Anionic Polymerization of Methyl Methacrylate with Lithium N-Benzyltrimethylsilylamide

Polymer Journal, Vol.35, No. 6, pp 539—543 (2003)

SHORT COMMUNICATIONS

† Tatsuki KITAYAMA and Takehiro KITAURA

Department of Chemistry, Graduate School of Engineering Science, Osaka University, Toyonaka, Osaka 560–8531, Japan

(Received February 21, 2003; Accepted March 27, 2003)

KEY WORDS Anionic Polymerization / End-functional Polymer / Lithium Amide / Aluminum Lewis Acid / Macromonomer /

End-functional polymers have attracted much atten- tion since they can be used for constructing more elab- orated polymer chain architectures.1 Living polymerization, the concept of which was first proposed by Szwarc2 in 1956, is one of the most promising ways to introduce a functional group at the polymer chain-end.3 Another means to obtain end-functional polymers is to polymerize a monomer with an initiator which com- prises a functional group or its precursor group.4 Equation 1. We have found stereospecific and living polymerization of methacrylates; isotactic-specific one diphenylethylenediamine monolithium amide in THF with tert-butylmagnesium bromide (t-BuMgBr),5, 6 afforded PMMA having secondary amino group at syndiotactic-specific one with tert-butyllithium (t- the initiating chain-end, though initiator efficiency and 7, 8 23 BuLi)/trialkylaluminum (R3Al), and heterotactic- MWD of the polymer were not fully discussed. Af- specific one with t-BuLi and bis(2,6-di-tert-butyl- ter introducing terminal carboxylic acid unit through 9–17 phenoxy)methylaluminum [MeAl(ODBP)2]. More the reaction with succinic anhydride, they carried out recently, we have reported that t-BuLi/bis(2,6-di-tert- end-cyclization of the hetero-telechelic PMMA to ob- butylphenoxy)ethylaluminum [EtAl(ODBP)2] initiat- tain cyclic PMMA. ing system was effective for syndiotactic-specific liv- Lithium bis(trimethylsilyl)amide (TMS2NLi) has ing polymerization of methacrylates13 and acrylates.18 been known as a strong base with low nucleophilic- 24 In all these cases, obtained polymers have t-Bu group ity, and used as a mild lithiation reagent. De- at the initiating chain-end of each polymer molecule. spite of its low nucleophilicity, Nagasaki et al. re- Lithium dialkylamides (R2NLi) such as lithium di- ported that TMS2NLi could initiate anionic polymer- 25 isopropylamide (LDA) have been known as useful an- ization of MMA. They declared that the polymer- ionic initiators for methacrylate polymerizations.19–22 ization provides a PMMA having a primary amino Antoun et al. reported that LDA/lithium chloride (LiCl) end-group after acid treatment. However, the poly- was an effective initiating system for anionic polymer obtained had broad MWD and the initiator effi- merization of methyl methacrylate (MMA) in tetrahy- ciency was low (ca. 0.07), probably owing to the in- drofuran (THF) and generated PMMA with narrow herently low nucleophilicity of TMS2NLi. Antoun et 20 22 molecular weight distribution (MWD) in high initia- al. and Couper et al. used TMS2NLi in combina- tor efficiency (> 0.9).20 The obtained polymer has the tion with several additives such as LiCl for polymer- tertiary amino group at the chain end, which is less ization of MMA. The initiator efficiency was still low feasible for further chemical modification than pri- (< 0.1), and MWD of the obtained polymers were rel- mary and secondary amino groups, however. Kubo et atively broad. As described above, we have found al. reported that polymerization of MMA with N,N- that several aluminum compounds are effective for the stereocontrol in methacrylate polymerizations. Thus, †To whom correspondence should be addressed. we have examined anionic polymerization of methacry-

