Monomers for non-bond crosslinking of vinyl polymers

Monomers for non-bond crosslinking of vinyl polymers

PERGAMON European Polymer Journal 35 (1999) 1159±1164 Monomers for non-bond crosslinking of vinyl polymers Anat Zada, Yair Avny, Albert Zilkha * Dep...

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PERGAMON

European Polymer Journal 35 (1999) 1159±1164

Monomers for non-bond crosslinking of vinyl polymers Anat Zada, Yair Avny, Albert Zilkha * Department of Organic Chemistry, The Hebrew University of Jerusalem, Jerusalem 91904, Israel Received 28 May 1997; accepted 16 October 1997

Abstract Monomers having on the one hand a polymerizable double bond and on the other hand a large macrocyclic ring (nr27±28) through which a growing chain may be threaded are shown to be a new type of crosslinking agents which lead to non-bond crosslinking of vinyl polymers. This is demonstrated by cyclic octaethylene glycol fumarate, a 29-membered ring obtained in the reaction of fumaryl chloride with octaethylene glycol under high dilution conditions, which upon copolymerization with styrene or methylmethacrylate led to crosslinked polymers. # 1999 Elsevier Science Ltd. All rights reserved.

1. Introduction The classical method of crosslinking vinyl polymers is copolymerization with a monomer containing two or more polymerizable double bonds, e.g. divinyl benzene or N,N 0 -methylene bisacrylamide. Since it is known [1] that chains may be threaded into macrocyclic rings through rotaxane formation, it seemed that a new type of crosslinking agent may be obtained by using a monomer (A) having on the one hand a polymerizable double bond and on the other hand a large macrocyclic ring, through which a polymer chain may be threaded in the course of the propagation reaction.

Calculations [2] have shown that for threading a straight paranic chain into a ring, a 22-membered cyclic structure is required. It can also be shown from

* Corresponding author.

molecular models that for threading a polystyrene or a polymethyl methacrylate chain a 27±28 membered ring may be required. This will lead to a new type of nonbond crosslinking of vinyl polymers (Fig. 1). This type of crosslinking will allow more degrees of freedom in the movement of segments in the crosslinked polymers, which may re¯ect in improved mechanical properties as regards impact strength or resisting stresses, as well as better swelling properties. These properties may ®nd use in improved ion exchange resins, gels for electrophoresis, hydrogels, hydrophilic contact lenses, Merri®eld resins for automated synthesis of peptides and nucleic acids, etc. Recently [3] it was shown in the condensation reaction of a diacid with a diamine, where one of them contained a large macrocyclic ring, that crosslinked insoluble polymers were formed due to threading. Preliminary information on some of the experimental work has been reported elsewhere [4].

2. Experimental 2.1. Materials Toluene was dried by azeotropic distillation and by standing over sodium. Dioxane was dried over sodium.

0014-3057/99/$ - see front matter # 1999 Elsevier Science Ltd. All rights reserved. PII: S 0 0 1 4 - 3 0 5 7 ( 9 8 ) 0 0 0 3 9 - 1

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Fig. 1. Non-bond crosslinking of polymer chains.

Polyethylene glycol PEG 400 (Aldrich) was dried in vacuo at 1308C. Octaethylene glycol was prepared [5] and similarly dried. Fumaryl chloride (Fluka) was used as received. Itaconyl chloride was prepared from itaconic acid and PCl5 [6]. 2.2. Instrumentation NMR spectra were recorded on a Bruker AMX-300 instrument. Mass spectra were carried out using chemical ionization. A Sage metering pump model 365 was used. Computer molecular modeling was carried out using the ``Macromodel-Interactive Molecular Modeling System'' from Columbia Innovation Enterprise. 2.3. Reaction between itaconyl chloride and PEG-400 Freshly distilled itaconyl chloride (898C/9 mm) (15 g, 0.09 mol) and PEG-400 (36 g, 0.09 mol) were both diluted to a volume of 42 ml with toluene, transferred to syringes and dropped simultaneously from a Sage

