PolymerChemistry

COMMUNICATION

Publ

ishe

d on

03

Apr

il 20

14. D

ownl

oade

d by

Uni

vers

ity o

f Z

uric

h on

30/

05/2

014

14:0

3:17

.

View Article OnlineView Journal | View Issue

aPhysical Sciences and Engineering Divis

Synthesis Laboratory, King Abdullah Univer

Thuwal 23955, Saudi Arabia. E-mail: nikolabPhysical Sciences and Engineering Division

Technology (KAUST), Thuwal 23955, Saudi

† Electronic supplementary informationadditional 1H NMR and MALDI-TOF10.1039/c4py00429a

Cite this: Polym. Chem., 2014, 5, 3750

Received 25th March 2014Accepted 27th March 2014

DOI: 10.1039/c4py00429a

www.rsc.org/polymers

3750 | Polym. Chem., 2014, 5, 3750–37

Sequential polymerization of ethylene oxide, 3-caprolactone and L-lactide: a one-pot metal-freeroute to tri- and pentablock terpolymers†

Junpeng Zhao,a David Pahovnik,a Yves Gnanoub and Nikos Hadjichristidis*a

Metal-free polymerization of ethylene oxide (EO) catalyzed by a

relatively mild phosphazene base (t-BuP2) was proven feasible, which

enabled the one-pot sequential polymerization of EO, 3-caprolactone,

and L-lactide. Using either 3-phenyl-1-propanol or water as an initi-

ator, the corresponding triblock or pentablock terpolymers were easily

prepared.

Epoxides and cyclic esters share the common feature of gener-ating and maintaining an alkoxide/hydroxyl end group asgrowing species upon undergoing ring-opening polymerization(ROP). At rst glance, their sequential polymerization seemsfacile and the synthesis of the corresponding block copolymersappears straightforward. However, initiating/catalytic systemsthat work for epoxides,1 typically alkali alkoxides, provoke chaintransfer reactions (i.e. transesterication on polymer) in thecase of cyclic esters, and those that are appropriate for the latter,typically tin or aluminum alkoxides,2 are irrelevant for theformer. On this basis, the synthesis of polyether–polyester blockcopolymers is possible but tedious as it generally requiresmultiple steps of synthesis, isolation and purication, etc.3

Organic initiators and catalysts have been employed for thepolymerization of a wide range of monomer types.4 Some ofthem have been found to be effective for both epoxides andcyclic esters, for example, N-heterocyclic carbenes (NHCs).5

However, to our knowledge, no attempt has been made yet tosequentially polymerize epoxides and cyclic esters using NHCs.Phosphazenes, a family of Bronsted neutral superbases,6 areanother class of organic catalysts that have been found to besuitable for the polymerization of a number of cyclic

ion, KAUST Catalysis Center, Polymer

sity of Science and Technology (KAUST),

os.hadjichristidis@kaust.edu.sa

, King Abdullah University of Science and

Arabia

(ESI) available: Experimental details,spectra, and kinetic plots. See DOI:

53

monomers.7 Due to their high basicity, low nucleophilic char-acter, good solubility in a wide range of solvents, phosphazenebases are used to generate reactive anions and/or improve thenucleophilicity of the initiator/chain end by complexation withthe counterion (e.g. proton or lithium cation). Rapid andcontrolled anionic/pseudo-anionic polymerization ensues,provided the phosphazene base is adequately chosen and nelytuned to the monomer to be polymerized. For example, t-BuP4,one of the strongest phosphazene base, is well suited for thepolymerization of epoxides,8 but brings about extensive chaintransfer reactions in the case of cyclic esters and siloxanes.9

The purpose of this study is to investigate the feasibility ofusing one catalyst to sequentially polymerize ethylene oxide(EO), 3-caprolactone (CL) and L-lactide (LLA) in one-pot. Asmentioned above, t-BuP4 is not an ideal catalyst for cyclic esters.Therefore, a relatively mild phosphazene base, t-BuP2, is used inthe present study to prepare triblock and pentablock terpoly-mers consisting of poly(ethylene oxide) (PEO), poly(3-capro-lactone) (PCL) and poly(L-lactide) (PLLA).

