Synthesis of Nanocomposite Organic/Inorganic Hybrid Materials Using Controlled/“Living” Radical Polymerization

The preparation of hybrid organic/inorganic nanocomposites comprised of well-defined polymers was reviewed. In particular, synthetic methods using controlled/"living" radical polymerization techniques, such as stable free-radical/nitroxide-mediated polymerizations, atom transfer radical polymerization, and reversible addition-fragmentation chain-transfer polymerization were described. The various approaches taken to prepare hybrid copolymers, nanoparticles, polymer brushes, dispersed silicate nanocomposites, and nanoporous materials were discussed.

1. Introduction
The synthesis of novel materials with improved properties and performance is a continually expanding frontier at the interface of chemistry and materials science. In this pursuit, the ability to control molecular structure on atomic and macroscopic dimensions is a key parameter in designing materials with preprogrammed activity. A significant advance in this area has been the synthesis of nanocomposites where the structural order within the material can be controlled on nanometer/submicron scales. While materials possessing such structural complexity are common in nature, robust and versatile methods to prepare synthetic nanocomposites remains an exciting challenge that is being tackled by research groups around the world.1 One approach to prepare nanocomposites has been the incorporation of well-defined organic and inorganic components into a singular material. In particular, the inclusion of well-defined polymers to inorganic substrates is of significance, because the functionality, composition, and dimensions of these macromolecules enable the design of specific properties into the resulting hybrid.2

Well-defined organic polymers have been attached to inorganic (co)polymers, particles, surfaces, glassy networks and interpenetrating polymer networks to prepare organic/inorganic hybrid materials. Additionally, polymers of controlled size, composition, and architecture have been used as shape templates in the synthesis of mesoporous inorganic networks (Figure 1). Polymers, such as poly(tetramethylene oxide) and poly(oxazolines), have been used to synthesize hybrid organic/inorganic nanocomposites.3 However, recent developments in controlled/"living" radical polymerization (CRP) have provided another valuable methodology to introduce well-defined organic (co)polymers to a variety of inorganic substrates. The scope of this review will cover hybrid organic/inorganic nanocomposites that have been made using CRP. For more fundamental discussions of organic/inorganic hybrid materials, the reader is directed to other reviews.2-5

2. Controlled Radical Polymerization
CRP has proved to be a versatile and robust method to prepare well-defined organic polymers. In the past decade, several techniques have been developed to synthesize well-defined polymers via radical polymerization. A major difference between conventional radical [i.e., azobis(isobutyronitrile)- or peroxide-initiated processes] and controlled radical polymerizations is the lifetime of the propagating radical during the course of the reaction. In conventional radical processes, radicals generated by decomposition of the initiator undergo propagation and bimolecular termination reactions within a second. In contrast, the lifetime of a growing radical can be extended to several hours in a CRP, enabling the preparation of polymers with predefined molar masses, low polydispersity, controlled compositions, and functionality.6,7 The mechanism invoked in CRP processes to extend the lifetime of growing radicals utilizes a dynamic equilibration between dormant and active sites with rapid exchange between the two states. Unlike conventional radical processes, CRP requires the use of persistent radical (deactivator) species, or highly active transfer agents to react with propagating radicals.

These persistent radicals/transfer agents react with radicals (deactivation or transfer reactions with rate constant, kd) to form the dormant species. Conversely, propagating radicals are generated from the dormant species by an activation reaction (with rate constant, ka).

2.1. Classification of CRP Systems. In the past decade, the field of CRP has seen tremendous development as evidenced by the wide range of materials that have been prepared using these techniques. In particular, three methods of considerable importance are the following: stable free-radical polymerization [SFRP; e.g., nitroxide-mediated processes (NMP)], metal-catalyzed atom transfer radical polymerization (ATRP), and degenerative transfer [e.g., reversible addition-fragmentation chain transfer (RAFT)]. While these three systems possess different components, general similarities in the CRP processess can be seen in the use of initiators, radical mediators (i.e., persistent radicals or transfer agents), and in some cases catalysts (Scheme 1). It is important to note that while SFRP and ATRP are subject to the persistent radical effect (PRE),8,9 degenerative processes, such as RAFT, do not conform to the PRE model because of the transfer-dominated nature of the reaction.

2.1.2. SFRP. In this type of CRP, alkoxyamine initiators10 (-PnX; eq 1 in Scheme 1) and nitroxide persistent radicals (X0; eq 1 in Scheme 1) have been effectively used to polymerize styrenes and acrylates. In certain systems, alkoxyamines have also been generated in situ by the initial use of conventional radical initiators (AIBN and peroxides) and nitroxide persistent radicals, which also led to a CRP process.11 A widely used nitroxide in the polymerization of styrene (Sty) is 2,2,6,6-tetramethylpiperidinyloxy (TEMPO), although recently developed nitroxides can also polymerize acrylates in a controlled fashion.12,13 The current limitation in this system lies in the inability to successfully polymerize methacrylate monomers, because of â-hydrogen elimination to the nitroxide radicals. Additionally, thiuram disulfides and dithiocarbamate "iniferter" systems have been used as agents for CRP with limited success.14,15

