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SYMPOSIA SECTION I. Symposium “ACS Award for Creative Invention Honoring Ralph Milkovich” 189th National Meeting of the American Chemical Society, Miami Beach, Florida, April 28-May 3, 1985 (Continued from June 1986 Issue)
Functionalization Reactions of Poly(styry1)lithium with p -(Dimethylamino)benzaldehydet Roderlc P. Quirk’ and Muhanad Alsamarrale Institute of Polymer Science, University of Akron, Akron, Ohio 44325
Functionalization reactions of poly(styry1)lithiumwith p-(dimethylamin_o)benzaldehydeproduce yellow-colored polymer products containing slgnificant amounts of coupling products for M , < 10 000. No coupling is observed for high molecular weight polymers (M, = 340 000). Mechanistic evidence is presented for dimer formation involving Cannizzaro reaction of the initial alkoxlde product with p-(dimethylamino)benzaldehydeto form carbonyl chain-end functionality, which reacts further with another molecule of poly(styry1)lithium. The products have been examined by UV (Amx 324 nm) and I R (Y 1660 cm-’) spectroscopy: in addition, p-(dimethy1amina)benzyl alcohol has been identified as one of the reaction products.
Introduction Alkyllithium-initiated anionic polymerizations of many styryl, dienyl, and polar vinyl monomers can be performed without the incursion of spontaneous termination or chain-transfer reactions (Morton, 1983; Young et al., 1984; Szwarc, 1983; Bywater, 1974, 1985). When suitable initiators are used (Quirk and Chen, 1982), these polymerizations yield polymers with predictable molecular weights and narrow molecular weight distributions. Because of the absence of termination and chain-transfer reactions, these polymerizations generate stable, carbanionic chain ends which, in principle, can be converted into a diverse array of functional end groups (Young et al., 1984; Quirk and Chen, 1982,1984;Quirk et al., 1985). Polymer chains with a copolymerizable chain-end functionality form the basis of the macromonomer appioach pioneered by Milkovich and co-workers for the preparation of comb-type graft polymers (Milkovich, 1981; Schulz and Milkovich, 1982, 1984). In order to exploit the full potential of these chain-end functionalized polymers, well-defined general procedures for quantitative chain-end functionalization must be available. Unfortunately, many of the reported examples of anionic chain-end functionalizations have not been well characterized (Young et al., 1984). Trepka (1984) recently reported studies of the use of p-(dimethylamino)benzaldehyde to introduce nitrogen functionality into lithiated, hydrogenated butadienelstyrene copolymers (eq 1). The product of this functionalization reaction contains both hydroxyl and dimethylamino functionality, and it was reported earlier by Zelinski et al. +Dedicated to the memory of Ralph Milkovich. 0196-4321/86/ 1225-0381$01.50/0
CHO
N(CH3)*
P-CHOH
N(CH3)z
(1963) that the corresponding telechelic functional polymers exhibited enhanced reactivity and better ultimate physical properties with diisocyanate curing compared to the corresponding telechelic dihydroxyl-terminated polymer. Trepka (1984) states that “p-(dimethy1amino)benzaldehyde was used because it is relatively free of reactive sites for competing and secondary reactions; e.g., labile hydrogens on the carbon adjacent to a carbonyl or ester group for elimination and subsequent coupling, etc.” Because of its apparent simplicity and utility, we undertook an investigation of this interesting functionalization reaction, and the results are reported herein. Experimental Section Materials. Styrene and benzene were carefully purified as described in detail previously (Quirk and Chen, 1982). Cyclohexane was purified by using the procedure described for benzene. Tetrahydrofuran (THF) (Fisher, Certified, ACS) was stirred over freshly crushed calcium hydride, degassed, and distilled onto a mixture of sodium dispersion (Alfa) and benzophenone. Fresh T H F was obtained from this dark purple solution by distillation directly into reaction flasks. Solutions of sec-butyllithium (Lithium Corp. of America, 12.0 wt % in cyclohexane) were analyzed by the double-titration method with 1,2-dibromoethane (Gilman and Cartledge, 1964). 4-(Dimethylamino)benz0 1986 American Chemical Society
382 Ind. Eng. Chem. Prod. Res. Dev., Vol. 25, No. 3, 1986 Table I. GPC Analysis of Functionalization Experiments of Poly(styry1)lithium (PSLI) with p -(Dimethylamino)benzaldehyde wt
expt 1
2 3 4
5
70
polym 12 3.7 8.4 7.6 7.8
[ArCHO]/ [PSLiIc 1.2 0.82
Mn (GPC) PSLI" 460
3 500 5 800 6 400 343 000
M w / M n(GPC) 1.15 1.05 1.01 1.02 1.05
[PSLi] X 11.7 0.95 1.2 1.3 0.021
wt % d coupled prod 33 19 10 10 0
1.1
5.9 3.4
wt % d uncoupled prod 67
81 90 90 100
Number-average molar mass for samples from methanol quenching of the poly(styry1)lithiums prior to functionalization. Molar concentration of chain ends. Stoichiometric molar concentration ratio of terminating agent to poly(styry1)lithium chain end. Determined by curve resolution of GPC traces.
