Grafting From - Surface Initiated Polymerization

The preparation of polymer brushes by controlled radical polymerization from appropriately functionalized polymer chains, surfaces or particles by a grafting from approach has recently attracted a lot of attention.742 743 The advantages of growing a polymer brush directly on a surface include well-defined grafts, when the polymerization kinetics exhibit living character, and stability due to covalent attachment of the polymer chains to the surface. Most work has used ATRP or NMP, though papers on the use of RAFT polymerization in this context also have begun to appear. 

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The same aplies to polymer brushes. The use of SAMs as initiator systems for surface-initiated polymerization results in defined polymer brushes of known composition and morphology. The different polymerization techniques, from free radical to living ionic polymerizations and especially the recently developed controlled radical polymerization allows reproducible synthesis of strictly linear, hy-perbranched, dentritic or cross-linked polymer layer structures on solids. The added flexibility and functionality results in robust grafted supports with higher capacity and improved accessibility of surface functions. The collective and fast response of such layers could be used for the design of polymer-bonded catalytic systems with controllable activity. 

In general, in the field of materials or condensed matter, the preparation of polymer brushes on solid surfaces is of great interest for surface modification and composite material preparation [4-6]. A number of model surface grafting techniques have been used on planar surfaces and particles and have been the subject of previous reviews. While a munber of polymer brush preparation methods have been reported using physisorption or chemisorption or so-called grafting onto methods, the emphasis of this review is on surface-initiated polymerization (SIP) methods or grafting from methods. 

A dense polymer brush is obtained using the grafting from techniques. Surface-initiated polymerization in conjunction with a living polymerization technique is one of the most useful synthetic routes for the precise design and functionalization of the surfaces of various solid materials with well-defined polymers and copolymers. Above all, surface-initiated living radical polymerization (LRP) is particularly promising due to its simplicity and versatility and it has been applied for the synthesis of Au NPs. 

In grafting from this limitation is overcome by polymerizing the polymer directly on the surface. The polymerization is carried out step-by-step after initiation by eligible chemical groups which are called initiators I. The scheme for a grafting-from polymerization is shown in Fig. 10.11. It is also called surface-initiated polymerization or SIP. 

When polymers are to be tethered to surfaces by covalent bonds, there are two fundamental approaches for achieving this grafting from and grafting onto. Grafting from means surface-initiated polymerization, whereby the polymer chains are created in situ. Grafting onto involves formation of covalent bonds between previously formed polymer chains and reactive groups at a surface (Fig. 9). Both methods have specific pros and cons. 

Grafting from methods (also called surface-initiated polymerization), by which the polymer is grown from the surface via attached initiator or chain transfer agent... 

The grafting-from technique involves the immobilization of initiators onto the substrate followed by in situ surface-initiated polymerization to generate a tethered polymeric phase. This approach has generally become the most attractive way to prepare thick, covalently tethered polymer brushes with a high grafting density. A variety of synthesis methods such as radical chain transfer reaction,reverse ATRP, living anionic surface-initiated polymerization, ATRP, " dispersion polymerization, and... 

Recently, the investigation of polymer brushes has been focused on the synthesis of new tethered polymer systems primarily through surface-initiated polymerization (SIP). Previously, the term polymer brushes has been limited to the investigation of block copolymers (qv) or end-functional linear polymers that have been physically or chemically adsorbed to surfaces, respectively (3,4). Recent synthetic efforts using different polymerization mechanisms have resulted in the discovery of many novel properties of polymer brushes. This has been aided no less than the use of innovative and unique surface-sensitive analysis methods as applied to flat substrates and particles. The study of polymer brushes has benefited from improved dielectric, optical, spectroscopic, and microscopic characterization methods. Understanding the chemistry of these grafting reactions and how... 

Fig. 5. Protocol for living anionic surface initiated polymerization (LASIP), with initiators attached to silica nanoparticles or clay (a) grafting of initiator and general polymerization scheme (b) nanoparticle grafting procedure. From Ref 36.

X. Fan, C. Xia, and R. C. Advincula, Grafting of polymers from clay nanoparticles via in situ free radical surface-initiated polymerization Monocationic versus bicationic mitiators. Langmuir, 19 (2003), 4381-9. 

Farhan et al. [25] reported surface-initiated polymerizations from polymeric surfaces of commercially important polyester films, poly(ethylene terephthalate) (PET), and poly(ethylene naphthalate) (PEN). Patterned self-assembled monolayers (SAMs) of the trichlorosilane initiator were first immobilized on the surface through a soft lithographic method of microcontact printing (ICP). Grafting from the surface was initiated via controlled ATRP, under aqueous conditions, to create patterned brushes of the ther-moresponsive polymer poly(N-isopropylacrylamide) (PNIPAm), as shown in Figure 1.8. 

Hoven et al. [34] used a chemically grafted tris(trimethylsiloxy)silyl (tris(TMS)) monolayer on a silicon oxide substrate as a template for creating nanoclusters of polymer brushes. Surface-initiated polymerization of 2-meth-acryloyloxyethyl phosphorylcholine (MFC) and tert-butyl methacrylate (t-BMA) was used to generate polymer brushes via ATRP from a-bromoester groups tethered to the residual silanol groups on the silicon surface, as... 

Synthetic strategies for the preparation of polymer brushes (A) physisorption, (B) grafting-to approach via reaction of appropriately end-functionalized polymers with complementary functional groups on the substrate surface, (C) grafting-from approach via surface-initiated polymerization. (Reprinted with permission from Barbey et al. 2009. Polymer Brushes via Surface-Initiated Controlled Radical Polymerization Synthesis, Characterization, Properties, and Applications. Chemical Reviews 109 (ll) 5437-5527 copyright (2009) American Chemical Society.)... 

The grafting-to approach is experimentally simple, but is limited by a low grafting density because of steric crowding of reactive sites by the adsorbed polymers. Grafting-from (also called surface-initiated polymerization) is a powerful method for the preparation of polymer brushes, involving polymerization of monomers from a substrate with surface-anchored initiator species or initiator sites (Barbey et al. 2009 Zhao and Brittain 2000). 

With the development of controlled radical polymerization techniques in recent years, the grafting-from approach has attracted considerable attention in the preparation of polymer brushes on solid substrates. The tethered polymer brushes can be prepared by the immobilization of initiators and subsequent surface-initiated polymerization (Zhao and Brittain 2000). 

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