Benzyl Potassium: An Efficient One-Pot Initiator for the Synthesis of Block Co- and Terpolymers of Ethylenoxide

Transcription

1 RAPID COMMUNICATION Benzyl Potassium: An Efficient One-Pot Initiator for the Synthesis of Block Co- and Terpolymers of Ethylenoxide NIKOS EKIZOGLOU, NIKOS HADJICHRISTIDIS Department of Chemistry, University of Athens, Panepistimiopolis Zografou, Athens, Greece Received 4January 2001; accepted 2February 2001 Published online 00 Month 2001 Keywords: benzyl potassium; anionic polymerization; polystyrene-b-polyethylenoxide; polyisoprene-b-poly(2-vinyl pyridine)-b-polyethylenoxide INTRODUCTION Block copolymers are fascinating materials because of their ability to phase-separate into welldefined periodic microstructures. 1 Among them, the amphiphilic block copolymers of polyethylenoxide (PEO) (a hydrophilic, nonionic, crystallinepolymer)arepromisingmaterialsforbiomedical and pharmaceutical applications. 2 Consequently, the synthesis of model block copolymers of PEO with different structural designs is very interesting. Unfortunately, the convenient lithium-based, in weak- or medium-polarity solvents, anionic polymerization is not suitable for ethylenoxide. The very strong lithium oxygen bond, produced during the initiation of ethylenoxide by the previous living block, forms ion pairs that are not able to polymerize ethylenoxide. 3 The following methodologies have been developed to overcome this problem: 1. The use of astrong polar solvent (e.g., dimethylsulfoxide)withpotassiumalkoxides 4 orlithiumcomplexingagents(e.g.,phosphazene bases). 5 These compounds solvate the Correspondence to: N. Hadjichristidis ( hadjichristidis@ chem.uoa.gr) JournalofPolymerScience:PartA:PolymerChemistry,Vol.39, (2001) 2001 John Wiley &Sons, Inc. lithium, thus pushing the equilibrium toward the free anions, which are capable of polymerizing ethylenoxide. 2. The replacement of the lithium counterion with potassium, which in contrary to lithiumissolvatedeveninmedium-polaritysolvents, such as tetrahydrofuran (THF) The use of apotassium initiator from the beginning. 7,8 The first methodology leads to block copolymers contaminated by PEO homopolymer, possiblybecausemorethanoneinitiatorspeciesforthe polymerization of ethylenoxide are present in the reaction solution. 4 Inthe case of t-bup 4 phosphazene base, no homopolymer PEO byproduct was observed; unfortunately because of equilibrium reasons, the ethylenoxide polymerization yield was 75%. 5 The second methodology is atwo-step procedure but the only one for the synthesis of block copolymers of PEO with polyolefins. 6 For the third methodology, cumyl potassium 7 (CK) and diphenyl methyl potassium 8 (DPMK) have been used as monofunctional initiators. The copolymers obtained by CK are contaminated by PEO homopolymer as aresult of the presence of CH 3 OKproducedduringthesynthesisofCKfrom cumyl methyl ether and potassium. On the other hand, because of its high reactivity, CK gives side products that contaminate the block copolymers. DPMK,aninitiatorwithlowreactivityasaresult 1198