metering pump into a 3-necked ¯ask containing toluene (400 ml) and ®tted with a magnetic stirrer, a re¯ux condenser with a CaCl2 guard tube and a gas inlet for bubbling N2. The ¯ask was heated in an oil bath at 758C. The reactants were added during 8.5 h and the mixture was heated for another 40 h. It was evaporated in vacuo and the residue (45 g) was dissolved in 45 ml ethyl acetate and was chromatographed on a column (3 cm diameter) packed with neutral alumina (230 g). Elution was carried out with a mixture of 80% ethyl acetate and 20% petroleum ether (40±608C). The fractionation was followed by TLC using alumina plates (Merck), the developing solvent was a mixture of 90% ethyl acetate and 10% ethanol and visualization was by iodine. Under these conditions PEG-400 shows Rf = 0.32; the ®rst fractions eluted showed Rf = 0.72, probably smaller rings, but later these were accompanied by other fractions having Rf = 0.55. No PEG-400 was eluted. The product (mass spectrum, Fig. 2) showed a tendency to polymerize on standing, so it was stored at low temperature. 2.4. Reaction between fumaryl chloride and PEG-400 Fumaryl chloride (4.1 g, 0.027 mol) and PEG-400 (10.7 g, 0.027 mol) each dissolved in toluene (40 ml) were dropped simultaneously during 20 h from syringes using a Sage metering pump into a 3-necked ¯ask containing dry toluene (200 ml). The mixture was stirred at 808C under nitrogen for another 48 h. TLC of the mixture on alumina with dioxane as a developing solvent showed a spot near the origin (probably a polymer fraction) and a long spot Rf = 0.8. PEG-400 under these conditions has Rf = 0.42. The mixture was

Fig. 2. Mass spectrum of cyclic esters formed in the reaction of PEG-400 with itaconyl chloride. Numbers in parentheses indicate the number of EO units in the macrocycle.

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1

evaporated to dryness, taken up in ethyl acetate and chromatographed on a column containing alumina (130 g) using ethyl acetate as eluent. The ®rst fractions that came out showed one spot in TLC, Rf = 0.8, yield 3.2 g. The mass spectrum (Fig. 3) showed formation of the cyclic products. A further 3 g of product were eluted in the later fractions, they showed Rf = 0.8 but were contaminated with dimeric products.

H-NMR (CDCl3): 6.91 (s, CH.CH, 2H), 4.37 (t, COOCH2, 4H), 3.76 (t, COOCH2CH2, 4H), 3.66 (m, OCH2CH2O, 24H). The mass spectrum gave M + + 1 at m/e 451 as required. Anal. calcd. for C20H34O11: C, 53.30; H, 7.61. Found: C, 52.95; H, 7.64.

2.5. Cyclic octaethylene glycol fumarate (B)

Dibenzoyl peroxide (5 mg, 210 ÿ 5 mol) was dissolved in the cyclic ester (100 mg, 2.1710 ÿ 4 mol), the solution purged with nitrogen and left for 3±4 days at 608C. The polymer obtained was soft and it was soluble in various solvents.

Fumaryl chloride (2.6 ml, 23.5 mmol) and dry octaethylene glycol (8.7 g, 23.5 mmol) each dissolved in dry dioxane (20 ml) were added dropwise simultaneously from syringes using a Sage metering pump during 72 h onto dry toluene (200 ml). The mixture was stirred under nitrogen for another 24 h and evaporated in vacuo at 40±508C. The residue (12 g) was puri®ed in two steps, the ®rst being a rough separation from polymeric products. It was taken up in a small volume of ethyl acetate and puri®ed on a column (3 cm diameter) packed with alumina (150 g) and eluted with ethyl acetate. The ethyl acetate was evaporated and the residue (3 g) was taken up in ethyl acetate and further chromatographed on a column (2 cm diameter) packed with alumina (75 g) and eluted with a mixture of 60% ethyl acetate±40% petroleum ether. The pure cyclic fumarate ester (1.0 g, 9.4% yield) showed one spot Rf = 0.54 (TLC on alumina developed with ethyl acetate). The later fractions were contaminated with traces of the dimeric cyclic ester which showed Rf = 0.21. The cyclic monomer was stored in the cold.

