I n d . Eng. Chem. Res. 1994,33, 235-238
235
Poly[ propylene-graft-(4-vinylpyridiniumdichromate)]: A Novel Fibrous Polymer-Supported Oxidizing Agent Robert T. Peltonen, Kenneth B. Ekman, and Jan H.Nasman' Laboratory
of
Polymer Technology, Abo Akademi University, Porthansgatan 3-5, SF-20500 Abo, Finland
Polypropylene nonwoven fabrics were radiation grafted with 4-vinylpyridine and then functionalized with chromium trioxide to obtain the corresponding poly[propylene-graft-(4-vinylpyridinium dichromate)]. The degree of functionalization was found to be in the range of 80-90% of the theoretical value. The insoluble functionalized polymer substrate possesses the desired characteristics of a useful polymer-supported reagent, including high accessibility of active sites and operational simplicity. The oxidation of alcohols was investigated to compare this polymer-substrate supported reagent with a commercially available cross-linked poly(viny1pyridinium) reagent. The reagent supported on the nonwoven fabric was found to be more efficient in the oxidation reactions than the commercially available poly(viny1pyridinium) reagent. This difference in reactivity is attributed to the non-cross-linked graft chains in the nonwoven supported reagent, which increase the accessibility to the active sites. The reagent is very easily separated from the reaction mixture since the fabric can be manually removed from the solution, which remains clear during and after the reactions.
Introduction Polymer-supported reagents and catalysts have many advantages, like ease of separation of the supported reagent from the reaction mixture, reuse of the catalyst or reagent after regeneration, and adaptability to continuous flow processes. Reduced toxicity and odor of supported species, compared with low molecular weight species, and chemical differences, such as prolonged activity or altered selectivity of a catalyst, compared with its soluble analog, are other advantages obtained with polymer-supported reagents. However, the industrial success of these reagents and catalysts has been limited since they cost more than low molecular mass analogs. Other major problems identified include the need to develop supports with high capacities where the functional sites are readily accessible. It is clear that the desirable features of high accessibility and high capacity tend to be mutually exclusive, thus for any particular application a compromise may have to be made between accessibility and capacity. For stoichiometric reactions high capacity is essential. On the other hand, for catalytic reactions accessibility is essential, since the turnover at each active site may be governed by the diffusion of substrate to the active site and of diffusion of products from the same site (Guyot et al., 1992). Difficulties in analyzing the structure of supported species and inability to separate polymer-bound impurities are other disadvantages that have contributed to the fact that polymer-bound reagents have not revolutionized industrial organic synthesis (Ford, 1986). 4-Vinylpyridine is a versatile monomer for the synthesis of functionalized polymers. Two other isomers of 4 4 nylpyridine, namely 2-vinylpyridine and 2-methyl-5vinylpyridine, exhibit similar properties, but are less frequently used in synthesis. The weakly basic nitrogen atom [pK, = 5.61 (Ellam and Johnson, 1971)l allows for a variety of reactions on vinylpyridine polymers, e.g. reactions with acids and quaternization and complexation of metals. Block and graft copolymers of vinylpyridine are important as emulsifying agents, thermoplastics, and membranes. Other important applications of vinylpyridine polymers is as polymer supports in reaction catalysis and for use as polyelectrolytes (Khan, 1990).