2 RAPID COMMUNICATION 1199 Table I. Results from Molecular Characterization Sample M n 10 3 (Calculated) a Molecular Weight ( 10 3 ) Composition (w/w) PEO Low PEO High b PS PEO c PS 54% PI P2VP PEO d PI 40% P2VP 29% a Molecular weights calculated by the ratio [M]/[I], where [M] is the monomer and [I] the initiator concentrations. b M n determined by SEC calibrated with PEO standards (Toyo Soda Manufacturing Co.). c M w determined by LALLS in THF. d M n determined by MO in toluene at 47 C. of the delocalization of the negative charge in the two phenyl rings, cannot efficiently initiate the polymerization of styrene or dienic monomers, thus leading to block copolymers with high polydispersity. Benzyl potassium is an efficient initiator for almost all common monomers, 9 and its reactivity lies between CK and DPMK. Consequently, it seemed logical to evaluate its efficiency for the polymerization of ethylenoxide. The block co- and terpolymers obtained were characterized by size exclusion chromatography (SEC), NMR spectroscopy, membrane osmometry (MO), and low-angle laser light scattering (LALLS). EXPERIMENTAL The purification of the monomers (isoprene and styrene), the solvents (benzene and THF), and the terminating agent methanol (to the high purity required for anionic polymerization) have been described in detail elsewhere Vinyl pyridine was first purified with CaH 2, then twice distilled over sodium mirror, and finally distilled over trioktyl aluminum. Ethylenoxide was purified with CaH 2 and then twice distilled over n-buli. s-buli was prepared from s-bucl as well as a lithium dispersion. t-buok (solution 1 M in THF, Aldrich) was used as received. All manipulations were performed under high vacuum in glass reactors. The addition of the reagents was made through break-seals. The apparatus for the preparation of the initiator was previously washed with n-buli benzene solution followed by rinsing with benzene. The apparatuses for all the polymerizations were washed with THF solution of n-buli and 1,1-diphenyl ethylene (Aldrich) in a 5/1 ratio, followed by rinsing with THF, which was the solvent for all polymerizations. The molecular weight distribution (I M w /M n ) of the final products was checked by SEC operating at 30 C in THF and triethylamine (1% v/v), with a differential refractive index and UV detector as well as three Phenomenex columns (continuous porosity ranged from Å). The use of triethylamine is needed to avoid absorption of PEO in the columns. 11 The content of each block in the final product was determined by 1 H NMR spectroscopy in CDCl 3 at 30 C with a Varian Mercury instrument at 200 MHz. The weight average molecular weight (M w ) was obtained by static light scattering measurements using a Chromatix KMX-6 (6 7 ) LALLS photometer operating at a wavelength of 633 nm. The number-average molecular weight (M n ) was obtained by MO using a Jupiter Instruments Co. osmometer at 47 C in toluene with a cellulose acetate membrane.the results of the molecular characterization are summarized in Table I. RESULTS AND DISCUSSION The metallation of toluene for the preparation of benzyl potassium was accomplished via the use of a super base. The super base was produced by the metal-exchange reaction between s-buli 12 and t-buok 13 according to the following reactions: s-buli t-buok 3 s-buk t-buoli (1) s-buk C 6 H 5 CH 3 3 C 6 H 5 CH 2 K BuH (2)

3 1200 J. POLYM. SCI. PART A: POLYM. CHEM.: VOL. 39 (2001) obtain a homogenous mixture of benzyl potassium and t-buoli, producing polystyrene (PS) with a low polydispersity. 14 We have chosen a 1/1 molar ratio of s-buli to t-buok as well as 4 h at room temperature. Toluene was used as the metallated reagent as well as the reaction solvent to avoid complications coming from an extra solvent. Figure 1. SEC of PEO obtained by benzyl potassium. Synthesis of PEO Homopolymer A series of seven PEO homopolymers ranging from ,000 was prepared with benzyl potassium as the initiator. The polymerizations were carried out in THF for 5dat50 C.The representative SEC chromatograph given in Figure 1 shows no byproducts. The low polydispersity usually obtained in anionic polymerization was also observed (I 1.05) in all cases. In Table I, only the lowest and highest molecular weight samples are provided. The formation of the exceptionally strong oxygen lithium bond provides the driving force to obtain the super base. The ratios of s-buli and t-buok as well as the temperature and reaction time should be taken into account to metallate only the methyl rather than the phenyl group of toluene. Previous studies concerning the synthesis of benzyl potassium have led to the following results: Synthesis of PS-b-PEO Copolymer A diblock copolymer of PS PEO was prepared with the same initiator. The polymerization of styrene was carried out in THF at 78 C for 1 h, 1. A 1/3 molar ratio of alkilithium to potassium alkoxide gave the best results The yield of the metallation increased with increasing temperature, and it was not changed appreciably in higher temperatures ( 55% at 78 C, 76% at 48 C, 84% at 20 C, and 81% at 65 C) The maximum degree of metallation (completion of the reaction) was reached within 2 min, and no further metallation was observed in the subsequent min. 14 Taking into account the aforementioned results and considering the following findings: 1. An excess of potassium alkoxide can initiate the polymerization of ethylenoxide, thus leading to copolymers with high polydispersities (two active centers) At least 2 h of extra time were needed to SEC of PS-b-PEO produced by benzyl po- Figure 2. tassium.