aldehyde (Aldrich,99% ) was purified by recrystallization from distilled water, followed by drying in a vacuum oven: mp 72-74 "C [lit. mp 73 "C by Boessneck (1885)l; one HPLC peak; 'H NMR (CDCI,) (60 MHz) 6 3.0 (CH,),N-), 6.5, 6.6, 7.5, 7.6 (aromatic, AB quartet), and 9.6 (-CHO). 4-(Dimethy1amino)benzyl alcohol was prepared by reduction of 3.012 g (0.02 mol) of 4-(dimethy1amino)benzaldehyde with 20 mL (0.061 mol) of a saturated solution of lithium aluminum hydride in tetrahydrofuran under an argon atmosphere. After 0.25 h, the solution was hydrolyzed with 0.025 N HCl, extracted by using diethyl ether, and the solvent was removed by using a rotary evaporator: 'H NMR (CDC1,) (400 MHz) 6 7.12, 7.10, 6.60, 6.58 (aromatic, 3.9 H), 4.44 (benzyl CH2, 2.0 H), 2.83 [(CH,),N- 6.0 HI, and 1.52, 1.39 (-OH, 1.5);IR (neat) absorption 3350 cm-' with no detectable carbonyl absorption; one HPLC peak. Polymerizations. sec-Butyllithium-initiated polymerizations were carried out at room temperature in all-glass, sealed reactors using breakseals and standard high vacuum techniques (Morton and Fetters, 1975). Functionalization reactions of poly(styry1)lithium were effected by adding benzene solutions of varying amounts of p-(dimethylamino)benzaldehyde via ampules with breakseals. The color changed almost immediately from reddish-orange to canary yellow. The reactions after quenching were terminated with degassed, anhydrous methanol. Metalation. To a 10.01-g sample of poly(p-methylstyrene-b-butadiene-b-p-methylstyrene) (Quirk, 1985) with block Mn'sof 14 000-82 000-14 000, respectively, dissolved in 400 mL of cyclohexane were added 4.55 mL (6.59 mmol) of sec-butyllithium and 1.01 mL (6.69 mmol) of N,N,N',N'-tetramethylethylenediamine (TMEDA) in an allglass, high-vacuum polymerization reactor. After 2 h at room temperature, the gelled metallation products were terminated by mixing with 3.9565 g (26.5 mmol) of solid p-(dimethy1amino)benzaldehydefollowed by 6.4 mL of degassed methanol for quenching. The yellow product contained a considerable amount of gel product which could not be dissolved in toluene. Characterization. Size exclusion chromatographic analyses (Waters 15043 were performed in THF by using six p-Styragel columns (IO2,5 x IO2, io3, io4, io5, lo6 A) for high molecular weight samples and four p-Styragel columns (two 100, 500, lo3 A) for low molecular weight samples after calibration with standard polystyrene samples. 'H NMR spectra were obtained by using Varian T-60 (60 MHz) and Varian XL 400 (400 MHz) spectrometers using (CH3)$i as internal standard. Ultraviolet spectra were obtained by using a Varian DMS-90 UV-vis spectrophotometer. Infrared spectra were obtained by using a Beckman Model FT-2100 FTIR spectrometer. HPLC analyses (Varian 5000 liquid chromatograph) were performed by using a MicroPak CH-10 column (30 cm x 4 mm) in 70/30 CH3CN/H20 at 120 atm with a Varian UV-50 variable-wavelength detector.
