Introduction. Durairaj Baskaran a, Axel H. E. Müller* b

Size: px

Start display at page:

Download "Introduction. Durairaj Baskaran a, Axel H. E. Müller* b"

Kory O’Neal’
5 years ago
Views:

1 390 Macromol. Rapid Commun. 21, (2000) Communication: A novel metal-free initiator, i.e. the salt of the tetrakis[tris(dimethylamino)phosphoranylidenamino]phosphonium (P 5+ ) cation with the 1,1-diphenylhexyl (DPH ) anion was prepared by cation metathesis. It initiates a very fast and controlled anionic polymerization of methyl methacrylate in THF. Kinetic investigations between 20 and +208C using a flow tube reactor provide nearly linear first-order time-conversion plots with halflives below 0.1 s, a linear dependence of the number-average degree of polymerization, and rather narrow molecular weight distributions (M w/m n L 1.2). C NMR measurements on a model of the active chain end (the P 5+ salt of ethyl isobutyrate) in THF-d 8 show 15 and 25 ppm upfield shifts of the a-carbon compared to the dimers and tetramers of the lithium ester enolate, respectively, indicating a non-aggregated structure and an increased charge density on the a-carbon. Anionic polymerization of methyl methacrylate using tetrakis[tris(dimethylamino)phosphoranylidenamino] phosphonium (P 5+ ) as counterion in tetrahydrofuran Durairaj Baskaran a, Axel H. E. Müller* b Institut für Physikalische Chemie, Universität Mainz, D Mainz, Germany (Received: October 22, 1999; revised: December 6, 1999) Introduction a b Present address: National Chemical Laboratory, Division of Polymer Chemistry, Pune , India New address: Universität Bayreuth, Makromolekulare Chemie II, D Bayreuth, Germany axel.mueller@uni-bayreuth.de The anionic polymerization of alkyl (meth)acrylates has been intensively studied in recent years. The nature of the counterion, their interaction with carbonyl groups as well as the association of ion pairs play a significant role in synthesizing well-defined poly(meth)acrylates 1, 2). The intramolecular solvation of the metal cation by the carbonyl group during propagation facilitates termination by an intramolecular Claisen condensation ( back-biting ) 3, 4) and slow equilibria between aggregated and nonaggregated ion pairs lead to broad molecular weight distributions 2, 5). Several new strategies have been used to circumvent the problem of termination and aggregation. One possibility is the use of various ligands in conjunction with classical anionic initiators. r-ligands, such as crown ethers 6), cryptands 7, 8), tertiary amines 9, 10) ; l-ligands, such as alkali alkoxides 11, 12) halides 2, ), perchlorates 14, 15), aluminum alkyls 16 19), and r,l-ligands, such as alkoxyalkoxides 20 23), aminoalkoxides 24), and silanolates 25 27) have been used with much success. Another possibility is the exchange of the counterion of the ester enolates which are the propagating species. Using silyl ketene acetals (silyl ester enolates) as initiators in presence of nucleophilic or Lewis acid catalysts, group transfer polymerization (GTP) paved a way for the living polymerization of methacrylates at ambient temperature 28). Although the mechanism is still under dispute, there is some indication that the active species in the presence of nucleophilic catalysts is an ester enolate with the metalfree counterion of the catalyst (e. g., tris[dimethylamino]sulfonium, TAS +, or tetrabutylammonium, TBA + ) 29). Reetz et al. 30) used salts of various metal-free cations, such as TBA + and hexamethylguanidinium, with thiolate, malonate, carbazolide, and other anions of weak C1H acids as initiators for the acrylate polymerization. Kinetic Macromol. Rapid Commun. 21, No. 7 i WILEY-VCH Verlag GmbH, D Weinheim /2000/ $ /0

