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Redalyc.HOMOPOLYMERIZATION of POLY(DIMETHYLSILOXANE Revista Mexicana de Ingeniería Química ISSN: 1665-2738 [email protected] Universidad Autónoma Metropolitana Unidad Iztapalapa México Morales-Cepeda, A.; Möller, M. HOMOPOLYMERIZATION OF POLY(DIMETHYLSILOXANE) MACROMONOMERS VIA FREE RADICAL POLYMERIZATION Revista Mexicana de Ingeniería Química, vol. 6, núm. 2, 2007, pp. 219-228 Universidad Autónoma Metropolitana Unidad Iztapalapa Distrito Federal, México Available in: http://www.redalyc.org/articulo.oa?id=62060211 How to cite Complete issue Scientific Information System More information about this article Network of Scientific Journals from Latin America, the Caribbean, Spain and Portugal Journal's homepage in redalyc.org Non-profit academic project, developed under the open access initiative REVISTA MEXICANA DE INGENIERÍA QUÍMICA Vol. 6, No. 2 (2007) 219-228 AMIDIQ HOMOPOLYMERIZATION OF POLY(DIMETHYLSILOXANE) MACROMONOMERS VIA FREE RADICAL POLYMERIZATION HOMOPOLIMERIZACIÓN DE MACROMONÓMEROS DE POLI(DIMETIL SILOXANO)VÍA POLIMERIZACIÓN POR RADICALES LIBRES A. Morales-Cepeda1* and M. Möller2 1 División de Estudios de Posgrado e Investigación del Instituto Tecnológico de Ciudad Madero Juventino Rosas y Jesús Urueta S/N, col. Los Mangos CP 89318, Ciudad Madero Tamaulipas, Mexico 2 Deutsches Wollforschungsinstitut an der RWTH Aachen, Veltmanplatz 8, D-52062 Aachen, Germany Recibido 19 de Enero 2006; Aceptado 15 de Marzo 2007 Abstract Hompolymers of poly(dymethylsiloxane) terminated with one methyl methacrylate end group (PDMS-MA) were synthesized by conventional free radical polymerization. The crucial step to polymerize the commercial PDMS- MA macromonomers was their purification using a free radical pre-polymerization technique. The separation of homopolymer and unreacted PDMS-MA macromonomer was achieved by precipitation in a mixture of diethyl ether/methanol (1:1). Kinetic studies of homopolymerization yielded the ratio between rate of propagation and ½ termination (kp/kt ). This ratio decreased with increasing the molecular weight of the macromonomers. The apparent rate of propagation was proportional to [I]2.2 and [M] for appropriate polymerizations conditions. The ceiling temperature were calculated for macromonomers with Mn = 1 and 5 Kg/mol, to be Tc= 160°C at [M1]= 0.88 and [M5]= 0.11 mol/L, for both homopolymers. Keywords: Macromonomer, Poly(dimethylsiloxane), free radical polymerization, homopolymerization, purification, pre-polymerization, polymerization, kinetic, ceiling temperature. Resumen Los homopolímeros de poli(dimetilsiloxano) con terminación metil metacrilato (PDMS-MA) fueron sintetizados por medio de polimerización de radicales libres. La purificación de estos macromonómeros PDMS-MA comerciales fue un paso crucial para esta investigación, éste se logró utilizando la técnica de pre-polimerización. La separación del homopolímero y el macromonómero PDMS-MA no reaccionado fue realizada mediante la precipitación de estos en dietil éter/ metanol (1:1). Los estudios cinéticos de la homopolimerización se realizaron con la relación de la velocidad de propagación y terminación (kp/kt½). Esta relación decrece al incrementarse el peso molecular de los macronómeros. La velocidad aparente de propagación fue proporcional a [I]2.2 y [M] para condiciones de polimerización apropiadas. La temperatura máxima de polimerización fue calculada para los macromonómeros con Mn = 1 and 5 Kg/mol, siendo Tc= 160°C para [M1]= 0.88 y [M5]= 0.11 mol/L, para ambos homopolímeros. Palabras clave: Macromonómero, poli(dimetil siloxano), polimerización de radicales libres, hompolimerización, purificación, pre-polimerización, cinética, temperatura máxima. 1. Introduction used in ink-paint and in the coating industry (Corey and Corey, 1998; Szycher, 1983; Koerner et al., Polyorganosiloxanes are industrially produced 1991). They can also be used as additives in in large quantities for various purposes depending on lubricants for auto and oil industry. The inherent molecular weight and composition. The hydrophobic nature of siloxane coupled with ability Poly(dimethylsiloxane) (PDMS) is the most widely to accumulate on the surface of polymers blends has used inorganic polymer due its thermal stability, low allowed the poly(dimethylsiloxane) to be used flammability, low viscosity change vs. temperature, widely to hydrophobice the surface of other low surface tension, high compressibility, shear materials, particularly in the case of polymers which stability, dielectric stability and low toxicity. The by themselves lack of such properties (LeGrand and highly viscous homopolymers have applications in Gaines, 1970; Gaines, 1981; Azrak, 1974; Pennings the personal care arena, e.g. in cosmetics. They are and Bosman, 1980). * Autor para la correspondencia: E-mail: [email protected] 219 Publicado por la Academia Mexicana de Investigación y Docencia en Ingeniería Química A.C. Morales-Cepeda and M. Möller / Revista Mexicana de Ingeniería Química Vol. 6, No. 2 (2007) 219-228 PDMS-MA macromonomers can be thermodynamic studies, ceiling temperature synthesized by anionic polymerization, and free estimation. radical polymerization has so far been unable to produce homopolymers of polydimethylsiloxane. 