2.1.3. ATRP. In these polymerizations, radicals are generated by the redox reaction of alkyl halides (-PnX; eq 2 in Scheme 1) with transition-metal complexes (Y; eq 2 in Scheme 1).16-18 Radicals can then propagate but are rapidly deactivated by the oxidized form of the transition-metal catalyst (X-Y0; eq 2 in Scheme 1). Initiators typically used are R-haloesters (e.g., ethyl 2-bromoisobutyrate and methyl 2-bromopropionate) or benzyl halides (e.g., 1-phenylethyl bromide and benzyl bromide). A wide range of transition-metal complexes, such as Ru-, Cu-, and Fe-based systems, have been successfully applied to ATRP. For Cu-based systems, ligands such as 2,2¢-bipyridine and aliphatic amines have been employed to tune both the solubility and activity of various ATRP catalysts. ATRP has been successfully applied for the controlled polymerization of styrenes, (meth)acrylates, (meth)acrylamides, acrylonitrile, and 4-vinylpyridine. ATRP systems are currently limited to monomers that do not strongly coordinate
to the catalyst.18

2.1.4. Degenerative Transfer. Radical polymerizations based upon a degenerative transfer system rely upon the rapid and reversible exchange of highly active transferable groups (-X; eq 3 in Scheme 1) and growing polymeric radicals (-Pm 0 and –Pn 0; eq 3 in Scheme 1). While conventional radical initiators (AIBN and peroxides) are used in this reaction, these compounds only serve as a radical source to drive reversible exchange reactions between active and dormant states. Transfer agents in this process contain moieties for both initiation and transfer which are generated in the presence of radicals. Controlled radical polymerizations from degenerative transfer reations have been done using alkyl iodides,19 unsaturated methacrylate esters,20 or thioesters as the transfer agents. In particular, the use of thioesters in the radical polymerization of vinyl monomers results in a RAFT polymerization.21 The RAFT process has proven to be a versatile method to polymerize functional styrenes, (meth)acrylates, and vinyl esters.

2.2. Synthetic Methodologies To Prepare Nanocomposite Hybrids Using CRP. A variety of CRP techniques have been developed to incorporate welldefined organic polymers to inorganic substrates. A key advantage of CRP processes is the facile functionalization/ deposition of initiator and polymerizable moieties onto inorganic polymers and surfaces.

2.2.1. Synthesis of Hybrid Homopolymers and Block, Graft, and Random Copolymers. In the preparation of organic/inorganic nanocomposite materials, hybrid copolymers are of particular interest because of the inherent incompatibility of the two segments. Thus, phase separation of these segments yields a variety of controlled nanostructures depending on the degree of incompatibility of the components, the composition, and the degree of polymerization in the final copolymer.

A wide range of polymeric structures have been made using different combinations of organic and inorganic components. In the simplest case, a vinyl monomer possessing an inorganic moiety (R ) inorganic moiety; Scheme 2a) can be homopolymerized in the presence of initiator (R-X) to prepare a hybrid homopolymer with organic main-chain and inorganic side-chain groups.

Alternatively, inorganic vinyl monomers can be copolymerized in the presence of organic monomers and R-X to make random copolymers (Scheme 2b). Chain extension of an organic macroinitiator prepared by CRP with an inorganic monomer yields a block copolymer (Scheme 2c). Conversely, an inorganic macroinitiator can be used in the CRP of an organic vinyl monomer to also prepare a hybrid block copolymer (Scheme 2d). Inorganic macroinitiators can be synthesized by end-capping growing inorganic chain ends obtained from ionic or step-growth processes with coupling agents that contain CRP initiator groups. In a similar fashion, hybrid graft copolymers are prepared by chain extending a multifunctional inorganic macroinitiator with an organic vinyl monomer (Scheme 2e). Additionally, inorganic macromonomers possessing vinyl end groups can be copolymerized with organic vinyl monomers by CRP, yielding graft copolymers with organic main-chain and inorganic side-chain
groups (Scheme 2f).

2.2.2. Synthesis of Hybrid Polymers from Inorganic Initiators. Another type of hybrid polymer is prepared by the use of a (multi)functional inorganic compound as an initiator for CRP. Various polymeric structures are possible depending on the functionality of the initiator. Monofunctional inorganic initiators would result in an end-functional polymer, while multifunctional initiators yield branched/star polymers (Scheme 3).

2.2.3. Synthesis of Nanocomposite Particles, Modified Surfaces, Dispersed Silicates, and Nanoporous Materials. The tethering of well-defined organic polymers to inorganic substrates such as particles (Scheme 4a) and surfaces (Scheme 4b) can be conducted using CRP. The attachment of prepared polymers was achieved by grafting to curved and flat surfaces.22-26 However, by the use of CRP methods, grafting organic polymers from surfaces can be performed by attachment of an initiator group (-X; Scheme 4) and polymerization of a vinyl monomer. The chemistry required for the functionalization of CRP initiator groups on curved and flat surfaces depends on the type of inorganic substrate used. In the case of gold and silicon wafers, the incorporation of initiating moieties to surfaces requires the tethering of functional thiols, or chloro/alkoxysilanes, respectively. By the use of CRP from inorganic surfaces, ultrathin films of precise thickness and composition can be grown. Organic/inorganic nanocomposites can also be prepared by CRP from the surfaces of layered silicates (Scheme 4c). CRP of an organic vinyl monomer between the inorganic layers yields a nanocomposite of inorganic platelets dispersed in a matrix of well-defined polymer. Finally, polymers of controlled size and compositions can be prepared by CRP and used as templates in the network polymerization of inorganic resins (Scheme 4d). When the molar mass and composition of the polymer template are tuned, using CRP enables control of both the hydrodynamic volume and the compatibility of the polymer with the inorganic matrix, respectively. Network formation around the polymeric templates, followed by degradation of the organic components, yields an inorganic glass with nanosized pores dispersed throughout the material.