Hydrogen Bond Interactions and Self-Condensation of Silanol

Chapter 28

Hydrogen Bond Interactions and Self-Condensation of Silanol-Containing Polymers in Polymer Blends and Organic-Inorganic Polymeric Hybrids 1

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Downloaded by UNIV LAVAL on September 28, 2015 | http://pubs.acs.org Publication Date: May 4, 2000 | doi: 10.1021/bk-2000-0729.ch028

Eli M . Pearce , T. K . Kwei , and Shaoxiang L u

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1Department of Chemical Engineering, Chemistry and Materials Science, and the Herman F. Mark Polymer Research Institute, Polytechnic University, Six Metro Tech Center, Brooklyn, N Y 11201 Revlon Research Center, 2121 Route 27, Edison, N J 08818 2

A new convenient polymer modification has been developed to synthesize a series of novel silanol-containing polymers by a selective oxidation of S i — H containing precursor polymers with a dimethyldioxirane solution in acetone. The silanol hydrogen bonding interactions in polymer blends as well as the silanol self-condensation to form siloxane semi-interpenetrating polymer networks in miscible polymer blends and organic-inorganic polymeric hybrids are discussed.

Silanol functional groups have been long recognized as reactive intermediates in silicon chemistry. They are formed in the hydrolysis of silanes with various silicon functional groups, such as halosilanes, alkoxysilanes, etc., and then transformed into siloxanes by spontaneous or catalytic condensation. Owing to this tendency, only a limited number of organosilanols have so far been synthesized. Organosilanols are stronger acids than analogous carbinols. ' They are known to form hydrogen-bonded complexes with phenol, ethers and ketones. On the other hand, the basicity of organosilanol does not prove to be inversely proportional to their acidity as that observed in the case of alcohols. Organosilanols are strongly selfassociated through hydrogen bonds even in very dilute solutions. The self-associated silanol hydrogen bonds are characterizated by a substantial width and great intensity in the silanol stretching vibration region of infrared spectroscopy. The combination of organic polymers with inorganic oxides by means of the sol-gel process is one of the most convenient and attractive ways to prepare 1

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© 2000 American Chemical Society

In Silicones and Silicone-Modified Materials; Clarson, S., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2000.

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organic-inorganic hybrids. " The incorporation of organic polymers into inorganic silicone oxide is of particular interest. Silanol groups produced in hydrolysis tend to further condense to form siloxane networks. On the other hand, silanols or residual silanols on the surface of silicon oxide micro-spheres readily form hydrogen bonds with hydrogen bond acceptor groups in the presence of organic polymers. It is shown that those hydrogen bond interactions between silanol groups and organic polymers are driving forces for formation of these hybrids, and influence the morphology and structure-property behavior of hybrids. In this chapter, we describe a new convenient polymer modification by the selective oxidation of Si—H containing polymers to synthesize silanol-containing polymers. The studies of silanol-containing polymer blends through silanol hydrogen bonding interactions and the self-condensation of silanols in either miscible hydrogen bonded polymer blends or organic-inorganic polymeric hybrids. Experimental Homopolymerization of 4-VinyIphenyldialkyl/arylsiIane. 4 g 4-vinylphenyldialkyl/arylsilane in 5 ml benzene was introduced into an ampule of 20 ml capacity in the presence of 2,2'-azobisisobutyronitrile (AIBN) (0.2 mole %). The ampule was thoroughly degassed by three freeze-thaw cycles, sealed in an argon atmosphere and then polymerized at 60 °C for 24 h. The resulting polymer was dissolved in methylene chloride and precipitated into methanol twice. A white polymer was obtained with yield greater than 75 % after dried under vacuum at 40 °C for 24 h. Copolymerization of 4-VinyIphenyIdialkyl/arylsilane With Styrene. About 20 g of a mixture of 4-vinylphenyldialkyl/arylsilane and styrene was introduced into an ampule of 25 ml capacity in the presence of AIBN (0.2 mole%). The polymerization procedure was the same as that used for homopolymerization. However, the conversion was limited to about 10% (about 4 h). The copolymer was isolated by precipitation of the methylene chloride solution into methanol and dried under vacuum at 40 °C for 6 h. Preparation of a Dimethyldioxirane Solution in Acetone. A 500 ml three-necked round-bottomed flask, containing a mixture of twice-distilled water (50 ml), acetone (40 ml), sodium hydrogen carbonate (24 g) and a magnetic stirring bar, was equipped with a gas inlet tube extending into the reaction mixture and an air condenser loosely packed with glass wool. The exit of the air condenser was connected to a condenser filled with dry ice-isopropanol. The bottom of the dry ice condenser was attached in succession to a receiving flask (50 ml) and two cold traps, all being kept in a dry iceacetone baths. Argon gas was gently passed through the reaction vessel. While applying a slight vacuum (180~220 Torr), 50 g potassium peroxomonosulfate (Oxone) was added quickly in one portion with vigorous stirring at room temperature. The slightly yellow solution (ca. 30 ml) of dimethyldioxirane in acetone (0.06 ~ 0.08 M) was collected in a receiving flask during a 45 minute period.


