankoné, P. J. Dijkstra, J. Feijen,Macromolecules29, 8296(1996)22) W. M. Stevels, M. J. K. Ankoné, P. J. Dijkstra, J. Feijen,Macromolecules29, 6132(1996)23) W. M. Stevels, M. J. K. Ankoné, P. J. Dijkstra, J. Feijen,Macromol.Chem.Phys.196, 1153(1995)24) B. Wurm, H. Keul, H. Höcker, Macromol.Chem.Phys.195,3489(1994)

Controlled ring-opening polymerization of w-pentadecalactone with yttrium isopropoxide as an initiator

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Controlled ring-opening polymerization of w-pentadecalactone with yttrium isopropoxide as an initiator

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