Scheme I VCH-CHZ-
~CH-CHZ-
I
I sec - C ~ H S L I
1. p-(CHg )2NCeH,CHO 2 . H20 *
CHOH
I
A Results and Discussion The reactions of organolithium compounds with carbonyl compounds, in general, and aldehydes, in particular, may be subject to several side reactions (Wakefield, 1974). As noted by Trepka (1984), there are no enolizable aprotons in p-(dimethy1amino)benzaldehyde;however, aldehydes undergo a number of side reactions in the presence of base (Patai, 1966). Therefore, it was of interest to carefully determine the nature of the p-(dimethylamino)benzaldehyde functionalization reaction. Preliminary functionalization experiments were performed by using lithiated (sec-BuLi/TMEDA/cyclohexane) poly(p-methylstyrene-b-butadiene-b-p-methylstyrene) (Quirk, 1985), analogous to the experiments reported by Trepka (1984) as illustrated in Scheme I. (Only metalation of the benzyl protons is illustrated in Scheme I; however, metalation of allylic protons in the polybutadiene block may also occur.) It was observed that the polymeric products of these reactions were yellow colored and that this color remained after repeated precipitations and silica-gel column chromatography. In addition, a significant amount of gelled polymer products was formed. In order to gain some understanding of the nature of the reactions responsible for the yellow color and gelation, functionalization experiments were performed with model poly(styryl)lithiums, prepared in benzene by using secbutyllithium as initiator (Scheme 11). The results of GPC analysis of a number of experiments with poly(styry1)lithiums of different molecular weights are listed in Table I. Several interesting conclusions can be drawn from the data in Table I. First, significant dimeric coupling products are observed with poly(styry1)lithiums in the reactive oligomer molecular weight range, Le., M,, < 10000. The formation of dimeric product in this molecular range is important because macromonomers and other reactive oligomers are generally in this molecular weight
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383
Q
s ~ c - C ~-€H CH2CHS ~ CH2 -CH
I
-CHOH I
35
I
I
I
1
I
1
36
37
38
39
40
41
41
43 ( m l l
ELUTION VOLUME (MLI
Figure 2. Size exclusion chromatograms for the base polymer (methanol-quenched PsLi) (-) and p-(_dimethylamino)benzaldehyde-functionalized polymer (- -) with M , = 343 000.
-
Scheme 111 o-LI+ CHO
I
PS-C-H
X
c
u
4 p: U W p:
22
24 25 ELUTION VOLUME(ML)
23
26
27
28
Figure 1. Size exclusion chromatograms for the base polymer (methanol-quenched PsLi) (- - -) and pidimethy1amino)benzaldehyde-functionalized polymer (-) with M , = 3500.
range. Size exclusion chromatograms for the base polymer and the products of the functionalization reaction for Mn = 3500 (expt 2) are shown in Figure 1. However, at high molecular weight (expt 5), no significant dimeric coupling products were observed as shown in Figure 2. These observations as well as the general trend in expt 1-4 indicate that there is a strong molecular weight dependence for the amount of coupling product observed. One possible mechanism for formation of coupling products in these functionalization reactions is shown in Scheme 111. This sequence of reactions involves oxidation of the initial alkoxide product (1) via a Cannizzaro reaction (Geissman, 1944) with p-(dimethy1amino)benzaldehyde (eq 2). The resultant carbonyl product (2) can then react with another poly(styry1)lithium chain to form the dimeric product 3 (eq 3). Evidence for Cannizzaro reactions in these systems was obtained by reversed-phase, HPLC analysis of the nonpolymeric products of these functionalization reactions. In all experiments, including expt 5 where no dimeric product was observed, HPLC analysis indicated the presence of significant amounts of p-(dimethylamino)-
benzyl alcohol, which was identified by comparison with an authentic sample (see Experimental Section). Additional evidence for the incursion of the reactions shown in Scheme I11 (eq 1-3) in these functionalization reactions was obtained by ultraviolet spectroscopic (UV) analysis of the polymeric products (Figure 3, expt 1). The UV spectrum exhibited a A, of 324 nm in carbon tetrachloride. One can calculate an expected A, for p-(dimethy1amino)acetophenone of 331 nm by using Woodward's Rules (Scott, 1964). In addition, the value of A,, is reported to be 337 nm in 95% ethanol (Kumler, 1946). Fourier transform infrared difference spectra (subtraction of methanol-quenched polystyrene samples from the p-(dimethy1amino)benzaldehyde-functionalized poly-
Ind. Eng. Chem. Prod. Res. Dev., Vol. 25, No. 3, 1986
384
&"""".>*
Scheme IV
H
O
PS -CH,-C-C
+
2a color less
OH PS -C H2 -C =C
&*N(cH3)2 yellow 2b
w J z
m PI v) 0
m Q
ilX
300
400
wavelength (nm!
Figure 3. Ultraviolet absorption spectra in CC1, for the polymeric products from reaction of PsLi (A?" = 640), with p-(dimethylamin0)benzaldehyde at t = 0 and at 1 = 0.5 h after preparation of solution.