2.6. Homopolymerization of cyclic octaethylene glycol fumarate (B)

2.7. Copolymerization of the cyclic ester (B) with styrene Cyclic ester (60 mg, 1.310 ÿ 4 mol) was dissolved in styrene (0.1 ml, 8.710 ÿ 4 mol) and dibenzoyl peroxide (10 mg, 410 ÿ 5 mol) was added. The solution was purged with nitrogen and left for 4 days at 608C. The solid copolymer was insoluble and it swelled in various solvents. Other experiments were carried out in which the molar ratio between the two monomers was varied between 4±27 and in all of them crosslinked polymers were formed. 2.8. Copolymerization of the cyclic ester (B) with methyl methacrylate Benzoyl peroxide (15 mg, 610 ÿ 5 mol) was dissolved in a solution of B (100 mg, 2.1710 ÿ 4 mol) in

Fig. 3. Mass spectrum of cyclic esters formed in the reaction of PEG-400 with fumaryl chloride. Numbers in parentheses indicate the number of EO units in the macrocycle.

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methyl methacrylate (0.2 ml, 1.910 ÿ 3 mol). The solution was purged with nitrogen and left for 4 days in an oven at 608C. The solid copolymer was insoluble and swelled in various solvents. 3. Results and discussion Several methods have been developed for the synthesis of rotaxanes, such as the directed synthesis and the statistical synthesis [1]. The latter is the simplest one, whereby chains and macrocyclic rings are mixed together and the threaded molecule, the rotaxane, is formed in yields which depend on statistical control [2]. Schill and co-workers [7] prepared a series of rotaxanes where they showed that the yields increased with increasing size of the macrocyclic ring (n = 21±29) and the length of the threaded chain (n = 10±38). We have previously studied [8] in detail the factors which determine the amount of threading of linear chain molecules in macrocyclic rings, using the statistical threading of poly(ethylene glycols) in ``crown polyethers''. The e€ects of molar ratios of rings to chains, length of chain, radius of the ring and volume of the system were studied and these were correlated in a mathematical expression which showed the e€ect of these factors on the amount of threading. Extensive work has lately been carried out utilizing the threading of growing chains in large macrocyclic rings for the formation of polyrotaxanes [9±13]. We contemplated the use of a monomer of type A, that will serve as a crosslinking agent. On the one hand it should have a polymerizable double bond and on the other hand a large macrocylic ring, through which a propagating chain of a vinyl polymer may be threaded into it (Fig. 1). Similar monomers, most of them having smaller rings that are good for metal complexation, have been synthesized before [14±16]. Now to synthesize macrocyclic rings having up to 18 members is relatively easy, but great diculties are encountered in the synthesis of larger macrocyclic rings (n2 30). The general procedure of preparing such rings is by the high dilution method, whereby difunctional monomers are reacted and there is competition between the cyclization and the polymerization process, high dilution assisting more the monomolecular cyclization reaction. The yields are usually not high because of the side reaction of polymerization. Carothers has demonstrated [17] that it is possible to use a catalytic method for the preparation of macrocyclic rings in high yield without dilution. Thus on heating polyesters in bulk at high temperature (02708C) in the presence of various metallic salt catalysts such as MgCl2, depolymerization accompanied by macrocyclization occurred leading to macrocyclic esters having up to 24 atoms in the ring in about 60% yield.

