New Polymeric Materials

Chapter 17

Downloaded by UNIV OF MINNESOTA on October 14, 2014 | http://pubs.acs.org Publication Date: September 29, 2005 | doi: 10.1021/bk-2005-0916.ch017

Preparation of Molecularly Imprinted Cross-Linked Copolymers by Thermal Degradation of Poly(methacryl-N,N'-diisopropylurea-co-ethylene glycol dimethacrylate) Radivoje Vuković, Ana Erceg Kuzmić, Grozdana Bogdanić, and Dragutin Fleš Research and Development Sector, INA-Industrija nafte d.d., Lovinčićeva bb, P.O. Box 555, 10002 Zagreb, Croatia

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Dedicated to the 70 birthday of Professor Frank E. Karasz

Crosslinked porous copolymers of methacryl-isopropylamide with ethylene glycol dimethacrylate, poly(MA-iPrA-coEDMA), were prepared by thermal degradation of crosslinked copolymers of methacryl-diisopropylurea with ethylene glycol dimethacrylate, poly(MA-DiPrU-co-EDMA), at temperature of 180-250°C in vacuum or under TGA conditions in nitrogen. Nonporous copolymers which represent the model compound of porous copolymers of the same structure were also prepared. Both, porous and nonporous model copolymers have almost the same thermal stability as shown by TGA measurements. At the same time the T of porous and nonporous model copolymers are different and indicate that nonporous copolymers have higher structural ordering. g

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In New Polymeric Materials; Korugic-Karasz, L., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2005.

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Introduction Synthesis of copolymers which contain porous cavities of defined geometry and distribution has attracted a large attention during the last few decades. Of special importance is that cavities in the polymer matrix contain definite recognition sides which have the affinity to the selected analytes. These polymers are mostly applied as enantioselective or structurally selective carriers in the chromatographic separations, as mimics of enzymes, or as carriers of drugs or catalyst in organic reactions or in biomedical applications. Of special interest is the use of porous copolymers as sensors in thefieldin which polymers are used as substitutes for biological materials (1-5). Molecularly imprinted polymers, which contain specific recognition groups, can be prepared by several well-described procedures. Two basically different methods are mostly used: thefirstmethod involves the formation of covalent adduct between print molecule and the functional monomers (d), while the second approach involves the use of non-covalent interaction between print molecules and functional monomers (7). One problem in both covalent and non-covalent molecular imprint procedure is the separation of imprint molecules from the template after the copoiymerizatioa Although the amount of templates, which after the extraction contain imprint molecules is small, and amounts to less than 1%, it usually causes difficulties in the analytical application of imprinted molecules (8). A new method, which enables the preparation of crosslinked molecularly imprinted polymers, was recently developed in our Laboratories. The method is based on the thermal degradation of crosslinked copolymers of acryl-and methacryl-disubstituted urea copolymerized with EDMA (9-11). Thefirstporous copolymers obtained by thermal degradation of crosslinked copolymers of disubstituted urea were prepared from the poly(acryldicyclohexylurea-co-EDMA), (9JO). Thermal degradation was performed by heating the crosslinked copolymer at temperature of 180-450°C. The decomposition proceeds by a two-step mechanism under the separation of volatile cyclohexylisocyanate (C H NCO), at temperature of 180-250°C, and formation of cavities in the crosslinked matrix. The amount of volatile fraction closely correlates to the amount of terminal cyclohexylamine in urea. -CONHQH„. Following the same experimental procedure we have recently prepared porous copolymers by thermal decomposition of methacryl-dicyclohexylurea (MA-DCU) with EDMA under the separation of QHnNCO (11). 6

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In continuation of our work on the thermal degradation of crosslinked copolymers which contain disubstituted urea derivatives, we prepared the porous copolymers of methacryl-isopropylamide (MA-iPrA) with EDMA, under the separation of isopropylisocyanate (iPrNCO).


