Grafting Functional Polymeric Molecules

The methods for obtaining polymer chains grafted onto the surface of nanoparticles can be divided into two classes grafting-from and grafting-to processes. In the former method, the nanoparticle surface is modified with functional groups by the reaction between molecules and particles, and then the molecules begin the polymerization processes via radical, cationic, or anionic polymerization. In the latter method, the groups at the end of preformed polymers react with the nanoparticle surface. 

This method of surface graft polymerization usually includes two steps surface activation and graft polymerization. Sites on the nanoparticle surface are first activated by high-energy electrons, plasma treatment, chemical reactions, etc. and, subsequently, the graft polymerization process is generated. 

Hong et al. [Ill] modified ZnO nanoparticles by grafting polystyrene (PS) onto the surface of particles. This procedure contained two steps first, the surface of ZnO nanoparticles was activated by KH570, and second, PS was grafted to the surface-activated ZnO nanoparticles through typical solution polymerization. Hong et al. [112] also investigated the radical polymerization of methyl methacrylate (MMA) on the surface of ZnO particles to which the functional double bond had first been introduced. 

The grafting-to method (without involving elaborate synthetic procedures) involves a chemical reaction between (end)-functionalized polymers and complementary reactive groups on the surface of nanoparticles. The reactive polymers can be synthesized by controlled radical, anionic, or other polymerization techniques. Compared with other polymer attachment techniques, the (end)-functionalized polymer can be thoroughly characterized via various chemical and physical methods. However, the major shortcoming of this method is the low maximum thickness of the obtained polymer layers [113]. 

Qin et al. [114] prepared functionalized single-walled carbon nanotubes (SWNTs) through treatment of polystyrene via the grafting-to method. The results showed that the PS was covalently attached to the side walls of SWNTs. The poly(vinylidene fluoride-co-hexafluoropropylene) (PVDF-HFP) was functionalized with glycidyl methacrylate (GMA) via atom transfer-radical polymerization (ATRP) and the BaTiOs nanoparticles were modified by amino-terminated silane molecules. Then, the nanocomposites with high dielectric coti-stant and high thermal conductivity were prepared by a grafting to method [115]. 

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When specific functionalities are desired, copolymerization or grafting reactions are common ways to add functionalities to polystyrenes. Other techniques used to chemically modify polystyrenes are grafting reactions, ionic polymerization, alkylation, amination, condensation reactions and ctosdinking reactions. Copolymerization sometimes involves the laborious synthesis of functionalized styrene molecules through conventional oiganic synthetic methods. In future years it will be difficult and expensive for companies to introduce new monomers due to tougher n ulations. 

Preformed silica particles have been used as supports to which imprinted layers have been grafted. In earlier approaches, polymerizable nnits were attached to the silica support followed by grafting to polymerization using template, functional monomer, cross-linker and soluble initiator. " It is now more common to attach initiator or iniferter units to the silica snpport, allowing for a grafting from approach. The template molecule may also... 

The A-B type iniferters are more useful than the B-B type for the more efficient synthesis of polymers with controlled structure The functionality of the iniferters can be controlled by changing the number of the A-B bond introduced into an iniferter molecule, for example, B-A-B as the bifunctional iniferter. Detailed classification and application of the iniferters having DC groups are summarized in Table 1. In Eqs. (9)—(11), 6 and 7 serve as the monofunctional iniferters, 9 and 10 as the monofunctional polymeric iniferters, and 8 and 11 as the bifunctional iniferters. Tetrafunctional and polyfunctional iniferters and gel-iniferters are used for the synthesis of star polymers, graft copolymers, and multiblock copolymers, respectively (see Sect. 5). When a polymer implying DC moieties in the main chain is used, a multifunctional polymeric iniferter can be prepared (Eqs. 15 and 16), which is further applied to the synthesis of multiblock copolymers. 

The fact that silsesquioxane molecules like 2-7 contain covalently bonded reactive functionalities make them promising monomers for polymerization reactions or for grafting these monomers to polymer chains. In recent years this has been the basis for the development of novel hybrid materials, which offer a variety of useful properties. This area of applied silsesquioxane chemistry has been largely developed by Lichtenhan et al With respect to catalysis research, the chemistry of metallasilsesquioxanes also receives considerable current interest. As mentioned above, incompletely condensed silsesquioxanes of the type R7Si70g(0H)3 (2-7, Scheme 4) share astonishing structural similarities with p-tridymite and p-cristobalite and are thus quite realistic models for the silanol sites on silica surfaces. Metal... 

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