539 T. KITAYAMA and T. KITAURA

N-Benzyltrimethylsilylamine (BnTMSNH) was prepared, according to the literature28 with a slightly modification, from benzylamine (Tokyo Chemical Industry) and trimethylsilyl chloride (1 equiv.) (Nacalai Tesque) in diethyl ether in the presence of CaH2 then fractionally distilled and dried over Molecular Sieves 4A. BnTMSNLi was prepared from BnTMSNH and n-BuLi (1 equiv.) in hexane at 0◦C.29 The solvent was removed under vacuum to leave a white solid, and the product was recrystallized several times by dissolving it in heptane and cooling to −20◦C. After most of the heptane was removed by a syringe and the residual heptane was evaporated under vacuum to dryness, the solid residue was dissolved in toluene giving a clear red solution. The concentration of this solution was determined by 1 ◦ H NMR in toluene-d8 at 35 C in combination with density measurements. Scheme 1. 2,6-Di-tert-butylphenol was fractionally distilled and used as a heptane solution. Trimethylaluminum lates with TMS2NLi/tributylaluminum (n-Bu3Al) or (Me3Al) (Aldrich Co. Ltd) and triethylaluminum 26 TMS2NLi/MeAl(ODBP)2 in toluene. The initiator ef- (Et3Al) (Kishida Chemical) were used as received. ficiency was improved as compared to the polymeriza- MeAl(ODBP)2 and EtAl(ODBP)2 were prepared ac- 11, 12, 30, 31 tion with TMS2NLi alone. In the case of TMS2NLi/n- cording to the literatures from Me3Al or Bu3Al, however, the initiating chain-end of the polymer Et3Al and 2,6-di-tert-butylphenol (2 equiv.) in toluene ◦ was n-butyl group instead of amino group to be derived at 0 C. Recrystallization procedures were similar to the from TMS2N-fragment, probably due to ate complex case of BnTMSNLi and used as toluene solutions. formation from TMS2NLi and n-Bu3Al, which initiated the polymerization by releasing n-butyl group. Polymerization On the other hands, Yamamoto and co-workers Polymerizations were carried out in glass ampoules have reported that lithium N-benzyltrimethylsilylamide filled with dried nitrogen passed through Molecular − ◦ (BnTMSNLi) reacted with methyl crotonate to generate Sieves 4A cooled at 78 C. Dried toluene and alkyl- a conjugate-addition product exclusively, while other aluminum were added to the glass ampoules, which were then cooled to polymerization temperature. Poly- lithium amides including LDA and TMS2NLi produced carbonyl-addition and γ-hydrogen-abstraction product merization was initiated either by adding monomer in addition to the conjugate-addition product.27 after BnTMSNLi or by adding BnTMSNLi after The present work aims at obtaining stereoregular monomer. The reactions were terminated by adding end-functional polymethacrylates by exploring the pos- methanol containing aqueous HCl at the polymeriza- sibility of utilizing BnTMSNLi as a functional initiation temperature. The reaction mixtures were poured into hexane, and the precipitates were collected by fil- tor in combination with n-Bu3Al, MeAl(ODBP)2,or tration, and then washed with hexane and 3 N HCl sev- EtAl(ODBP)2 for the polymerization of methacrylates. eral times. The recovered polymers were dried under ◦ EXPERIMENTAL vacuum at 60 C. Acrylation Reaction Materials PMMA [number average molecular weight MMA (Nacalai Tesque) and acryloyl chloride (M ) = 2800, M /M = 1.07, 100 mg (0.036 mmol)], (Aldrich Co. Ltd) were purified by fractional distilla- n w n obtained by the anionic polymerization with tion under reduced nitrogen pressure, dried over cal- BnTMSNLi/n-Bu Al, was dissolved in 3 mL dry cium dihydride (CaH ) and vacuum-distilled just be- 3 2 benzene, and then acryloyl chloride (1.11 mmol) fore use. Triethylamine (Nacalai Tesque) was distilled and triethylamine (3.0 mmol) were added in order at under nitrogen, dried over CaH , and vacuum-distilled. 2 ambient temperature. After 20 h, the reaction mixture Toluene, benzene and heptane were purified in the usual was poured into hexane. The precipitate collected by manner, mixed with a small amount of n-BuLi (Nacalai filtration was dissolved in chloroform and shaken with Tesque), and distilled under high vacuum.