Experimental and Discussion of Results Synthesis of N-MethaciyI-N,N'-diisopiOpyIuiea (MA-DiPrU) The title compound was prepared by condensation of 6.3 g (0.05 mol) of diisopropyicarbodiimide in 10 mL of tetrahydrofuran with 4.3 g (0.05 mol) of M A A and 0.2 g hydroquinone in 10 mL of THF at room temperature. The crystalline diisopropylurea was filtered off, the mother liquor evaporated to dryness and treated with 30 mL of petroleum ether (bp. 40-60°C) yielding 3.4 g (32.1%) of MA-DiPrU melting at 77-78°C. Analysis ofMA-DiPrU: Calculated for CuH N O (212) (%): C, 62.26; H, 9.43; N, 13.21; Found: C, 62.18; H, 9.43; N, 13.06. The monomer structure was determined by NMR spectroscopy measurements. 20

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Copolymerization of MA-DiPrU with EDMA at Molar Ratio of 0.5 to 0.5 in the Feed To a mixture of 0.636 g (0.003 mol) ofMA-DiPrU and 0.594 g (0.003 mol) of EDMA was added 6 mL of butanone which contains 2% (0.025 g) of BZ2O2 as initiator, and the reaction mixture was heated for 48 hours at 70°C in a stream of nitrogen After cooling, the copolymer was washed with butanone, followed by methanol yielding 0.832 g (67.7%) of the crosslinked copolymer. Under the described conditions of copolymerization, the ratio of comonomers in the crosslinked copolymer is 0.26 molar ratio of MA-DiPrU and the molar ratio of EDMA is 0.74. Calculated amount of nitrogen in copolymer is 3.66% while the experimental value of nitrogen in copolymer has a value of 3.44%. Thermogram of crosslinked copolymer of MA-DiPrU-co-EDMA at a monomer-to-monomer ratio of 0.5 to 0.5 in the feed is shown in Figure 1. It is


232 evident that the copolymer decomposes by a two-step mechanism losing 11.5% of weight at 250°C.


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—,— —,—ι—,—,—,—,—,—ι—ι—j(

I 1 1 t_J ι I , I ι 1 . 1 1 0 0 1 S D 2 0 0 2 5 D 3 0 0 3 S ) 4 B 4 5 D

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Figure L Thermogram of crosslinked copolymer ofMA-DiPrU with EDMA at molar ratio of 0.26 to 0.74 in copolymer (0.5 to 0.5 in thefeed).

Preparation of Porous Crosslinked Copolymer of Methacryl-isopropylamide (MA-iPrA) with EDMA by Thermal Degradation of Poly(MA-DiPrU-coEDMA) Porous copolymers MA-iPrA with EDMA were prepared by thermal degradation of pofy(MA-DiPrU-co-EDMA) in vacuum or in thermogravimetric analyzer in nttrogea In vacuum experiment, a sample of 0.702 g of crosslinked copolymer poly(MA-DiPrU-co-EDMA) prepared at monomer ratios of 0.5 to 0.5 in the feed, was heated for 60 min at 200-210°C in vacuum of 0.26 kPa, yielding 0.615 g (87.60%) of solid residue and 0.087 g (12.40%) of volatile fraction identified as iPrNCO. Elemental analysis of solid residue (%): C, 59.83; H, 7.55; N , 1.97. Based on nitrogen the copolymer contains 0.18 molarfractionof MA-iPrA Thermal degradation of the same copolymer was also performed in the stream of nitrogen at heating rate of 10°C/min in TGA instrument: A sample of



233 2.206 mg of copolymer was heated for 5 mm at 250°C yielding after cooling 1.940 mg (87.94%) of solid porous copolymer and 0.266 mg (12.06%) of volatile fraction. Volatile fraction was calculated as the difference to 100% of starting copolymer sample. Elemental analysis of solid residue (%): C, 59.16; H, 7.35; N , 2.16. Based on nitrogen the copolymer contains 0.19 molarfractionof MA-iPrA. It is of interest to note that the amount of volatile fraction obtained by thermal degradation of MA-DiPrU-co-EDMA in vacuum or by the discontinuous measurements in thermogmvimetric analyzer is practically the same: 12.40% and 12.06% respectively. Closely related is also the loss of weight by TGA measurement at 250°C shown in Figure 1, which amounts 11.50%. In order to compare the proparties of porous crosslinked copolymers with those of nonporous copolymer of the same composition, we prepared the nonporous model compound: pory(MA-iPrA-co-EDMA) by coporymerization of MA-iPrA with EDMA.