3-Phenyl-1-propanol (PPA) was used as a representativemonohydroxyl initiator (Scheme 1a) with ca. 1 equiv. of t-BuP2 tocatalyze the polymerization of the three monomers (see ESI† forexperimental details). The products were analyzed by sizeexclusion chromatography (SEC) and nuclear magnetic reso-nance (NMR) spectroscopy. As PEO standards were used forcalibration, the molecular weights of PEO samples determinedby SEC (Mn,SEC) can be considered accurate. Matrix-assistedlaser desorption/ionization time of ight mass spectrometry(MALDI-TOF MS) was also used for the analysis of PEO. Aerheating at 50 �C for 3 days, the obtained PEO shows a Mn,SEC of4600 g mol�1 and a low polydispersity index (Mw/Mn ¼ 1.03).Compared to the molecular weight calculated from the feedratio (Mn,theo ¼ 5000 g mol�1), this indicates an EO conversionof 92%.10 On the le side of the main RI signal, a small peak isseen which represents ca. 1.5% of the total peak area andexhibits a doubled molecular weight (Fig. 1, le). However,there is no corresponding UV signal for this peak (Fig. 1, right),indicating that the byproduct formed may well be due to the

This journal is © The Royal Society of Chemistry 2014

Fig. 1 SEC traces of the aliquots withdrawn from the one-potsequential polymerization of ethylene oxide (blue), 3-caprolactone(green) and L-lactide (orange) using 3-phenyl-1-propanol as an initi-ator in the presence of t-BuP2.

Fig. 2 Conversion vs. time plots of 3-caprolactone (green) and L-lactide (orange) in their one-pot sequential polymerization withethylene oxide using 3-phenyl-1-propanol as an initiator in the pres-ence of t-BuP2.

Scheme 1 Schematic illustration of one-pot sequential polymeriza-tion of ethylene oxide, 3-caprolactone and L-lactide catalysed byt-BuP2 toward (a) the triblock terpolymer using 3-phenyl-1-propanolas an initiator and (b) the pentablock terpolymer using water as aninitiator.

Fig. 3 Upper figure: 1H NMR spectra of PEO (blue), PEO-b-PCL(green) and PEO-b-PCL-b-PLLA (orange) prepared from one-potsequential polymerization of the three monomers using 3-phenyl-1-propanol as an initiator in the presence of t-BuP2; lower figure:magnified areas showing signals from the end groups and monomericunits linking different blocks.

Communication Polymer Chemistry

Publ

ishe

d on

03

Apr

il 20

14. D

ownl

oade

d by

Uni

vers

ity o

f Z

uric

h on

30/

05/2

014

14:0

3:17

. View Article Online

presence of traces of impurity acting as a difunctional initiator.This actually inspired us to use water as an initiator for thepreparation of the pentablock terpolymer, as will be discussedlater. The 1H NMR spectrum of the isolated PEO sample showsall the characteristic signals of both PEO and PPA moieties(Fig. S1†). The molecular weight calculated from the peakintegrals (Mn,NMR ¼ 4600 g mol�1) ts well with Mn,SEC. MALDI-TOF MS also conrms the molecular characteristics of the PEO(Fig. S2†).

CL was added to the reaction mixture containing PEO andt-BuP2 at room temperature to grow the PCL block. Aliquotswere withdrawn for SEC and 1H NMR characterization. SECanalysis shows a steady increase in molecular weight and a lowdispersity (Fig. 1), indicating the formation of a well-denedPEO-b-PCL diblock copolymer from the PEO precursor. Corre-spondingly, 1H NMR characterization shows a continuouslyincreasing CL conversion (Fig. 2), which reaches 60% aer 17 h

This journal is © The Royal Society of Chemistry 2014

of reaction. An aliquot was withdrawn and the isolated PEO-b-PCL diblock copolymer was characterized by 1H NMR spectrawhich exhibited all the characteristic signals of PPA, PEO andPCL (Fig. 3). The presence of residual EO seemed not to affectthe growth of the PCL block.

To the remaining reaction mixture containing PEO-b-PCLand t-BuP2, LLA was introduced as a THF solution. The analysisof the withdrawn aliquots shows that nearly complete conver-sion of LLA is reached in 10 min (the rst aliquot), aer whichthe SEC traces and 1H NMR spectra (monomer conversion)

Polym. Chem., 2014, 5, 3750–3753 | 3751

Polymer Chemistry Communication

Publ

ishe

d on

03

Apr

il 20

14. D

ownl

oade

d by

Uni

vers

ity o

f Z

uric

h on

30/

05/2

014

14:0

3:17

. View Article Online

remain unchanged (Fig. 1 and 2). The reaction was quenchedaer 24 h. The isolated nal product shows, in its 1H NMRspectrum, all the expected characteristic signals with ttingintegrals from the three blocks (Fig. 3, upper), from the endgroups, and from monomeric units linking different blocks aswell (Fig. 3, lower).