2. Experimental Only one patent has been reported by Gornowicz et al. (1998) for the homopolymerization of acrylate or 2.1 Materials and methods: methacrylate functionalized polydiorganosiloxane by means of emulsion polymerization as a method to The molecular weight of the obtain the homopolymer. They used different poly(dimethylsiloxane) macromonomer was varied surfactants (ionic and anionic) and AIBN as the between Mn=1, 5 and 10 kg/mol (supplied by Chisso initiator. After 68 hours of reaction time, they and Shin-Etsu). The macromonomers were purified obtained a product with high conversion of 82-wt%. by filtration over a two-layer column of silica gel (20 The size exclusion chromatography (SEC) data in cm) and aluminum oxide (Al2O3) (20 cm) using toluene and chloroform showed bimodal peak absolute chloroform as the mobile phase. 2,2’- distributions i.e. the presence of un-reacted PDMS- Azobisisobutyronitrile (AIBN, Aldrich) and 1,1’- MA. azobiscyclohexane-1-carbonitrile (CAN, Aldrich) Several kinetic aspects will need to be were re-crystallized from methanol, tert-butyl considered before homopolymerizing the peroxide (TBP, Aldrich) was re-crystallized from macromonomer: chloroform and dimethyl 2,2’ –azobisisobutyrate (V- Their molecular weight being rather high 601, Wako) was used as received. The solvents 3 4 (10 <M<10 ), in most cases, the molar concentration benzene and toluene were distilled over LiAlH2, of the polymerizable group that can be used in methanol and diethyl ether were used after polymerization experiments is bound to be low (~ 1 distillation (using Mg and LiAlH2 as drying agents … 0.1 mmol/g macromonomer). respectively). The intrinsic reactivity of the terminal reactive SEC chromatography: was performed in double bond (characterized by propagation rate chloroform (for analysis grade) using refractive constant kp) may be smaller than that in a low index (RI, Waters) as detector and a Waters-515 molecular weight monomer exhibiting the same pump (flow rate = 1 mL/min) connected to μ- degree of instauration. Styragel Waters columns (106, 105, 104, 103 Å If the repeating unit of the macromonomer can nominal pore size). 20mg of the reaction mixture be a target prone to transfer reactions, the probability were dissolved in 1-mL of chloroform and filtered for such side reactions to occur increases with the through 0.45-μL Spartan filters to remove any gel degree polymerization of the macromonomer fraction prior to injection. (Tsukahara, 1993). To measure the conversion of It could be expected that the rate of poly(dimethylsiloxane) (PDMS-MA) macromono- polymerization of macromonomers is reduced not mers, the relationship between the polymer only by the low concentration of the active sites in a concentrations and their relative intensities in the macromonomer solution, but also to the lower SEC was studied. The refractive index (RI) of accessibility of the growing radical end groups to the PDMS-MA (nD ≈ 1.42) is lower than that of terminal unsaturated bonds further macromonomer chloroform (nD ≈ 1.446), hence the RI signal of the molecules. PDMS-MA macromonomer was negative in the On the other hand, the bimolecular chloroform-chromatogram. Since the refractive index termination reaction is hindered by the sterical of poly (PDMS-MA) (nD ≈ 1.42) is very close to that demands of the macromolecular units attached to the of chloroform, it appeared as a small positive signal reactive sites. in SEC traces. The monomer conversion was For this propose the investigation of the determined by calculating the decrease in the area of homopolymerization behavior of PDMS-MA the SEC peaks of the macromonomer throughout the macromonomers by conventional free radical polymerization. polymerization was evaluated. Polymerization of 1H-NMR measurements were performed with PDMS-MA macromonomer bearing methyl a Bruker 400 MHz spectrometer, in CDCl3 as the methacrylate end group was studied in homogeneous solvent. A concentration of compound 20-30 mg/mL solution with respect to the following points: (i) was applied. characterization of commercial available macro- Matrix-assisted laser desorption/ionization monomers, (ii) optimization of homopolymerization time of flight mass spectrometry (MALDI-TOF MS) conditions, (iii) separation of homopolymer from analysis: The sample was analyzed using a Bruker- unreacted macromonomer,
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