Synthesis of Methacyl-isopropylamide (MA-iPrA) To a solution of 5.9 g (0.08 mol) of isopropylamine in 30 mL of ether was added 15.6 g (0.1 mol) of MA-anhydride and the reaction mixture was left overnight at room temperature. The ether solution was washed with aq. NaOH solution, followed by water, dried with Na S0 and ether was evaporated to dryness. The residue was treated with petrolether yielding 2.09 g (20.0%) of MA-iPrA; m.p. 89-90°C. The structure of the product was proved by elemental analysis and by NMR spectroscopy. Analysis ofMA-iPrA: Calculated for C H NO (127) (%)*. N, 11.02; Found: N, 10.59. 2

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Copolymerization of MA-iPrA with EDMA at Molar Ratio of 0.5 to 0.5 in the Feed (Nonporous Model Compound) Comonomers mixture of 0.127 g (0.001 mol) of MA-iPrA with 0.198 g (0.001 mol) of EDMA in 2 mL, of butanone with 2% BZ2O2 was heated for 48 hours at 70°C under a stream of nitrogen. Crude polymer was washed with butanone, yielding 0.295 g (90.67%) of crosslinked polymers. Elemental analysis (%): C, 59.85; H, 8.12; N , 3.67. Based on the nitrogen content, the copolymer contains of 0.33 molar ratio of MA-iPrA and 0.67 ratio of EDMA. Thermograms of porous copolymers of MA-iPrA with EDMA obtained as a solid residue after the removal of iPrNCO from poly(MA-iPrU-co-EDMA)


234 prepared at molar ratio of comonomer of 0.5 to 0.5 in thefeed,and nonporous model compounds prepared at the same molar ratio of 0.5 to 0.5 in the feed of MA-iPrA with EDMA are shown in Figure 2. Thermograms in Figure 2 indicate that both copolymers have similar thermal stability, thus indicating that the porous structure has no significant influence to the thermal stability of crosslinked copolymers. In order to prove the difference between crosslinked copolymers prepared by thermal degradation of MA-DiPrU-co-EDMA and model copolymers MA-iPrAco-EDMA, we compared T 's of the mentioned crosslinked copolymers. It is shown that the T of copolymer obtained after the removal of iPrNCO is 207°C, while the T of model compound exhibits the value of 190°C and a transition at 267°C, thus indicating that model compound has higher structural ordering. Preliminary results obtained by comparison of x-ray diffractograms of both porous and nonporous copolymers, also indicates the higher structural ordering of model copolymers. It is of interest to note that in the preliminary experiments, it is shown that the density of crosslinked poly(MA-DiPrU-co-EDMA) is higher than that of porous crosslinked poly(MA-iPrA-co-EDMA). g

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Figure 2. Thermograms ofporous residue obtained after removal ofiPrNCO from the copolymer ofMA-DiPrU with EDMA (1) and model copolymer of MA iPrA with EDMA (2). Both copolymers are prepared at molar ratios of 0.5 to 0.5 of comonomers in thefeed.


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The possible mechanism of the thermal degradation of poly(MA-DiPrU-coEDMÂ) is shown in Scheme 1.

Scheme L Mechanism of thermal degradation of crosslinked copolymers ofMADiPrU with EDMA. By applying the coiled structure of the terminal chain in repeating unit of the MA-DiPrU trapped in the crosslinked copolymer matrix, the numbering of atoms can be represented by die Rule of Six proposed by Newman (12) as indicated by the Scheme la. As represented by the structure of the terminal unit in the Scheme la, the hydrogen atom 6 is close to the nitrogen atom 3 and at the temperature of about 200°C hydrogen 6 reacts with iPrN= 3 thus causing the separation of iPrNCO as shown in the Scheme lb. In Experimental part of this article is shown that iPrNCO which has a boiling point of 74°C can be easily removed in vacuum at a temperature of200-210°C, or by heating the crosslinked copolymer at 250°C in nitrogen After the removal of volatile iPrNCO, the nanopores, which contain the recognition groups -CO-NHCH(CH ) are formed, thus indicating that the 3

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236 obtained porous copolymers described in this article could be of interest in various practical applications. It is further of interest to note that since various N,N'-disubstituted ureas can be easily prepared, the procedure in the article can be considered as a general method for the preparation of porous crosslinked copolymers which contain the desired recognition groups in pores.