LLA is known to polymerize much faster than CL,11 whichenables the achievement of pure PLLA blocks even in thepresence of residual CL monomers. Nevertheless, it wassurprising to see that aer the complete conversion of LLA, theresidual CL does not polymerize from the hydroxyl end groupsof the PLLA blocks (Fig. 2). The reason is still under investiga-tion, however, this can be considered and utilized as anadvantage of this synthetic system, as failure to quench thereaction in time does not result in transesterication (andconsequently increased polydispersity) or undesired furtherpolymerization of residual monomers.

Water was then used as a difunctional initiator to preparethe PLLA-b-PCL-b-PEO-b-PCL-b-PLLA pentablock terpolymer(Scheme 1b). The PEO obtained in this case shows a tailing in itsSEC peak (Fig. 4), indicating a slow initiation caused by poornucleophilicity of water compared to a primary alcohol. A lowermolecular weight (Mn,SEC ¼ 3500 g mol�1) and a higher poly-dispersity index (Mw/Mn ¼ 1.12) were consequently obtained.12

Mn,SEC and Mn,theo indicate a ca. 70% EO conversion, however,the following polymerization reactions of CL and LLA appear tobe unaffected by the residual EO.

The MALDI-TOF spectrum conrms the telechelic dihydroxyfeature of this PEO (Fig. S2†). As shown by the SEC traces (Fig. 4)and monomer conversion plots (Fig. S3†), the polymerizationreactions of CL and LLA proceed similarly to that of the triblockterpolymer, i.e., steady and continuous growth of the PCLblocks with 72% of CL conversion reached at 22 h, nearly

Fig. 4 SEC traces given by the aliquots withdrawn from the one-potsequential polymerization of ethylene oxide (blue), 3-caprolactone(green) and L-lactide (orange) using water as an initiator in the pres-ence of t-BuP2.

3752 | Polym. Chem., 2014, 5, 3750–3753

instant completion of LLA polymerization followed by anunchanged reaction mixture. 1H NMR spectra of the isolatedproducts present all the characteristic signals and tting inte-grals (Fig. S4†), which further conrms the formation of theCBABC type pentablock terpolymer.

Conclusions

One-pot sequential polymerization of EO, CL and LLA has beenproven feasible with a mild phosphazene base (t-BuP2) as thecommon catalyst. Triblock and pentablock terpolymers can bereadily obtained using a monohydroxyl compound or water asan initiator. The polymerization of EO, which takes a long timein this case, needs to be conducted rst because if PCL or PLLAwere used as an initiator for EO, inter/intra-macromoleculartransesterication reaction would occur extensively duringthe polymerization of EO, which would lead to poorly denedproducts. CL needs to be polymerized before LLA due tomuch lower reactivity and the fact that it cannot be initiatedby the end group of PLLA in this case. Therefore, it is essentialto follow the order of EO-CL-LLA. This study adds to theapplication of phosphazene bases in polymer synthesis andmacromolecular engineering based on organocatalyticpolymerization.

Notes and references

1 A.-L. Brocas, C. Mantzaridis, D. Tunc and S. Carlotti, Prog.Polym. Sci., 2013, 38, 845.

2 (a) O. Dechy-Cabaret, B. Martin-Vaca and D. Bourissou,Chem. Rev., 2004, 104, 6147; (b) O. Coulembier, P. Degee,J. L. Hedrick and P. Dubois, Prog. Polym. Sci., 2006, 31,723; (c) A. P. Dove, Chem. Commun., 2008, 6446; (d)M. Labet andW. Thielemans, Chem. Soc. Rev., 2009, 38, 3484.

3 N. Hadjichristidis, S. Pispas and G. Floudas, BlockCopolymers, John Wiley & Sons, Hoboken, 2003.

4 (a) N. E. Kamber, W. Jeong, R. M. Waymouth, R. C. Pratt,B. G. G. Lohmeijer and J. L. Hedrick, Chem. Rev., 2007,107, 5813; (b) M. K. Kiesewetter, E. J. Shin, J. L. Hedrickand R. M. Waymouth, Macromolecules, 2010, 43, 2093; (c)A. P. Dove, ACS Macro Lett., 2012, 1, 1409; (d) M. Fevre,J. Pinaud, Y. Gnanou, J. Vignolle and D. Taton, Chem. Soc.Rev., 2013, 42, 2142.