* Author to whom correspondence should be addressed. E-mail:
[email protected]. 0S88-5885/94/2633-0235$04.50/0
Polymer-supported reagents suitable for oxidation of alcohols to corresponding aldehydes and ketones in simple one-step processes have been developed during the last 20 years. Poly(viny1pyridinium chlorochromate) (Fre'chet et al., 1981),poly(viny1pyridiniumdichromate) (Coreyand Schmidt, 1979; Fre'chet et al., 1981), and poly(viny1pyridinium permanganate) (John and Rajasekharan, 1989) are examples of such reagents. The polymer-supported reagents commonly used are cross-linked solid-phase powders that still have to be filtered from the solution after the reaction is completed. A major improvement is to graft the organicreagent onto an inert polymer substrate, which will allow an easy manual removal of the substrate from the reaction vessel. Futhermore, the graft chains are not cross-linked, which increases the accessibility of the active sites. Several workers have reported the use of such graft copolymers as supports (Garnett et al., 1981; Hartley et al., 1982; Akelah et al., 1983) and some have used radiation-induced grafting for the preparation. Radiation grafting of 4-vinylpyridine onto polyolefins has gained widespread interest (Odian et al., 1980;Hartley et al., 1981; Arai et al., 1985; Chuanyin, 1987; Kaur and Barsola, 1990),but 4-vinylpyridine has also been radiation grafted onto several other polymer backbones, e.g. styrene butadiene-styrene copolymer (Yang and Hsiue, 199Oac), polytetrafluoroethylene (Chapiro et al., 19761,poly(viny1 chloride) (Hegazy et al., 1985; El Dessouky et al., 1986) and polypyrrole (Arca et al., 1988). In order to facilitate the introduction of polymersupported reagents to industry one should (1)improve mechanical properties of polymer supports, (2) prepare polymer supports with high accessibility, capacity, and adaptability to flow processes, and (3) support active reagents and catalysts to the novel materials and demonstrate their use in organic synthesis. In this paper we report the preparation of an easily accessible polymer support by radiation grafting of 4 4 nylpyridine onto polypropylene nonwoven fabrics, which have an open architecture compared to films. The oxidation of different alcohols with poly [propylene-graft(4-vinylpyridinium dichromate)] was done with ease compared to a commercially available analog. 0 1994 American Chemical Society
236 Ind. Eng. Chem. Res., Vol. 33, No. 2, 1994
Experimental Section Materials. Polypropylene nonwoven fabrics (20 g/m2) were received from Neste Chemicals (Neste Chemicals, PL 320, 06101 Porvoo, Finland). The fabrics were extracted with chloroform overnight and then stored in an excicator after being dried. 4-Vinylpyridine (95% , purity checked by GC) inhibited with 100-ppm hydroquinone was purchased from Aldrich and used without further purification. Poly(4-vinylpyridiniumdichromate), benzyl alcohol, cinnamyl alcohol, 2-butene-1,4-diol, and iron(I1) sulfate heptahydrate were also purchased from Aldrich and used as received. Cyclohexanolwas purchased from Fluka. Chromium(V1) oxide, cyclohexane, chloroform, phosphoric acid, and sulfuric acid were purchased from Merck. Irradiation. The fabrics were irradiated under nitrogen atmosphere (02 < 300 ppm) using an Electrocurtain electron accelerator (Energy Sciences Inc.), operating at a beam energy of 175 keV. Preparation of Poly[propylene-graft-(4-vinylpyridine)] (Abbreviated as PP-PVP). Extracted and preweighed polypropylene nonwoven fabrics were irradiated with doses up to 500 kGy and then transferred to glass flasks where 4-vinylpyridine (neat)had been purged with nitrogen for more than 30 min a t ambient temperature. After stopping the reaction the grafted samples were extracted with chloroform for 24 h to remove any unreacted monomer and homopolymer. The grafted samples were then dried in an oven at 50 "C until a stable weight was reached. The percentage of grafting was calculated from the initial increase in weight of the original sample by the equation extent of grafting (% ) = m2-m l x 100 m1 where ml and m2 are the weights of original sample and grafted sample, respectively. Functionalization of PP-PVP with Chromium Trioxide (Abbreviated as PP-PVPDC). The PP-PVP fabric was added to a solution of chromium trioxide in water and left stirring at ambient temperature overnight. The stirring was stopped after 16 h after which the fabric was washed repeatedly with water until the filtrate was clear. The fabric was then used without drying or dried