Polymer brushes chain transfer

Beckert, F., et al., Sulfur-functionalized graphenes as macro-chain-transfer and RAFT agents for producing graphene polymer brushes and polystyrene nanocomposites. Macromolecules, 2012. 45(17) p. 7083-7090. 

While in most of the reports on SIP free radical polymerization is utihzed, the restricted synthetic possibihties and lack of control of the polymerization in terms of the achievable variation of the polymer brush architecture limited its use. The alternatives for the preparation of weU-defined brush systems were hving ionic polymerizations. Recently, controlled radical polymerization techniques has been developed and almost immediately apphed in SIP to prepare stracturally weU-de-fined brush systems. This includes living radical polymerization using nitroxide species such as 2,2,6,6-tetramethyl-4-piperidin-l-oxyl (TEMPO) [285], reversible addition fragmentation chain transfer (RAFT) polymerization mainly utilizing dithio-carbamates as iniferters (iniferter describes a molecule that functions as an initiator, chain transfer agent and terminator during polymerization) [286], as well as atom transfer radical polymerization (ATRP) were the free radical is formed by a reversible reduction-oxidation process of added metal complexes [287]. All techniques rely on the principle to drastically reduce the number of free radicals by the formation of a dormant species in equilibrium to an active free radical. By this the characteristic side reactions of free radicals are effectively suppressed. 

To make further use of the azo-initiator, tethered diblock copolymers were prepared using reversible addition fragmentation transfer (RAFT) polymerization. Baum and co-workers [51] were able to make PS diblock copolymer brushes with either PMMA or poly(dimethylacrylamide) (PDMA) from a surface immobihzed azo-initiator in the presence of 2-phenylprop-2-yl dithiobenzoate as a chain transfer agent (Scheme 3). The properties of the diblock copolymer brushes produced can be seen in Table 1. The addition of a free initiator, 2,2 -azobisisobutyronitrile (AIBN), was required in order to obtain a controlled polymerization and resulted in the formation of free polymer chains in solution. 

Due to the relative ease of control, temperature is one of the most widely used external stimuli for the synthesis of stimulus-responsive bmshes. In this case, thermoresponsive polymer bmshes from poly(N-isopropylacrylamide) (PNIPAM) are the most intensively studied responsive bmshes that display a lower critical solution temperature (LOST) in a suitable solvent. Below the critical point, the polymer chains interact preferentially with the solvent and adopt a swollen, extended conformation. Above the critical point, the polymer chains collapse as they become more solvophobic. Jayachandran et reported the synthesis of PNIPAM homopolymer and block copolymer brushes on the surface of latex particles by aqueous ATRP. Urey demonstrated that PNIPAM brushes were sensitive to temperature and salt concentration. Zhu et synthesized Au-NPs stabilized with thiol-terminated PNIPAM via the grafting to approach. These thermosensitive Au-NPs exhibit a sharp, reversible, dear opaque transition in solution between 25 and 30 °C. Shan et al. prepared PNIPAM-coated Au-NPs using both grafting to and graft from approaches. Lv et al. prepared dual-sensitive polymer by reversible addition-fragmentation chain transfer (RAFT) polymerization of N-isopropylacrylamide from trithiocarbonate groups linked to dextran and sucdnoylation of dextran after polymerization. Such dextran-based dual-sensitive polymer is employed to endow Au-NPs with stability and pH and temperature sensitivity. 

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, surface modification techniques for polymer chains have progressed a great deal with the development of a new polymer synthesis method. In particular, surface-initiated atom transfer radical polymerization (SI-ATRP) is one of the most effective modification methods for preparing a well-defined dense polymer brush structure, or polymer brush, on solid substrates. Thus, a self-oscillating polymer brush prepared by SI-ATRP can be expected to create a novel self-oscillating surface with autonomous function, which will lead to potential applications in transporting systems for nanomaterials of flow control in microfluidics. 

CRP is usually divided into three categories (i) ATRP (ii) reversible radical addition fragmentation chain transfer (RAFT) and (iii) nitroxide-mediated polymerization (NM P). All three techniques permit the polymer molecular weight, the polydispersity, and the polymer architecture to be accurately controlled, and have been used to build up highly dense polymer brushes from inorganic particles. A list of macro-initiators which have been developed recently for this purpose is provided in Table 4.6. 

To overcome these problems, a surface-initialed living radical polymerization technique is used for preparing polymer brush surfaces, where precisely controlled polymer chains are densely tethered. The technique also can be used to construct block copolymer on the surfaces by sequentially grafting another polymer via the active polymer-end groups of a first-grafted polymer brush. As a typical surface-initiated Uving radical polymerization, atom transfer radical polymerization (ATRP) and reversible addition-fragmentafion chain transfer radical (RAFT) polymerization are... 

---