2 Anionic polymerization of methyl methacrylate using studies show that the polymerization of n-butyl acrylate initiated by TBA + malonate proceeds with considerable induction periods and exhibits very poor initiator efficiencies and broad MWD s 31). Bidinger and Quirk 32) used TBA + 9-methylfluorenide at low concentration as an initiator for the polymerization of MMA at ambient temperature resulting in polymers with rather broad MWD. Bandermann et al. 33) also reported induction periods and broad MWD s as well as the presence of residual initiator in MMA polymerization using TBA malonate as initiator. They also found evidence for Hofmann elimination as a termination reaction. Recent studies of MMA polymerization using the TBA + salts of 9-ethylfluorene and 1,1- diphenylhexane as initiators show that initiator efficiency decreases with the ionic radius of the counterion and they indicate that initiation is in fact an equilibrium process 34). Zagala and Hogen-Esch 35) successfully used the tetraphenylphosphonium (TPP + ) counterion in the polymerization of MMA and obtained polymers with narrow molecular weight distribution at ambient temperature. Our kinetic studies 36) supplemented by NMR evidence 37) revealed that TPP + ion pairs are in a dynamic equlibrium with dormant ylides. Pietzonka and Seebach 38) reported the use of the phosphazene base tert-butyl-p 4 to deprotonate ethyl acetate and the successful use of this initiator for the polymerization of MMA to polymers with narrow MWD. However, their initiator efficiencies were quite low. Börner and Heitz 39) showed that t-bu-p 4 is not strong enough to deprotonate acetates quantitatively. At 50 8C only 11% of ethyl acetate and 5% of methyl isobutyrate were deprotonated. Consequently, very low efficiencies with methyl isobutyrate/t-bu-p 4 as an initiator for MMA polymerization are obtained 40). Börner and Heitz used the the same system to initiate the polymerization of n-butyl acrylate. Their results indicate various side reactions, especially the formation of P 4+ butoxide, attributed to a direct abstraction from the monomer leading to an alleneketene 39). Möller et al. 41, 42) used t-bu-p 4 to deprotonate alcohols and to subsequently initiate the fast anionic ringopening polymerization of ethylene oxide and octamethylcyclotetrasiloxane. They also showed that addition of t-bu-p 4 to butyllithium enhances the anionic polymerization of ethylene oxide 43). We now wish to present results of a kinetic investigation on the anionic polymerization of MMA in THF between 208C and +208C using a new metal-free initiator, namely the salt of the tetrakis[tris(dimethylamino)- phosphoranylidenamino]phosphonium (P 5+ ) cation with the 1,1-diphenylhexyl (DPH ) anion (see formula). This initiator is easily and quantitatively obtained by a metathesis reaction between P 5+ chloride and 1,1-diphenylhexyllithium. Experimental part Materials Methyl methacrylate (MMA, Röhm GmbH) was stirred over CaH 2 for 12 h and then fractionated in presence of the nonvolatile stabilizer Irganox 1010 (Ciba-Geigy) under reduced pressure over a 1 m column filled with Sulzer packing. The distillate was collected over fresh CaH 2, degassed and stirred for 12 h at 08C and stored at 208C. This pre-purified MMA was recondensed just before polymerization. No impurities were detectable by GC. Diphenylethylene (DPE, Aldrich) was titrated with a small amount of BuLi and distilled under high vacuum. Ethyl a-lithioisobutyrate (EIBLi) was synthesized acording to Lochmann 44). Octane was distilled over Na-K alloy and used as internal standard for GC. Fractionated THF was purified by refluxing over K metal and stored over fresh Na-K alloy on the vacuum line. Initiators Butyllithium (BuLi, 1.6 M in hexane, Aldrich) was used after determining its concentration by Gilman double titration. 1,1-Diphenylhexyllithium (DPHLi) was prepared by the reaction of a known amount of BuLi with a slight excess of diphenylethylene (DPE) in hexane at room temperature. DPHLi separated as a red precipitate and was washed twice with dry hexane and dried under vacuum for 4 h. It was characterized using NMR spectroscopy in THF-d 8 : 1 H NMR (THF-d 8 ): d = 0.91 (CH 3 1CH 2 1); 1.33 (m, 1CH 2 1); 2.32 (m, 1CH 2 1C1, anion); 5.55 (t, para-aryl); 6.44 (t, meta-aryl); 6.83 (d, ortho-aryl). C NMR: (CH 3 1CH 2 1); 23.87, 28.02, 33.68, (1CH 2 1); (1C1, anion); (para-aryl); (ortho-aryl); (meta-aryl); (quaternary-aryl). Tetrakis[tris(dimethylamino)phosphoranylidenamino]phosphonium chloride (P 5+ Cl, Fluka) was dried under high vacuum for 14 h at 110 8C. 1,1-Diphenylhexyltetrakis[tris(dimethylamino)phosphoranylidenamino]phosphonium (DPHP 5 ) was prepared in situ in THF at 208C by cation exchange reaction of DPHLi with P 5+ Cl. 1 H and C NMR (Fig. 1) in THF-d 8 showed that this initiator is stable for weeks in a refrigerator. 1 H NMR (THF-d 8 ): d = 0.91 (CH 3 1CH 2 1); 1.34 (m, 1CH 2 1); 2.35 (m, 1CH 2 1C1, anion); 2.60 and 2.62 (d, P1N(CH 3 ) 2 ); 5.51 (t, para-aryl); 6.41 (t, meta-aryl); 6.85 (d, ortho-aryl). C NMR (THF-d 8 ): d = (CH 3 1CH 2 1); 23.96, 28.18, 33.76, (1CH 2 1); and 37.45