for storage. Oxidations Using PP-PVPDC. The followinggeneral procedure for oxidation of alcohols was used. The PPPVPDC fabrics were soaked in water for 5 min, blotted dry, and transferred to glass tubes with 4 mL of cyclohexane. The desired alcohol was then added in millimolar quantities and stirred at 75 "C for several hours. The experiments with 2-butene-1,4-diolwere made in the same way at ambient temperature but using water as solvent. To evaluate the performance of this polypropylenesupported reagent in oxidation reactions, experiments with the commercially available PVPDC reagent from Aldrich were also made. In these experiments the PVPDC resin was soaked in water for 30 min and filtered prior to use. Analysis. The conversion of the reactions were determined by gas chromatography using a Varian 1400 gas chromatograph equipped with a DB1 column (length 30 m, inner diameter 0.32 mm, film thickness 0.25 pm) from J&W Scientific. The reaction producb were also identified by GC-MS using a Hewlett Packard 5890 gas chromatograph equipped with a HP1 column (length 25 m, inner diameter 0.32 mm, film thickness 0.17 pm) together with a Hewlett Packard 5970 mass selective detector. 1,2-
Figure 1. The structure of poly(4-vinylpyridinium dichromate). Table 1. Atomic Surface Concentrations of Elements in the 650% Grafted Fabric Analyzed by ESCA concentration Cr/N Cr/O (%) Cr/N Cr/O (theor) (theor) element C 77.37 0.85 0.27 1.00 0.29 N 4.56 0 14.19 Cr 3.88
Dichlorobenzene was used as internal standard for quantitative gas chromatographic analysis. Determination of the surface atomic concentration of carbon, oxygen, nitrogen, and chromium in the fabric was made on ground materials using a Perkin-Elmer ESCA system. The photoelectrons were generated by X-ray photons of energy 1253.6 eV (Mg K,) from a spot of 1 mm in diameter and analyzed in a PHI spherical capacitor energy analyzer having an Omni Focus 54 lens. The takeoff angle was chosen to be 60 ". Atomic composition data were determined by using sensitivity factors tabulated in the PHI software: Cis, 0.296; Ols, 0.711; Nls, 0.477; and CrzP, 2.427. The amount of dichromate attached to the fabric was alternatively determined by direct titration of the chromate displaced from the fabric by reaction with aqueous 2 M sodium hydroxide overnight. In a typical titration, a freshly prepared solution of ferrous sulfate was used to reduce the chromate after acidification with phosphoric acid and sulfuric acid (Haartman and Harju, 1981). Ferroin was used as an indicator. Nonbonded chromium salts left in the reaction solution after an oxidation reaction were determined using a plasma emission spectrophotometer, Spectra Span 111. N
Results and Discussions The nonwoven fabrics have an open architecture and probably because of this fact they had to be treated with doses of 500 kGy to ensure high extents of grafting. After 6 h the average weight increase of the fabrics were 650%, but the weight increase continued to grow almost linearly until loads up to 1800% were achieved. After extraction with chloroform and drying, the color of the fabrics was slightly yellow. Upon drying the material also became brittle, but the flexibility of the fabric could be restored by immersing in water for a few minutes. The functionalization of PP-PVP with chromium trioxide proceeded smoothly. The formed reagent could then be washed free of nonbonded chromium and be used directly in oxidation reactions or dried for storage. The capacity of the reagent was measured both by titration with a ferrous sulfate solution and with ESCA analysis. The amount of oxidation agent [expressed as mmol ( P V P H ) ~ + C ~ Z O (Figure ~ ~ - 1) per gram of dry polymer] varied with the extent of grafting and was found by titration to be 1.68 mmol/g for 650% grafted fabrics (82.9% ofthe theoreticalvalue) and 1.88mmoVgfor1OOO% grafted fabrics (88.6% of the theoretical value). The ratio of chromium and nitrogen determined from ESCA analysis on 650% grafted fabrics was 0.85 (Table l), which is slightly higher than the result from the titration. The amount of oxygen in the fabric is higher
Ind. Eng. Chem. Res., Vol. 33, No. 2,1994 237 Table 2. Alcohols Used and Their Corresponding Reaction Prodnets in the Different Experiments Using P P - P W D C substrate
- Dichlormethane
product
- Tetrahydrofuran
(3)
0
14)
20
Reaction time (h)
FiureZ. Acompariaonofdifferentsolventsontherateofoxidation of c h a m y l alcohol(O.20 mmol) a t ambient temperature using the PP-PVPDC reagent (0.35 g, 900% grafted fabric).