4 RAPID COMMUNICATION 1201 followed by distillation of ethylenoxide, and then raising the temperature up to 50 C for 5 d.the color of the solution was changed from the deep red of the PS anion to the colorless anion of ethylenoxide. From the SEC chromatograph in Figure 2, it is obvious there are no traces of homopolymers, and the polydispersity is low as expected for anionic polymerization (I 1.06). Unfortunately, a sample of the PS block could not be extracted from the solution with flame cutting because heating may cause pyrolysis of THF to products that can neutralize some active centers. The 1 H NMR spectra of the final product show the distinguish broad peaks of PS at 7.2 and 6.8 ppm as well as the single peak of PEO at 3.7 ppm. Synthesis of PI-b-P2VP-b-PEO Copolymer Figure 3. SEC of PI-b-P2VP-b-PEO produced by benzyl potassium. A triblock terpolymer of polyisoprene (PI) poly(2- vinyl pyridine) (P2VP) PEO was synthesized with the initiation of benzyl potassium as well as the sequential addition of monomers. The polymerization of I s and 2VP was carried out in THF at 78 C. The reaction time was 1 h for each monomer. The change of the yellow color of the PI anion to the red color of P2VP is an indication of the polymerization of the consequent monomers. Ethylenoxide was distilled into the solution, and after the color disappeared, the temperature was increased up to 50 C for 5 d. The SEC chromatograph in Figure 3 illustrates a single narrow peak (I 1.04) free of byproducts. Nevertheless for the Figure 4. 1 H NMR spectrum of PI-b-P2VP-b-PEO prepared with benzyl potassium showing the characteristic peak of PEO (3.7 ppm) and the characteristic peaks of PI 1,2 (5.7 ppm) and PI 3,4 (4.5 5 ppm) as well as the broad peaks of P2VP ( , 7 7.2, 6.6 7, and ppm).

5 1202 J. POLYM. SCI. PART A: POLYM. CHEM.: VOL. 39 (2001) reasons mentioned previously, samples during the different steps of the reaction could not be withdrawn. The 1 H NMR spectra (Fig. 4) ofthe final product exhibit the characteristic peak of PEO (3.7 ppm) and the characteristic peaks of PI 1,2 (5.7 ppm) and PI 3,4 (4.5 5 ppm) as well as the broad peaks of P2VP ( , 7 7.2, 6.6 7, and ppm). CONCLUSION Benzyl potassium is an efficient one-pot monofunctional initiator for the synthesis of block coand terpolymers of ethylenoxide. In a forthcoming article, the synthesis of more complicated structures based on PEO will be explained. The authors acknowledge Prof. J. W. Mays for the helpful discussions. The financial support of the Ministry of Education through the Operational Program and Initial Educational Vocational Training on Polymer Science and its Applications is also greatly appreciated. REFERENCES AND NOTES 1. Lohse, D. J.; Hadjichristidis, N. Curr Opin Colloid Interface Sci 1997, 2, Cammas, S.; Kataoka, K. In Solvents and Self- Organization of Polymers, NATO ASI Series, Series E Applied Sciences; Kluwer Academic Publishers: Norwell, MA, 1996; Vol. 327, p Quirk, R. P.; Ma, J.-J. J Polym Sci Part A: Polym Chem 1988, 26, Quirk, R. P.; Kim, J.; Kausch, C.; Chun, M. Polym Int 1996, 39, Esswein, B.; Möller, M. Angew Chem Int Ed Engl 1996, 35, Hillmyer, M. A.; Bates, F. S. Macromolecules 1996, 29, Hruska, Z.; Hurtez, G.; Walter, G. S.; Riess, G. Polymer 1992, 33, Candau, F.; Afchar-Taromi, F.; Rempp, P. Polymer 1977, 18, Rempp, P.; Mervill, E. W. Polymer Synthesis, 2nd ed.; Hüthing & Wepf: 1994; Chapter 5, p Hadjichristidis, N.; Iatrou, H.; Pispas, S.; Pitsikalis, M. J Polym Sci Part A: Polym Chem 2000, 38, Reuter, H.; Höring, S.; Ulrbricht, J. Eur Polym J 1989, 25, Steinke, J. H. G.; Haque, S. A.; Fréchet, J. M. J.; Wang, H. C. Macromolecules 1996, 29, Lochmann, L.; Petránek, J. Tetrahedron Lett 1991, 32, Bucca, D.; Gordon, B., III Polym Prepr (Am Chem Soc Div Polym Chem) 1990, 31, 509.