Hybrid materials composed of inorganic nanoparticles and organic surface groups possess interesting optical,50 magnetic,51 and blending52 properties. These hybrids containing nanoparticles have been prepared by other synthetic routes by trapping colloids within cross-linked matrixes,53 "grafting to" particles with functional molecules/ polymers,25 or "grafting from" particles using a living or controlled polymerization process.54 Controlled radical polymerization techniques have also been introduced to colloidal materials by the attachment of ATRP initiating groups to the particle surface. Subsequent ATRP of vinyl monomers yielded core-shell particles with well-defined homopolymers and block copolymers tethered to a colloidal initiator.55,56 The properties of hybrid nanoparticles prepared from this method can be tuned by varying the particle size of the colloidal initiator, changing the composition of the particle core, or tethering (co)polymers with novel composition/functionality. An interesting feature of hybrid nanoparticle ultrathin films has been the formation of ordered two-dimensional arrays of particles, with a spacing dependent on the radius of gyration of the tethered (co)polymer. The general methodolgy for the synthesis of hybrid nanoparticles from ATRP is presented in Scheme 11.
Siloxane-based nanoparticles have been successfully applied to ATRP systems to prepare well-defined hybrid nanoparticles. In the first step of the process, nanoparticles were synthesized via the base-catalyzed hydrolysis and condensation of tetralkoxysilanes (i.e., the Sto¨ber process)57,58 or by microemulsion polymerization of trialkoxysilanes.59 Condensation reactions of surface silanol groups with functional silanes yielded colloidal initiators bearing benzyl chloride, 2-bromopropionate, or 2-bromisobutyrate groups. The synthesis of hybrid nanoparticles was then conducted by using the colloidal initiators in the ATRP of various vinyl monomers.
5.1. Hybrid Nanoparticles from Silica Colloids.
A modification of the Sto¨ber process was developed to prepare silica (SiO2) colloidal initiators for ATRP.55 Dynamic light scattering (DLS) and transmission electron microscopy (TEM) revealed that functional silica particles with an average effective diameter (Deff) of 70 nm were obtained. Silica colloids with benzyl chloride groups on the surface were used in the ATRP of Sty.
DLS and TEM confirmed that Deff of the pSty hybrid nanoparticles increased with monomer conversion. SEC of pSty chains cleaved from the particle confirmed the synthesis of well-defined polymers of low polydispersity (Mw/Mn <> 1 ím) has been conducted using CRP. Previously, surface-initiated polymerizations from micron-sized particles had been conducted using conventional radical polymerization.61-64
However, by the use of CRP from particles, polymer coatings of controlled thickness and functionality were prepared. In particular, hybrids from larger particles were synthesized as potential chromatographic stationary phases65-68 and templated supports.69 In the preparation of stationary phases for liquid chromatography, thin films (_100 Å) of poly(acrylamide) were grown from benzyl chloride functional porous silica particles (Deff ) 5 ím and pore sizeave ) 860 Å) using ATRP. The successful separation of various proteins using these poly(acrylamide) particles pointed to the successful grafting of polymers, without significant clogging of particle pores.65 Similarly, poly(methacrylates) possessing nucleotide side-chain groups were grown from porous silica particles using ATRP. The immobilization of oligonucleotides has been demonstrated as an attractive approach for the templated synthesis of nucleic acids, with the primary goal being control of both the degree of polymerization and sequence distribution in the final product. Toward this endeavor, methacrylate derivatives of uridine and adenosine were synthesized and polymerized using a 2-bromoisobutyrate functional silica particle.69 In a different templated system, poly- (benzyl methacrylate) was grafted to a silica surface using ATRP and treated with hydrofluoric acid to prepare hollow polymeric colloids.70 Hybrid particles with tethered pSty were also prepared using ATRP from 2-chloro-2-phenylacetate functional silica particles.71
Asignatura: CRF
Fuente: www.eng.uc.edu/~gbeaucag/.../MatyjaszewskiReviewPolyonCer.pdf
Ver: http://nanocompositescrf.blogspot.com/
Ver: http://nanocompositescrf.blogspot.com/
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