Grafting To - Use of End-Functional Polymers

The synthesis of end functional polymers by NMP, ATRP and RAFT has already been discussed in Section 9.7. The grafting to approach involves the covalent attachment of an end-funetionalized polymer with reactive surface groups on the substrate. The approach is inherently limited by the crowding of chains at the surface and the limit this places on the final graft density. 

RAFT polymerization lends itself to the synthesis of polymers with thiol end groups. Several groups have utilized the property of thiols and dilhioesLers to bind heavy metals such as gold or cadmium in preparing brushes based on gold film or nanoparticles1 8 761 763 and cadmium selenide nanoparticles.763 76 1 

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There are additional factors that may reduce functionality which are specific to the various polymerization processes and the particular chemistries used for end group transformation. These are mentioned in the following sections. This section also details methods for removing dormant chain ends from polymers formed by NMP, ATRP and RAFT. This is sometimes necessary since the dormant chain-end often constitutes a weak link that can lead to impaired thermal or photochemical stability (Sections 8.2.1 and 8.2.2). Block copolymers, which may be considered as a form of end-functional polymer, and the use of end-functional polymers in the synthesis of block copolymers are considered in Section 9.8. The use of end functional polymers in forming star and graft polymers is dealt with in Sections 9.9.2 and 9.10.3 respectively. 

Functional and end-functional polymers are precursors to block and graft copolymers and, in some cases, polymer networks. Copolymers with in-chain functionality may be simply prepared in copolymerizations by using a functional monomer. However, obtaining a desired distribution requires consideration of the chain statistics and, for low molecular weight polymers, the specificity of the initiation and termination processes, l hese issues are discussed in Section 7.5.6... 

Fig. 9.19 Preparation of polymer brushes on solid surfaces by a) chemical grafting of end-functionalized linear polymers or selective adsorption of asymmetric block copolymers and b) by surface-initiated polymerization (SIP) using initiator functions on the solid surface. The depicted SAM bearing to-functionalities...

To the best of our knowledge, no literature work has been reported describing the use of photoinduced grafting onto technique, which seems almost impossible to achieve. Grafting Onto is best defined as the attachment of end-functional polymeric chains, namely, telechelics, onto specific sites on polymer backbones or surfaces via a chemical reaction (Scheme 13.6). 

The grafting onto technique involves the use of a backbone polymer with randomly distributed functionalities along the chain. The branches with chain-end functional groups are synthesized and characterized separately. The coupling of branches to the backbone leads to the formation of the graft copolymer. In most of the cases of PLL-based graft copolymers the polypeptide was used as a backbone and its E-NH2 side groups were used for... 

The grafting-to strategy refers to the reaction of a prefabricated, end-functionalized polymer chain with a functional group on the solid substrate to form a tethered polymer brush. The covalent or ionic linkage formed between the surface and polymer chain end makes the polymer brushes robust and resistant to common environmental conditions (Zhao and Brittain 2000). This method has been widely used in the preparation of ferrocene polymer brushes. 

We will start our discussion with the grafting to approach, which may be characterized as the simplest. Depending on the nature of the surface of the porous membrane that we want to decorate with a brush, we must choose an end-functionalized polymer chain (or a highly asymmetric block copolymer of an amphipathic nature) that may strongly adsorb from its functionalized part on the inner surface. Then in principle we may use the same method as for the case of a flat surface, i.e., dip the membrane in a solution that contains these polymers and then wait for the full formation of the brush. However, it has been shown [21] that self-assembly inside the pores is prohibited when the ratio of pore diameter to chain radius of gyration... 

In the grafting-to technique, end-functionalized polymer reacts from solution onto a suitable substrate surface to form polymer brushes. This method is particularly suitable for homogeneous brushes using linear polymers with a narrow molecular weight distribution. However, the amount of polymer that can be attached to the substrates is low, giving very thin layers of limited density. 

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