We have shown before [18] that the same cyclization can be obtained starting with monosuccinates, by-passing the polymerization step. We looked into the possibility of preparing polymerizable cyclic esters using the above method of catalytic cyclization, since it may a€ord a simple relatively high yield method for their preparation. Cyclic esters of maleic or itaconic acid (n 2 30)

may a€ord suitable monomers for polymerization, besides the large cyclic ring will allow threading. It was recently shown [19] that even very bulky fumarate esters are able to undergo free radical homopolymerization as well as copolymerization so that the polymerization of these cyclic monomers should not be a problem. Maleic anhydride was reacted with PEG-400, a cheap available source of diol, average DP = 8.7 at 1008C for 1 h to obtain the mono-PEG ester of maleic acid and its cyclization was tried at high temperature >2308C under high vacuo (0.5 mm), but no pure cyclic ester could be isolated; instead, maleic anhydride distilled out, indicating preferential ring closure to the ®ve-membered ring. We tried the use of a polyester prepared from maleic anhydride and PEG-400 by heating for several hours at 1708C, but here also the polymer with MgCl2 depolymerization occurred on heating and only maleic anhydride but no pure cyclic ester could be isolated. We tried similarly to use itaconic anhydride and PEG-400, but in the depolymerization of the polyester prepared from them at high temperature and high vacuo, no pure cyclic ester could be isolated. After the Carothers method proved unsuccessful, we used the high dilution method for the preparation of the cyclic esters. Itaconyl chloride or fumaryl chloride and PEG-400 dissolved in dry toluene were added dropwise simultaneously from a Sage metering pump into stirred toluene over a period of about 10±20 h. The reaction was carried out in a 3-necked ¯ask ®tted with a magnetic stirrer a re¯ux condenser with a CaCl2 guard tube and ¯ushed continuously with nitrogen. When Et3N or powdered CaCO3 was added to neutralize the HCl gas produced in the reaction, a fast polymerization of itaconyl chloride (or fumaryl chloride) occurred possibly through an intermediate formation of a ketene or diketene:

Due to this side reaction, the reaction between the diacyl chlorides and polyethylene glycol was carried

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out in the absence of any catalyst. The reaction was carried out in an oil bath held at 1108C, for 40 h. The toluene was evaporated in vacuo, the residue was taken up in ethyl acetate and chromatographed on alumina to separate the cyclic monomers. The cyclic monomers polymerized on heating with benzoyl peroxide and gave insoluble copolymers with styrene and methyl methacrylate. Since the insolubility could be due also to copolymerization with any dimer present, we analyzed by mass spectrometry the products formed. PEG-400 itself is a mixture and its analysis by GC showed [20] it to be composed of polyethylene glycols DP = 4±13, the largest fraction being that of DP = 8± 9. Mass spectrometry utilizing chemical ionization techniques to limit fragmentation of the macrocyclic esters showed clearly (Fig. 2) in the case of the product

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obtained from itaconyl chloride, the presence of macrocyclic rings having ring sizes, n = 20±44 formed from 5±13 EO units, the largest fractions being those formed from 7±10 EO units, rings having n = 26±35. In addition, a small fraction of dimers of these macrocyclic monomers were formed having ring sizes (n = 38±65) corresponding to 11±20 EO units. Altogether a mixture of about 17±18 products was formed. Similarly, with fumaryl chloride and PEG-400, cyclic monomers were obtained containing 5±12 EO units (Fig. 3). This is interesting since the trans con®guration of the acyl chloride units did not a€ect appreciably the ring closure. Thus, due to the presence of a small fraction of dimeric cyclic esters containing two double bonds in

Fig. 4. Computer model showing a polystyrene chain threaded into cyclic octaethylene glycol fumarate.