3 392 D. Baskaran, A. H. E. Müller Fig. 1. C NMR spectrum of DPHP 5 in THF-d 8 (1P1N(CH 3 ) 2 ); (1C1, anion); (para-aryl); (ortho-aryl); (meta-aryl); (quaternaryaryl). Similarly, tetrakis[tris(dimethylamino)phosphoranylidenamino]phosphonium enolate (EIBP 5 ) was synthesized by reacting the lithium enolate with little excess P 5+ Cl in THF at 208C and the excess salt was filtered off. C NMR, see Tab. 2. The stability of EIBP 5 is found to be rather poor at room temparature as evidenced from a new signal arising at the expense of the intensity of the a-carbon and carbonyl signal with time. These new signals may correspond to a product resulting from a Claisen condensation between two EIBP 5 molecules. Kinetic experiments All experiments were carried out in a specially designed flow-tube reactor with a mixing time of a1 ms and reaction times of 4 ms f s f 0.6 s 23, 36, 45). The effective temperature, T eff, of each experiment was determined using Eq. (1) 46) T eff = T m DT (1) where T m is the temperature of the mixing jet and DT is the temperature rise between mixing and quenching jets due to the exothermicity of the polymerization. Monomer conversion was determined by GC using octane as internal standard. After evaporation of the solvent, the polymer was dissolved in benzene, filtered and freeze-dried. Molecular weights and MWD s were determined using GPC equipped with two UV detectors with a variable wavelength, an RI detector, and two 60 cm 5 l PSS SDV-gel columns, Å, 16linear: Å, with THF as eluent at room temperature. The calibration was performed using PMMA standards. Results and discussion The results of kinetic experiments of the anionic polymerization of MMA using DPHP 5 as an initiator at 20, 0, and +20 8C are summarized in Tab. 1. The polymerizations were performed at the initial initiator concentration [I] 0 = 1.4 N 10 3 mol/l. The reaction is extremely fast with half-lives ranging between 0.05 and 0.15 s depending on the temperature. The semi-logarithmic time-conversion plots (Fig. 2) show a very slight downward curvature, indicating fast initiation and very little termination, presumably back-biting. However, at all temperatures A98% conversion is reached. Rate constants of propagation, k p, were obtained by dividing the initial slope of the first-order plot (apparent rate constant, k app ) by the concentration of active centers, [P*] (see below); these results are given in Tab. 1. The values obtained are extremely high and will be discussed further below. The linearity of the plot of the number-average degree of polymerization, P n, versus conversion, x p, shows the absence of transfer reactions or slow initiation during the polymerization (Fig. 3). However, the obtained P n values strongly deviate from the theoretical line (P n;th = [M] 0 x p / [I] 0 ) indicating a low initiator efficiency, f = 0.2, indepen-