(5)
-PP-PVPDC
A'
A
A
1
0.18
0.24
0
0
0
Table 3. Oxidation of Various Alcohols with PP-PVPDC in 4 mL of Cyclohexane substrate no. mmol
60
40
- PVPDC
0
0 U
0
20 fabrie (mg)
T ('C)
355
b b
139
0.38
136
0.35
132
1.87
1821C
i (min)
15
conversion (76)
b b
180
29 63 83
ARO ...
9fi
52 52 74 74 74 74 25
31 60 10 60 210 1680 3360
99 74 99 75 99 27
60
_.._ 77
*
O650%graftedfabries. Ambienttemperature. Waterassolvent
than it stoichiometricallyshould be and is probably due to oxidation of the fabric itself. Thedifferentalcoholsused in theoxidationexperiments and the corresponding products are presented in Table 2. During the experiments small amounts of the solution were withdrawn for chromatographic analysis at appro. priate intervals to follow the conversion of the alchols to the correspondingproducts. The results are presented in Table 3. 2-Butene-1,4-diol was converted to 2-(5H)furanone, and no other by products were detected. The other alcohols used were all neatly converted to the corresponding aldehydes. The influence of different solvents on the rate of oxidation of cinnamyl alcohol was also studied and the results are presented in Figure 2. Nonpolar hydrocarbons like cyclohexane gave the most satisfactory results regarding the rate of reaction. The use of more polar solvents resulted in a marked decrease in the rate of reaction. Compared with the commercially available PVPDC the new PP-PVPDC resin performed well in the experiments. By using a 2-fold excess of PP-PVPDC or PVPDC resin less time was required for the former resin to complete the reaction. This is presented in Figure 3. The greatest advantagewith the reagent supported by apolypropylene nonwoven matrix was the ease of separation. The reagent could be manually removed from the solution and no filtration was needed. Very low leakage of chromate ions from the resin was observed. Typically less than 1 ppm of free chromium salts were left in the solution after complete reaction. During the oxidation reactions in organic solvents the
Y
0
10
20
30
40
50
Reaction time (min)
FmreS. AcompariaonoftheefficiencybetweenPP-PWDC(900% grafted fabric) and P W D C in the oxidation of 0.555 mmol benzyl alcohol to benzaldehyde in 4 mL of cyclohexane: (A)PP-PWDC, 0.610 g; (0)PWDC, 0.505 g.
fabricalmost immediatelyturned dark and became brittle. However, the flexibility could be restored by soaking the fabric in water for a few minutes. Because of the favorable characteristics of this supported reagent we feel it would be a strong candidate for use in a flow process. Literature Cited Akelnh. A,; Hefferman, S. B.; Kinpton, S. B.; Shemngton, D. C. Rigid polar composite supporta for w e in d i d phase synthesis. J. Appl. Polym. Sei. 1983,28,3137-3144. Arai, K.; Ogiwara, Y.;S h o t o , T. Behaviorofpoly(4-vinylpyridine) grafts as ligands, introduced by gas phase graft copolymerization into an insoluble polymer. Makromol. Chem. 1985,5,837-840. Ar~,M.;Arca,E.;Yildiz,A.;Guven,0.Preparationofanelect.rcactive copolymer by radiation induced grafting of N-vinyl-&pyridine onto polypyrrole. Radiat. Phys. Chem. 1988,31,647-651. Chapiro, A,; Jendrychowsh-Bonamour, A. M.; Mizrahi, S. Preparation of mosaic membranes by radiochemical grafting in poly. tetrafluoroethylene films. Eur. Polym. J . 1976,12,773-780. Chuanyin, D. Preparation of charge-mosaic membrana by radiation induced graft copolymerization of PE films with styrene and 4-vinylpyridine. Effects of pre-irradiation dosage, atmosphere. storagetime and storagetemperature ofgrafting of44nylpyridine onto PE-films. Desalination 1987,62,275-282. Corey, E. J.; Schmidt, G. Useful procedures for the oxidation of alcohols involving pyridinium dichromate in eprotic media. Tetrahedron Lett. 1979,5,39H02. El-Dessouky, M.M.;Hegazy, E.4. A,; Dessouki, A. M.; El-Saw, N. M. Electrical conductivity of anionic graft copolymers obtained by radiation grafting of 4-vinylpyridineonto poly(viny1chloride). Radiat. Phys. Chem. 1986,26, (No.2). 157-163. Ellam, G. B.; Johnson, C. D. Substituent effects on the basicity of pyridine. Elucidation of the electroniccharacter of @substituted vinyl groups. J. Org. Chem. 1971,36,2284-2288. Ford,T. W. Polymerichagents andCatalysts. PolymericReagents ond Catalysts: An overview; Ford, T. W., Ed.; ACS Symposium Series, 308; American Chemical Society: Washington, DC, 1986; pp 1-16.