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the ring, it was essential to remove these products in order to make sure that the crosslinking was due only to threading. The separation of this large mixture using various chromatography techniques proved very dicult, so it was decided to use pure octaethylene glycol for the cyclization instead of PEG-400. This would lead to a macrocyclic ring having n = 29 and any dimeric ester that would be formed in a small amount would have n = 58 and it would be much easier to separate them. The reaction between fumaryl chloride and octaethylene glycol was similarly carried out under high dilution conditions. Fumaryl chloride and octaethylene glycol both dissolved in dry dioxane were added dropwise from a Sage metering pump over 72 h at room temperature into dry toluene. The pure cyclic octaethylene glycol fumarate was isolated by column chromatography on alumina using ethyl acetate/petroleum ether as eluent in about 10% yield. Its structure was con®rmed from mass spectra, NMR and elemental analysis.

Cyclic octaethylene glycol fumarate homopolymerized in the presence of benzoyl peroxide at 608C and led to soft low melting polymers which dissolved in various solvents such as toluene, chloroform, THF, DMF and water. The cyclic monomer copolymerized with vinyl monomers such as styrene and methyl methacrylate yielding insoluble crosslinked polymers. Thus the copolymer with styrene swelled in various solvents, including toluene, chloroform, THF and DMF. In toluene the copolymer swelled to about 15 times its size and it remained swollen for months. The crosslinking could have only originated from threading of the vinyl polymer chain in the ring of the fumaryl ester in the copolymer, since no monomer with two polymerizable double bonds was present in the system. It was possible to hydrolyze the insoluble copolymer of styrene and the cyclic ester by swelling it in THF and heating it in the presence of several equivalents of 5 N sodium hydroxide and a few drops of DMSO. This led to cleaving of the ester groups and unthreading of the entangled chains, so that the hydrolysis product became soluble. Computer models (Fig. 4) have actually shown that the ring of cyclic octaethylene glycol fumarate is large

enough to allow threading of a polystyrene or a polymethyl methacrylate chain. Acknowledgements The authors wish to thank Dr Claude N. Cohen from Synergix, Jerusalem, Israel for carrying out the computer modeling. References [1] Schill G. Catenanes, Rotaxanes and Knots. New York: Academic Press, 1971. [2] Harrison IT. J Chem Soc Perkin Trans I 1974;301. [3] Delaviz Y, Gibson HW. Macromolecules 1992;25:4859. [4] Zada A, Avry Y, Zilkha A. American Chemical Society Meeting San Francisco, April 1997. Polymer Preprints 1997;38(1):145. [5] Coudert G, Mpassi M, Gillaumet G, Selve C. Synth Commun 1986;16:19.

[6] Organic Syntheses Collect, Vol. IV. New York: Wiley, 1963. p. 554. [7] Schill G, Beckman N, Schweickert N, Fritz M. Chem Ber 1986;119:2647. [8] Agam G, Graiver D, Zilkha A. J Am Chem Soc 1976;98:5206. [9] Amabilino DB, Parsons IW, Stoddart JF. Trends Polymer Sci 1994;2:146. [10] Raymo FM, Stoddart JF. Trends Polymer Sci 1996;4:208. [11] Gibson HW, Liu S, Lecavalier P, Wu C, Shen YX. J Am Chem Soc 1995;117:852. [12] Gibson HW, Nagvekar D, Yamaguchi N, Bryant WS, Bhattacharjee S. Polymer Preprints 1997;38(1):64. [13] Gibson HW, Nagvekar D, Bryant WS, Powell J, Bhattacharjee S. Polymer Preprints 1997;38(1):115. [14] Kopolow S, HogenEsch TE, Smid J. Macromolecules 1973;6:133. [15] Thompson MD, Bradshaw JS, Nielsen SF, Bishop CT, Cox FT, Pore PE, Maas GC, Izatt RM, Christensen JJ. Tetrahedron 1977;33:3317. [16] Gibson HW, Nagvekar D, Bhattacharjee S. Polymer Preprints 1997;38(1):481. [17] Spangel EW, Carothers WH. J Am Chem Soc 1935;57:929. [18] Bachrach A, Zilkha A. Eur Polym J 1982;18:421. [19] Laschewsky A, Cochin D. Eur Polym J 1994;30:891. [20] Vitali CA, Masci B. Tetrahedron 1989;45:2201.