4 Anionic polymerization of methyl methacrylate using Tab. 1. Kinetic results of MMA polymerization initiated by DPHP 5 in THF. [M] 0 = 0.2 mol/l, [I] 0 = 1.4 N 10 3 mol/l Run P Š610 3 a) T eff mol=l 8C b) t max c) x p,max P n;gpc s d) at x p,max M w/m n f e) k app at x p,max s 1 k p L N mol 1 N s a) [P*] = f N [I] 0, where f is the initiator efficiency. b) Longest reaction time. c) Conversion obtained at t max. d) P n;gpc = (M n,gpc M init )/M mon. e) Initiator efficiency, f = P n;th /P n;gpc from the ratio [M] 0 /[I] 0 and slope of the plot of P n;gpc vs. conversion. Fig. 2. First-order time conversion plots for the anionic polymerization of MMA in THF initiated by DPHP 5 at various temperatures. [M] 0 = 0.2 mol/l, [I] 0 = 1.4 N 10 3 mol/l Fig. 3. Dependence of number-average degree of polymerization, P n, and polydispersity index, M w/m n, versus conversion, x p for the anionic polymerization of MMA in THF initiated by DPHP 5 at 208C (F), 08C (9), and +208C (H). ( ) theoretical line. [M] 0 = 0.2 mol/l, [I] 0 = 1.4 N 10 3 mol/l Fig. 4. GPC eluograms of PMMA samples of the polymerization of MMA in THF initiated by DPHP 5 at +208C, obtained at different conversions, x p. (1) t = s, x p = 0.63, P n = 461, M w/m n = 1.23; (2) t = 0.07 s, x p = 0.78, P n = 528, M w/m n = 1.21; (3) t = 0.11 s, x p = 0.90, P n = 584, M w/m n = 1.21 dent of the reaction temperature. The active center concentration, [P*] = f[i] 0, was determined from the slope of the plot of P n versus conversion. The polydispersity indices, M w/m n of the PMMAs obtained somewhat decrease with conversion (Fig. 3) reaching a limiting value M w/m n L 1.2. The GPC data of PMMA obtained at various times during the polymerization (Fig. 4) show the absence of oligomers indicating that the observed low initiator efficiencies are not due to slow initiation. The low initiator efficiency raised the question of the stability of the initiator DPHP 5. 1 H NMR spectra of the initiator show the presence of a small amount (a1%) of an a-methine proton as a triplet at 3.9 ppm which may have formed due to the unavoidable destruction of the initiator during sample preparation. The intensity of the a-methine proton signal did not change for several days. This confirms the stability of the initiator and the lower initiator efficiency can not be attributed to its decomposition. It is assumed that only a fraction of initiator is used for initiation and the rest did not take part for yet unknown reasons. Incomplete initiation appears to be an inherent property of metal-free anionic polymerizations of MMA. In a recent publication this topic was comprehensively assessed taking into account ion-pairing effects and initiation equilibria 34). However, usually this goes along with slow initiation. For an initiation process which is so slow that only 20% of initiator is converted before all monomer is consumed strongly upwards curved time-conver-