238 Ind. Eng. Chem. Res., Vol. 33, No. 2, 1994 Fre'chet, J. M. J.; Darling, P.; Farrall, M. J. Poly(viny1pyridinium dichromate): An inexpensive recyclable polymeric reagent. J. Org. Chem. 1981, 46, 1728-1730. Garnett, J. L.; Levot, R.; Long, M. A. UV and radiation grafting of p-styryldiphenyl-phosphineto synthetic polymers and the use of the resulting copolymers in insolubilisation processes. J.Polym. Sci., Polym. Lett. 1981, 19, 23-28. Guyot, A.; Hodge, P.; Sherrington,D. C.; Widdecke, H. Recent studies aimed at the development of polymer-supported reactants with improved accessibility and capacity. Relatiue Polymers 1992,16, 233-259. Haartman, C.; Harju, J. Manual for quantitative analysis, Abo Akademi University, Finland, 1981. Hartley, F. R.; McCaffrey, D. J. A.; Murray, S.G.; Nicholson, P. N. y-Radiation produced supported metal complex catalysts. 11. Cobalt carbonyl hydroformulation catalysts supported on polypropylene containingpyridine side chains. J.Organomet. Chem. 1981, 206,347-359. Hartley, F. R.; Murray, S.G.; Nicholson, P. N. y-Radiation produced supported metal complex catalysts. 111. Phosphinated polypropylene supports. J. Polym. Sci., Chem. Educ. 1982, 20, 23952408. Hegazy, E.-S. A,; Dessouki, A. M.; El-Dessouky, M. M.; El-Sawy, N. M. Crosslinked grafted PVC obtained by direct radiation grafting. Radiat. Phys. Chem. 1985,26 (No. 2), 143-145. John, K. J.; Rajasekharan Pillai, V. N. Functionalization of crosslinked with poly(4-vinylpyridine) and poly(4-vinylpyridine-co-styrene)
permanganate species: Preparation of poly [I-vinyl(pyridinium permanganate)]^ and their use as oxidizing reagents. J. Polym. Sci., Polym. Chem. Educ. 1989,27,2897-2906. Kaur, I.; Barsola, R. Grafting onto polypropylene. Graft copolymerization of 4-vinylpyridine and binary mixture of 4-vinylpyridine with acrylonitrile by preirradiation method. J. Appl. Polym. Sci. 1990,41, 2067-2076. Khan, I. M. Vinylpyridine polymers. In Concise Encyclopedia of Polymer Science and Engineering; Kroschwitz, J. I., Ed.; Wiley: New York, 1990; pp 1284-1285. Odian, G.; Derman, A.; Imre, K. Diffusion controlled reaction. Radiation graft polymerization of 4-vinylpyridine to polyethylene. J.Polym. Sci., Polym. Chem. Educ. 1980,18,737-748. Yang, J.-M.; Hsiue, G.-H. SBS/VP homograft membrane for oxygen enrichment. Angew. Makromol. Chem. 1990a, 179,99-111. Yang, J.-M.; Hsiue, G.-H. Radiation induced graft polymerization of 4-vinylpyridine to styrene-butadiene-styrene triblock copolymer. J. Appl. Polym. Sci. 1990b, 39, 1475-1484. Yang, J.-M.; Hsiue, G.-H. Oxygen permeation in SBS-g-VPmembrane and effect of facilitated oxygen carrier. J.Appl.Polym. Sci. 1990c, 41, 1141-1150. Received for review May 25, 1993 Revised manuscript received October 12, 1993' Abstract published in Advance A C S Abstracts, December 15, 1993.
