Hemi-Telechelic Polystyrene-POSS Copolymers as Model Systems for the Study of Well-Defined Inorganic/Organic Hybrid Materials

The combination of different inorganic additives and organic polymers, in conjunction with the appropriate processing methods, can produce hybrid materials with new properties for electrical, optical, structural, or related applications.1-3 Typically, properties such as the glass transition temperature, Tg, melting point, and stress-strain behavior or microphase separation can be altered by introducing an inorganic component into an organic polymer matrix.4,5 Layered organic/inorganic nanocomposites6 and silica gel-derived hybrids7,8 are but two examples of organic/inorganic composites. In what is commonly called the “top-down” approach the various components are combined together with mechanical work and/or thermal energy. Blended systems suffer from a lack of further processability and control due to difficulties with dispersion of the inorganic component. On the other hand, systems such as silica gel-derived hybrids are a result of postpolymerization formation of a network that is a function of pH of the solution or the structure of the precursor. This methodology gives rise to hybrid organic/inorganic polymers which can conveniently be used for membrane or coating applications.8 Nonetheless, the final inorganic structure is poorly defined due to the randomness of the network crosslink formation. These two approaches, component blending or postreaction cross-linking, are the most commonly taken to prepare hybrid organic-inorganic systems. Both approaches suffer from a lack of control over the microstructure giving rise to an “ill-defined” material. An alternative approach to generating organic/ inorganic hybrid structures would be to start at the molecular level using well-defined building blocks. Relatively few studies detailing the synthesis and characterization of controlled architectures using this concept have been completed.9-15 Expanding the scope of novel hybrid materials is of interest not only from the standpoint of the overall synthetic challenge but also critical in order to validate the self-assembly properties predicted from computational investigations.16-18 The use of anionic polymerization to produce well-defined organic-inorganic hybrid structures has not yet been fully investigated. The synthetic utility of anionic polymerization is well documented for the synthesis of organic block copolymers.19 Replacement of one of the organic domains in a block copolymer architecture with a well-defined inorganic moiety presents the opportunity to compare, or contrast, the new hybrid copolymers obtained with their conventional organic counterparts. New chemical and physical properties are likely to be found from the resultant hybrid copolymers. Polyhedral oligomeric silsesquioxanes (POSS) are inorganic nanosized particles and are potential candidates to control microstructure. These building blocks are of particular interest due to their molecularly precise structure as well as their solubility in common organic solvents.20,21 Experimental protocols have been developed over the past 40 years so as to have good yield and control over the structure of the POSS macromer. The so-called T8 POSS series has a cubic core with eight silicon atom at each corner and an oxygen bridge between each silicon atom. Seven silicon atoms bear an organic group that provides solubility, and a reactive group is generally attached to the eighth silicon atom. It is possible to incorporate this ∼1.5 nm diameter macromonomer into organic polymers. Random copolymers incorporating POSS have been prepared that are either thermosets or thermoplatics.22-35 These represent a category of new hybrid polymers with a tremendous technological potential.36,37 Control over the placement of the POSS within an organic polymer is possible using living/controlled polymerization methodologies. Matyjaszweski9,10,38 has incorporated the POSS inorganic particle into both linear and star systems using ATRP. Amphiphilic telechelic polymers having a poly(ethylene oxide) backbone and POSS end groups have been independently prepared by Mather and Frey.12,39 The tethering of POSS to an anionically synthesized polymer will extend the range of materials as well as the morphologies that can be achieved. The living anionic synthesis of hydroxyl-terminated polystyrene has been described in the literature.40-42 The molecular weight range of the hydroxyl-terminated polystyrene samples was chosen so that the radius of gyration of the polymer was comparable to that of the POSS particle.43,44 Polystyrene chains with a number-average molecular weight, Mn, of 900 and 16 000 g/mol have radii of gyration of 8.5 and 34 Å, respectively, vs a radius of approximately 7.5 Å for the POSS particle. Tethering of the isocyanate-modified POSS particle (POSS-NCO) to the polystyrene chains was achieved through the formation of a urethane linkage (Figure 1). The syntheses of amphiphilic telechelic polymers having a poly(ethylene oxide) backbone and POSS end groups are straightforward as the purification of the final product can be achieved based on solubility differences of the reagents. In our case an efficient, highly yielding coupling reaction must be used due to the comparable solubility of POSS and polystyrene in common organic solvents. To avoid extensive purification, an equimolar amount of POSS-NCO with respect to the hydroxyl-terminated polystyrene was used in the urethane formation chemistry. Urethane formation was promoted by the use of dibutyltin dilaurate at 90 °C in toluene solution.45 A representative synthesis of the hemi-telelchelic hybrids is the conversion of hydroxyl-terminated polystyrene PSOH 4, Mn and PDI of 4890 g/mol and 1.11, to PS-POSS 4 with a Mn and PDI of 6040 g/mol and 1.06, respectively (entry 4, Table 1). The increase in molecular weight is as expected for all samples given the molar mass of POSS-NCO (1058.56 g/mol). The retention of the narrow molecular weight distribution indicates the absence of side reactions (Figure 3). An equimolar amount of polystyrene and POSS-NCO was * To whom correspondence should be addressed. E-mail: coughlin@mail.pse. umass.edu. 5123 Macromolecules 2004, 37, 5123-5126