5 394 D. Baskaran, A. H. E. Müller Tab. 2. C chemical shifts of lithium and P 5+ ester enolates in THF-d 8 T 8C C2O C a CH 3 1OCH 3 /1OCH 2 1CH 3 reference MIBLi tetramer , ) EIBLi (tetramer) , , 61.1 this work MIBLi dimer ) MIB, [Li, 211] + cryptated unimer , ) EIBP , , 59.3 this work Fig. 5. Arrhenius plot of the propagation rate constants, k p, in the anionic polymerization of MMA with P 5+ counterion in THF and of the reported rate constants for other counterions sion plots and strongly downwards curved plots of P n vs. conversion should be expected. Moreover, Gold 47) has calculated a polydispersity index of M w/m n L 1.33 for such a case, however, values below 1.25 were found in this study. Fig. 5 shows the Arrhenius plot of the propagation rate constants, k p, compared to other counterions. The rate constants in presence of the P 5+ cation are much higher than those obtained with metal cations 1) and even higher than for the tetraphenylphosphonium ion where dormant ylides have been shown to be the majority of species 36). Ylide formation is not possible for the P 5+ cation. The k p values roughly correspond to the values for the bulky cryptated sodium ion, [Na,222] +, or the free anion 7) expected at these temperatures (assuming comparable activation energies). The plot displays a linearity with an activation energy, E a,app = 19.2 l 0.9 kj mol 1 and a frequency exponent, log A app = 8.4 l 0.2. These values are apparent only because dissociation into free ions was not taken into account and the observed k p values are only averages over those of the ion pairs and the free anion, k p ˆ k l k k l a with the fraction of free anions, a L (K d /[P*]) 1/2 1). Since the concentration dependence of the polymerization rates was not investigated the dissociation constant for enolates with large counterions could only be estimated as K d L (1... 5) N 10 5 mol/l 7). Then, the fraction of free anions at [P*] = 2.8 N 10 4 mol/l is very considerable, a L In order to investigate the structure of the active species, a model compound of the growing PMMA chain end, e. g., ethyl a-tetrakis[tris(dimethylamino)phosphoranylidenamino]phosphonium isobutyrate (EIBP 5 ) was prepared. C NMR shifts are shown in Tab. 2 in comparison with other systems reported in literature. It is known that lithium enolates like methyl a-lithium isobutyrate (MIBLi) in THF exist as tetrameric aggregates in equilibrium with dimeric aggregates in the absence of additives 48). The presence of chelating additives such as DME, glyme-3, 12-crown-8, HMPA, or TMEDA does not deaggregate lithium enolates effectively as evidenced from kinetic and NMR results. However, addition of the powerful chelating agent cyptand 211 leads to a pronounced upfield shift of the a-carbon atom of MIBLi from 72.7/65.4 ppm (tetramer/dimer) to 59.9 ppm, indicating the existence of monomeric cryptated lithium enolate 48). Similarly, the spectra of EIBP 5 show the resonance for the a-carbon at 48.8 ppm which is at even higher field than the shift for MIB, [Li,211] +. This is almost a 15 and 25 ppm upfield shift from dimeric and tetrameric MIBLi or EIBLi species, respectively, which indicates a substantial increase of the charge density at the a-carbon in presence of the P 5+ counterion, due to a higher interionic distance. Thus, the EIBP 5 ion pair most probably exists as in a monomeric form and a similar structure is expected for PMMA, P 5+. This is in coincidence to the high reaction rates. Acknowledgement: This work was supported by the Deutsches Bundesministerium für Bildung, Wissenschaft, Forschung und Technologie (grant no. 03N 3006 A5) and BASF AG. D. B. acknowledges a grant by the German Academic Exchange Service (DAAD). 1) A. H. E. Müller, in: Comprehensive Polymer Science, Vol. 3, G. Allen, J. C. Bevington, Eds., Pergamon, Oxford 1988, p ) D. Kunkel, A. H. E. Müller, L. Lochmann, M. Janata, Makromol. Chem., Macromol. Symp. 60, 315 (1992)