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Polymer Modification of the =Si—H Containing Precursor Polymers with a Dimethyldioxirane solution in acetone. To a methyl ethyl ketone solution of s=Si—H containing precursor polymers or copolymers, a cold solution (ca. -10 °C) of dimethyldioxirane in acetone was quickly added and reacted for 30 min at 0 °C. The mole ratio of dioxirane to polymer was ca. 1.2~1.3. The resulting silanol polymers or copolymers were obtained either in solution and used as is or precipitated into hexane followed by vacuum drying at 40 °C for 24 h. Preparation of blends, semi-interpenetrating polymer networks (semi-IPNs) and organic-inorganic polymeric hybrids. Blends were prepared by mixing appropriate amounts of each polymer solution in a common solvent while stirring. The resulting blend solutions were stirred at room temperature overnight. Blend films were prepared by solution casting onto glass slides. After the solvent was slowly evaporated at room temperature, all the films were vacuum dried at 80 °C for 3 days unless otherwise specified in the text. In the case of mutual precipitation of two polymers took place while mixing, the precipitates were filtered and washed with chloroform and acetone, respectively, followed by vacuum drying at 80 °C to constant weight. Thermal analysis. Differential scanning calorimetry (DSC) was performed by means of either the T A 2920 DSC or the Perkin-Elmer DSC-7 calorimeter. Sample weights of 8 - 12 mg and a heating rate of 20 °C/min were used. The glass transition temperature of the blend was taken from the second scan unless otherwise specified in the text. FT-IR spectroscopy. Fourier Transform Infrared Spectroscopy was performed with the use of the Perkin-Elmer 1600 series FT-IR or Digilab FTS-60 spectrometer. A minimum of 64 scans at a resolution of 2 cm" was signal-averaged. Samples for FTIR studies were prepared by casting blend solutions onto KBr windows followed by vacuum drying at 80 °C for 3 days. For the precipitated inter-polymer complexes, KBr discs were prepared. 1

Results and Discussion Synthesis and Characterization of Silanol-Containing Polymers and Copolymers. Polymer modification is of particular interest when the desired polymer is not readily available from its corresponding monomer by conventional polymerization methods. The conventional method for the synthesis of organosilanols can be accomplished by the hydrolysis of the appropriate substituted silane in the presence of catalysts such as an acid or a base. This synthetic route, however, has some difficulty when applied to the synthesis of silanol polymers which demanded not only high conversion of the functional groups for polymer modification but also resistance to the transformation of silanols to siloxane by self- or catalytic condensation during the preparation. A new convenient method for the synthesis of silanol polymers was developed by the selective oxidation of corresponding precursor polymers containing Si—H moiety with a dimethyldioxirane solution in acetone. . A series of styrene1

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422 based silanol polymers and copolymers were synthesized (Scheme 1). The reaction was carried out by an addition of a dimethyldioxirane solution in acetone to the precursor S i — H containing polymer solustion. This reaction resulted in a rapid and selective conversion of the =Si—H to =Si—OH bonds.

Scheme 1


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C

H

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Hydrogen Bond Interactions and Self-Condensation of Silanol

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