5 (a) E. F. Connor, G. W. Nyce, M. Myers, A. Mock andJ. L. Hedrick, J. Am. Chem. Soc., 2002, 124, 914; (b)A. P. Dove, R. C. Pratt, B. G. G. Lohmeijer, D. A. Culkin,E. C. Hagberg, G. W. Nyce, R. M. Waymouth andJ. L. Hedrick, Polymer, 2006, 47, 4018; (c) O. Coulembier,M. K. Kiesewetter, A. Mason, P. Dubois, J. L. Hedrick andR. M. Waymouth, Angew. Chem., Int. Ed., 2007, 46, 4719; (d)L. Guo and D. Zhang, J. Am. Chem. Soc., 2009, 131, 18072;(e) J. Raynaud, C. Absalon, Y. Gnanou and D. Taton, J. Am.Chem. Soc., 2009, 131, 3201; (f) J. Raynaud, W. N. Ottou,Y. Gnanou and D. Taton, Chem. Commun., 2010, 46, 3203;(g) E. J. Shin, H. A. Brown, S. Gonzalez, W. Jeong,J. L. Hedrick and R. M. Waymouth, Angew. Chem., Int. Ed.,2011, 50, 6388.

This journal is © The Royal Society of Chemistry 2014

Communication Polymer Chemistry

Publ

ishe

d on

03

Apr

il 20

14. D

ownl

oade

d by

Uni

vers

ity o

f Z

uric

h on

30/

05/2

014

14:0

3:17

. View Article Online

6 (a) R. Schwesinger and H. Schlemper, Angew. Chem., Int. Ed.,1987, 26, 1167; (b) R. Schwesinger, C. Hasenfratz,H. Schlemper, L. Walz, E.-M. Peters, K. Peters andH. G. von Schnering, Angew. Chem., Int. Ed., 1993, 32, 1361;(c) R. Schwesinger, J. Willaredt, H. Schlemper, M. Keller,D. Schmitt and H. Fritz, Chem. Ber., 1994, 127, 2435.

7 (a) S. Boileau and N. Illy, Prog. Polym. Sci., 2011, 36, 1132; (b)J. Zhao, N. Hadjichristidis and Y. Gnanou, Polimery, 2014, 59,49.

8 (a) B. Esswein andM.Moller, Angew. Chem., Int. Ed., 1996, 35,623; (b) B. Eßwein, N. M. Steidl and M. Moller, Macromol.Rapid Commun., 1996, 17, 143; (c) H. Schlaad, H. Kukula,J. Rudloff and I. Below, Macromolecules, 2001, 34, 4302; (d)H. Misaka, R. Sakai, T. Satoh and T. Kakuchi,Macromolecules, 2011, 44, 9099; (e) J. Zhao and H. Schlaad,Macromolecules, 2011, 44, 5861; (f) H. Misaka, E. Tamura,K. Makiguchi, K. Kamoshida, R. Sakai, T. Satoh andT. Kakuchi, J. Polym. Sci., Part A: Polym. Chem., 2012, 50,1941; (g) T. Isono, K. Kamoshida, Y. Satoh, T. Takaoka,

This journal is © The Royal Society of Chemistry 2014

S.-i. Sato, T. Satoh and T. Kakuchi, Macromolecules, 2013,46, 3841; (h) J. Zhao, H. Alamri and N. Hadjichristidis,Chem. Commun., 2013, 49, 7079; (i) J. Zhao, D. Pahovnik,Y. Gnanou and N. Hadjichristidis, Macromolecules, 2014,47, 1693.

9 (a) A. Molenberg and M. Moller, Macromol. Rapid Commun.,1995, 16, 449; (b) H. Yang, J. Xu, S. Pispas and G. Zhang,Macromolecules, 2012, 45, 3312.

10 Another experiment was done at 35 �C for 3 days. Only 20%EO conversion was achieved as indicated by the theoreticaland obtained molecular weights.

11 (a) L. Zhang, F. Nederberg, J. M. Messman, R. C. Pratt,J. L. Hedrick and C. G. Wade, J. Am. Chem. Soc., 2007, 129,12610; (b) L. Zhang, F. Nederberg, R. C. Pratt,R. M. Waymouth, J. L. Hedrick and C. G. Wade,Macromolecules, 2007, 40, 4154.

12 Attempt was made to polymerize propylene oxide with waterand t-BuP2. No polymer was formed aer heating at 50 �C for3 days.

Polym. Chem., 2014, 5, 3750–3753 | 3753