Conclusion Copolymer of MA-DiPrU with EDMA at the monomer-to-monomer molar ratio of 0.5 to 0.5 in the feed was prepared. The yield of crosslinked copolymer, which contains 0.26 to 0.74 molar ratios of comonomers, was 67.7%. In the thermogravimetric analysis it is shown that the prepared crosslinked copolymer decomposes by two-step mechanism under almost quantitative evaporation of isopropylisocyanate (iPrNCO) at a temperature of 250°C, The residue obtained after the removal of iPrNCO is thermally stable, and decomposes by a one-step mechanism between 280 and 450°C After the removal of volatile iPrNCo, the crosslinked porous copolymer which contains the recognition groups -CO-NHCH(CH )2 are formed. In view of the possibility to modify the copolymer composition and geometry of Ν,Ν^-disubstituted urea, we consider that these copolymers may be of interest as host specific for desired molecules for various applications, especially in the thin layer chromatography or as carriers of various functional analytes. 3

Acknowledgement The Ministry of Science and Technology of Croatia supported this work.

References 1. Mosbach, Κ. Molecular Imprinting. Trends Biochem. Sci. 1994, 19, 914. 2. Shea, J. K. Molecular Imprinting of Synthetic Network Polymers: The De Novo Synthesis of Macromolecular Binding and Catalytic Sites. Trends Polym. Sci. 1994, 2(5), 166-173. 3. Kempe, M.; Mosbach, K. Molecular Imprinting Used for Chiral Separations. J. Chromatogr. 1995, A694, 3-13.



237 4. Wulff, G. Molecular Imprinting in Cross-Linked Materials with Aid of Molecular Templates-Α Way Toward Artificial Antibodies. Angew. Chem. Int. Ed.Engl.1995, 34, 1812-1832. 5. Muldoon, T. M.; Stanker, H. L. Plastic Antibodies: MolecularlyImprinted Polymers. Chem. Ind. 1996, 24, 204-207. 6. Wulff G.; Biffis, A. Molecular Imprinting with Covalent or Stoichiometric Non-covalent Interactions. In: Molecularly Imprinted Polymers: Man-Made Mimics of Antibodies and Their Applications in Analytical Chemistry. Ed.; Sellergren, B. Elsevier: Amsterdam. 2001., pp. 71-111. 7. Sellergren, B. The Noncovalent Approach to Molecular Imprinting. In: Molecularly Imprinted Polymers: Man-Made Mimics of Antibodies and Their Applications in Analytical Chemistry.Ed.;Sellergren, B. Elsevier: Amsterdam. 2001., pp. 113-184. 8. Sellergren, B.; Shea, J. K. Influence of Polymer Morphology on the Ability of Imprinted Network Polymers to Resolve Enantiomers. J. Chromatogr. 1993, 635, 31-49. 9. Erceg Kuzmić, A.; Vuković, R; Bogdanić, G.; Fleš, D. Separation of Cyclohexylisocyanate from the Crosslinked Copolymers of N-AcrylDicyclohexylurea with Ethylene Glycol Dimethacrylate or Divinyl Benzene. J.Macromol.Sci.-Pure Appl.Chem. 2003, A40(1), 81-85. 10. Erceg Kuzmić, A.; Vuković,R.;Bogdanić, G.; Fleš, D. Synthesis of Nanoporous Crosslinked Poly(acryl-N-cyclohexylamide-co-ethylene glycol dimethacrylate) by Thermal Degradation of Poly(acryl-N,N'dicyclohexylurea-co-ethylene glycol dimethacrylate). J. Macromol. Sci.PureAppl.Chem. 2003, 40(8), 747-754. 11. Erceg Kuzmić, A.; Vuković,R.;Bogdanić, G.; Fleš, D. Preparation of Nanoporous Crosslinked Poly(Methacryl-N-cyclohexylamide-co-ethylene glycol dimethacrylate) by Thermal Degradation of Poly(Methacryl-N,N'-dicyclohexylurea-co-ethylene glycol dimethacrylate). J. Macromol. Sci.-Pure Appl. Chem. 2004, A41(8)(in Press.). 12. Newman S. Melvin. Some Observations Concerning Steric Factors. J. Am. Chem. Soc. 1950, 72, 4783-4785.


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