540 Polym. J., Vol.35, No. 6, 2003 Poly(methyl methacrylate) with Benzylamino End-Group

Table I. Polymerization of MMA with BnTMSNLi/aluminum compounds in toluene at −78◦C for 24 ha d e Mn Mw Tacticity/% run Al compound d e SEC NMR Calcd. Mn mm mr rr 1b 4200 4800 6.37 79.6 16.4 4.0 none 2c 4400 4800 9.10 81.5 15.6 2.9 ------3b 6700 8600 1.42 4.1 18.3 77.6 n-Bu Al 4c 3 2800 2800 1.07 6.0 13.1 80.9 ------2600 ------5b 3100 3300 1.21 13.3 64.7 22.0 MeAl(ODBP) 6c 2 3000 3100 1.21 13.8 65.8 20.4 ------7b 2700 2800 1.09 6.0 13.8 80.2 EtAl(ODBP) 8c 2 2900 2900 1.10 5.3 14.6 80.1 aToluene 10 mL, BnTMSNLi 0.4 mmol, Al/Li = 3, MMA 10 mmol, yield 100%. bAddition order; aluminum compounds, BnTMSNLi, MMA. cAddition order; aluminum compounds, MMA, BnTMSNLi. dDetermined by SEC in THF (PMMA standards). eDetermined by 1 ◦ 400 MHz H NMR (CDCl3,55 C).

3 N HCl, and then the chloroform was evaporated to the MWD was relatively broad (run 3). We antic- dryness. ipated that BnTMSNLi formed a complex with n- Bu3Al such as ate complex and lost nucleophilic addi- Measurements tion reactivity, while MeAl(ODBP)2 and EtAl(ODBP)2 1H NMR spectra were measured in chloroform-d at were too bulky to form such complexes at least 55 ◦C on a JEOL JNM400 spectrometer operated at at −78 ◦C. To avoid such kind of complex forma- 400 MHz. Tacticities of PMMAs were determined from tion prior to the initiation reaction, we attempted to 1 α-methyl hydrogen signals. Mn was determined by H change the addition order of the reagents as alu- NMR spectroscopic end-group analysis, by using ben- minum compounds, monomer and BnTMSNLi so that zylmethylene proton signal (4.2 ppm) of the initiator the initiation reaction takes place before the com- fragments and methoxy methyl proton signal (3.7 ppm). plex formation. The polymerizations with BnTM- Mn and molecular weight distribution (MWD) were SNLi alone and with BnTMSNLi/MeAl(ODBP)2 or also determined by size exclusion chromatography BnTMSNLi/EtAl(ODBP)2 were not affected signifi- (SEC) using a JASCO TRI ROTAR-V chromatograph cantly by the change of the addition order (runs 2, 6, 8). equipped with Polymer Laboratories SEC columns On the contrary, the addition order exerted great influ- MIXED-C × 2 (7.5 mm i.d. × 300 mm) using THF as ence on the polymerization with BnTMSNLi/n-Bu3Al an eluent at 40 ◦C. SEC chromatograms were calibrated (run 4). The initiator efficiency was so much improved against standard PMMA samples (Shodex). that the Mn value became close to the calculated one and the MWD was narrower. These results support RESULTS AND DISCUSSION above-mentioned anticipation that BnTMSNLi forms the complex with n-Bu3Al but not with MeAl(ODBP)2 Polymerization of MMA with BnTMSNLi in combi- or EtAl(ODBP)2. 1 nation with n-Bu3Al, MeAl(ODBP)2 or EtAl(ODBP)2 Figure 1a illustrates H NMR spectrum of PMMA ◦ (1/3) was conducted in toluene at −78 C and the re- prepared with BnTMSNLi/n-Bu3Al (Table I, run 4). sults were summarized in Table I. In the first series Since the polymerization was terminated with HCl– of experiments, the reagents were added to toluene in MeOH, it is expected that the trimethylsilyl group the following order; aluminum compounds, BnTM- in the initiator fragment is easily eliminated and hy- SNLi, and monomer, as reported for the polymer- drochloride salt of PMMA with benzylamino end- ization with t-BuLi/aluminum compounds.7–18 The group is produced. In fact, the PMMA did not polymerization with BnTMSNLi alone gave PMMA show trimethylsilyl group signal around 0 ppm, and ex- in a good yield but with broad MWD in low ini- hibited ammonium proton signals at lower magnetic tiator efficiency (run 1). In contrast, the polymer- field. Aromatic and benzyl methylene proton sig- izations with BnTMSNLi/MeAl(ODBP)2 (run 5) and nals, which derived from the initiator fragment, were BnTMSNLi/EtAl(ODBP)2 (run 7) afforded PMMAs observed around 7.6, 7.4, and 4.2 ppm, respectively. which had controlled molecular weights and narrow Moreover, the spectrum showed methylene proton sig- MWDs. In the case of BnTMSNLi/n-Bu3Al, how- nals of the first monomer unit, which attached to the ever, Mn was fairy larger than the expected one and amino end-group. Mn estimated from integral in-