styrene) indicated the presence of an absorption band a t 1660 cm-l (thin film); p(dimethy1amino)acetophenone exhibits an absorption band a t 1646 cm-' in CC1, (Bergmann and Pinchas, 1952). Thus, the observed UV and infrared absorption spectra for the polymeric products are consistent with the formation of a significant amount of polymer product with structure 2, the expected product of the Cannizzaro reaction. All of the available evidence is consistent with the reaction mechanism shown in Scheme I11 to explain the formation of dimer in the functionalization of poly(styry1)lithium with p(dimethy1amino)benzaldehyde. However, Scheme I11 does not provide an explanation, per se, for the consistent observation of yellow-colored products from all of these functionalization reactions, even in expt 5 (Table I) where no coupling product was observed. The key to explaining the yellow color in the polymeric products was the observation that the yellow color, and an ultraviolet absorption at ca. 400 nm, faded with time in CC1, solution (see Figure 3). This suggested the possibility that some type of tautomeric equilibrium was responsible for the color and the time dependence of the color. A possible tautomeric equilibrium, consistent with the expected products of the Cannizzaro reaction Scheme 111) is shown in Scheme IV. It would be anticipated that the enolization of the acetophenone-type carbonyl end group would depend on the solvent and that nonpolar solvents might favor formation of the nonpolar ketonic form 2a (Toullec, 1982). In conclusion, the functionalization reaction of poly(styry1)lithium with p(dimethy1amino)benzaldehyde is not a simple carbonyl addition process as suggested in previous literature. Evidence presented herein indicates that this functionalization reaction is complicated by Cannizzaro reactions, dimerization, and tautomeric equilibria. Differences in extent of dimerization reactions have also been noted for functionalization reactions of oligomeric anions (A?,, < 10000) and higher molecular weight species (A?,, =
340000). Thus, one cannot necessarily use reactions of oligomeric species as models for the corresponding high molecular weight species. As this work indicates and as noted previously (Young et al., 1984),many of the reported applications of functionalization reactions to anionic polymers have not been well characterized. It is not acceptable to simply assume that this type of functionalization reaction proceeds as expected from textbooks or on the basis of reactions of model compounds including oligomers.
Acknowledgment We gratefully acknowledge the generous support of this research by the Exxon Education Foundation and Owens-Corning Fiberglas Corp. Registry No. Poly(styryl)lithium, 36345-04-7;p(dimethy1amino)benzaldehyde, 100-10-7;p(dimethy1amino)benzyl alcohol, 1703-46-4.
Literature Cited Bergmann, E. D.; Pinchas, S.J . Cbim. Pbys. 1952, 4 9 , 537. Boessneck, P. Ber. Dtscb. Cbem. Ges. 1885, 18, 1516. Bywater. S.Prog. Polym. Sci. 1974, 4 , 27. Bywater, S. "Anionic Polymerization" in Encyclopedia of Polymer Science and Engineering, 2nd ed.; Wiley-lnterscience: New York, 1985; Vol. 2, p 1. Geissman, T. A. Org. React. 1944, 2 , 94. Gilman, H.; Cartledge, F. K. J . Organomet. Cbem. 1964, 2, 447. Kumler, W. D. J . Am. Cbem. SOC. 1946, 6 8 , 1184. Milkovich, R. I n Anionic Polymerization: Kinetics, Mechanism and Syntbesis; McGrath, J. E., Ed.; ACS Symposium Series 166; American Chemical Society: Washington, DC, 1981: Chapter 3, p 41. Morton, M. Anionic Polymerization : Principles and Practice; Academic: New York, 1983. Morton, M.; Fetters, L. J. Rubber Cbem. Tecbnol. 1975, 4 8 , 359. Patai, S., Ed. The Chemistry of the Carbonyl Group; Interscience: New York, 1966. Quirk, R. P.; Chen, W.-C. Makromol. Cbem. 1982, 183, 2071. Quirk, R. P.; Chen, W.-C. J . Polym. Sci., Polym. Chem. Ed. 1984. 22, 2993. Quirk, R . P. Polym. Prepr. ( A m . Cbem. SOC.,Div. Polym. Chem.) 1985, 26(2), 14. Quirk, R. P.; Chen. W.-C.; Cheng, P.-L. Reactive Oligomers; Harris, F., Spinelli, H., Eds.; ACS Symposium Series 282; American Chemical Society: Washington, DC, 1985; Chapter 12, p 139. Schulz, G. 0.; Milkovich, R. J . Appl. Polym. Sci. 1982, 27, 4773. Schulz, G. 0.; Miikovich, R . J . Polym. Sci.. Polym. Chem. Ed. 1984, 22, 1633. Scott. A. I . Interpretation of the Ultraviolet Spectra of Natural Products; Pergamon: Frankfurt, Germany, 1964; p 109. Szwarc, M. Adv. Polym. Sci. 1983, 4 9 , 1. Touliec, J. Adv. Pbys. Org. Cbem. 1982, 78. 1. Trepka, W. J. Macromolecules 1984, 77, 497. Wakefield, B. L. The Chemistry of Organolitbium Compounds ; Pergamon: Oxford, England, 1974: p 129. Young, R . N.; Quirk, R . P.; Fetters, L. J. Adv. Polym. Sci. 1984, 5 6 , 1. Zelinski, R. P.; Hsieh, H. L.; Strobel, C. W. US. Patent 3 109871, 1963.
Received for review October 8, 1985 Accepted February 3, 7986