6 Anionic polymerization of methyl methacrylate using ) H. Schreiber, Makromol. Chem. 36, 86 (1960) 4) L. Lochmann, M. Rodová, J. Petránek, D. Lím, J. Polym. Sci., Polym. Chem. Ed. 12, 2295 (1974) 5) G. Litvinenko, A. H. E. Müller, Macromolecules 30, 1253 (1997) 6) S. K. Varshney, R. Jérôme, P. Bayard, C. Jacobs, R. Fayt, P. Teyssié, Macromolecules 25, 4457 (1992) 7) C. Johann, A. H. E. Müller, Makromol. Chem., Rapid Commun. 2, 687 (1981) 8) J. S. Wang, R. Jérôme, P. Teyssié, Macromolecules 27, 3376 (1994) 9) D. Baskaran, S. Chakrapani, S. Sivaram, Macromolecules 28, 7315 (1995) 10) J. Marchal, Y. Gnanou, M. Fontanille, Makromol. Chem., Macromol. Symp. 107, 27 (1996) 11) L. Lochmann, J. Kolarik, D. Doskocilova, S. Vozka, J. Trekoval, J. Polym. Sci., Polym. Chem. Ed. 17, 1727 (1979) 12) L. Lochmann, A. H. E. Müller, Makromol. Chem. 191, 1657 (1990) ) S. K. Varshney, J. P. Hautekeer, R. Fayt, R. Jérôme, P. Teyssié, Macromolecules 23, 2618 (1990) 14) D. Baskaran, S. Sivaram, Macromolecules 30, 1869 (1997) 15) D. Baskaran, A. H. E. Müller, S. Sivaram, Macromolecules 32, 56 (1999) 16) C. B. Tsvetanov, D. T. Petrova, P. H. Li, I. M. Panayotov, Eur. Polym. J. 14, 25 (1978) 17) T. Kitayama, T. Shinozaki, T. Sakamoto, M. Yamamoto, K. Hatada, Makromol. Chem., Suppl. 15, 167 (1989) 18) D. G. H. Ballard, R. J. Bowles, D. M. Haddleton, S. N. Richards, R. Sellens, D. L. Twose, Macromolecules 25, 5907 (1992) 19) H. Schlaad, B. Schmitt, A. H. E. Müller, Angew. Chem. 110, 1497 (1998) 20) J. S. Wang, R. Jérôme, P. Teyssié, Macromolecules 27, 4902 (1994) 21) J. S. Wang, R. Jérôme, P. Teyssié, Macromolecules 27, 4615 (1994) 22) P. Bayard, R. Jerome, P. Teyssié, S. K. Varshney, J. S. Wang, Polym. Bull. (Berlin) 32, 381 (1994) 23) A. Maurer, X. Marcarian, A. H. E. Müller, C. Navarro, B. Vuillemin, Polym. Prepr. (Am. Chem. Soc., Div. Polym. Chem.) 38(1), 467 (1997) 24) J. Marchal, M. Fontanille, Y. Gnanou, Abstracts, International Symposium on Ionic Polymerization, Paris 1997, p ) T. Zundel, P. Teyssié, R. Jérôme, Macromolecules 31, 2433 (1998) 26) T. Zundel, C. Zune, P. Teyssié, R. Jérôme, Macromolecules 31, 4089 (1998) 27) T. Zundel, P. Teyssié, R. Jérôme, Macromolecules 31, 5577 (1998) 28) O. W. Webster, W. R. Hertler, D. Y. Sogah, W. B. Farnham, T. V. RajanBabu, J. Am. Chem. Soc. 105, 5706 (1983) 29) R. P. Quirk, J. S. Kim, J. Phys. Org. Chem. 8, 242 (1995) 30) M. T. Reetz, Angew. Chem. 100, 1026 (1988) 31) M. Janata, A. H. E. Müller, unpublished results 32) R. P. Quirk, G. P. Bidinger, Polymer Bull. (Berlin) 22, 63 (1989) 33) A. Fieberg, D. Broska, C. Heibel, F. Bandermann, Des. Monomers Polym. 1, 285 (1998) 34) D. Baskaran, S. Chakrapani, S. Sivaram, A. H. E. Müller, T. E. Hogen-Esch, Macromolecules 32, 2865 (1999) 35) A. P. Zagala, T. E. Hogen-Esch, Macromolecules 29, 3038 (1996) 36) D. Baskaran, A. H. E. Müller, Macromolecules 30, 1869 (1997) 37) D. Baskaran, A. H. E. Müller, H. Kolshorn, A. P. Zagala, T. E. Hogen-Esch, Macromolecules 30, 6695 (1997) 38) T. Pietzonka, D. Seebach, Angew. Chem. 105, 741 (1993) 39) H. G. Börner, W. Heitz, Macromol. Chem. Phys. 199, 1815 (1998) 40) D. Baskaran, A. H. E. Müller, unpublished results 41) A. Molenberg, M. Möller, Macromol. Rapid Commun. 16, 449 (1995) 42) B. Esswein, N. M. Steidl, M. Möller, Macromol. Rapid Commun. 17, 143 (1996) 43) B. Esswein, M. Möller, Angew. Chem., Int. Ed. Engl. 108, 623 (1996) 44) L. Lochmann, M. Rodová, J. Trekoval, J. Polym. Sci., Polym. Chem. Ed. 12, 2091 (1974) 45) T. Hofe, A. Maurer, A. H. E. Müller, GIT Labor-Fachz. 42, 1127 (1998) 46) G. Löhr, B. J. Schmitt, G. V. Schulz, Z. Phys. Chem. (Frankfurt am Main) 78, 177 (1972) 47) L. Gold, J. Chem. Phys. 28, 91 (1958) 48) J. S. Wang, R. Jérôme, R. Warin, H. Zhang, P. Teyssié, Macromolecules 27, 3376 (1994)

Anionic polymerisation of methyl methacrylate with complex initiator system

Anionic polymerisation of methyl methacrylate with complex initiator system Turgut Nugay 1, Nihan Nugay 1, Robert Jérôme 2 1 Department of Chemistry/Polymer Research Center, Bogazici University, 80815,