Polym. J., Vol.35, No. 6, 2003 541 T. KITAYAMA and T. KITAURA

1 ◦ Figure 1. H NMR spectra of PMMA (CDCl3,55 C). (a) α-Benzylamino-PMMA hydrochloride salt, (b) α-benzylamino-PMMA, (c) α-(N-acryloylbenzylamino)-PMMA. tensity ratio of benzyl and methoxy proton signals and its feasibility of further derivatization, the PMMA was close to the expected value, suggesting high ini- was reacted with acryloyl chloride to convert the end tiator efficiency (> 0.9). The fact that the Mn esti- group to N-acryloylbenzylamino group. Figure 1c il- mated by 1H NMR end-group assay also agreed with lustrates the spectrum of the obtained polymer, which the value determined by SEC analysis indicates that demonstrates the existence of acrylamide group. Mn each polymer molecule has one benzyl amino group. calculated from integrations of methoxy and olefinic The polymers obtained by BnTMSNLi/MeAl(ODBP)2 protons was 2800, which is consistent with the value or BnTMSNLi/EtAl(ODBP)2 gave similar spectra. of the starting polymer. Thus, it is proved that the reac- Tacticities of the produced polymers were isotac- tion was quantitative, and that the amino end-group had tic rich (mm ≈ 80%) in the case of BnTMSNLi high reactivity enough for quantitative conversion. alone. Heterotactic rich polymers (mr ≈ 65%) were obtained with BnTMSNLi/MeAl(ODBP)2, and syndio- REFERENCES tactic rich ones (rr ≈ 80%) with BnTMSNLi/n-Bu3Al and BnTMSNLi/EtAl(ODBP)2. The polymerization 1. R. Jerome, M. Henrioulle-Granville, B. Boutevin, and J. J. of MMA with t-BuLi/n-Bu3Al gives the polymer with Robin, Prog. Polym. Sci., 16, 837 (1991). higher rr content (rr > 90%).7, 8 Syndiotacticity of the 2. M. Szwarc, Nature, 178, 1168 (1956). polymers obtained in the present work was estimated 3. A. Hirao and M. Hayashi, Acta Polym., 50, 219(1999). 4. H. L. Hsieh and R. P. Quirk, “Anionic Polymerization: Prin- relatively low, partly due to the overlap signals of ciples and Practical Applications”, Marcel Dekker Inc., New monomer units at and near chain ends. York, N.Y., 1996. In order to confirm the existence of the amino end- 5. K. Hatada, K. Ute, K. Tanaka, T. Kitayama, and Y. Okamoto, group, a chloroform solution of the polymer was shaken Polym. J., 17, 977 (1985). with aqueous NaHCO3 to convert the ammonium salt 6. K. Hatada, K. Ute, K. Tanaka, Y. Okamoto, and T. Kitayama, to free amine. In the 1H NMR spectrum of the polymer Polym. J., 18, 1837 (1986). 7. T. Kitayama, T. Shinozaki, E. Masuda, M. Yamamoto, and K. after treatment with NaHCO3 (Figure 1b), ammonium proton signals around 10 ppm disappeared and methy- Hatada, Polym. Bull., 20, 505 (1988). lene proton signals at 3.1 and 2.6 ppm shifted to higher 8. T. Kitayama, T. Shinozaki, T. Sakamoto, M. Yamamoto, and K. Hatada, Makromol. Chem. Suppl., 15, 167 (1989). field. Aromatic proton signals around 7.6 and 7.4 ppm 9. T. Kitayama, Y. Zhang, and K. Hatada, Polym. Bull., 32, 439 shifted to overlap CHCl3 signal, and benzyl methylene (1994). proton signal at 4.2 ppm was assumed to shift to overlap 10. T. Kitayama, Y. Zhang, and K. Hatada, Polym. J., 26, 868 methoxy signal. When this polymer was treated with (1994). aqueous HCl, the 1H NMR spectrum became identical 11. T. Kitayama, T. Hirano, and K. Hatada, Polym. J., 28,61 to that in Figure 1a, which indicates the reversible pro- (1996). tonation of amino group. 12. T. Kitayama, T. Hirano, Y. Zhang, and K. Hatada, Macromol. To further confirm the presence of the amino group Symp., 107, 297 (1996).

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