Block copolymers RAFT approach

An alternative approach uses reversible addition-fragmentation chain transfer (RAFT), which has fewer limitations in the selection of monomers (Chong et al, 1999) so that monomers with hydroxyl, t-amino and acid functionality may be polymerized into narrow-polydispersity block copolymers. The RAFT agent is a dithioester of the general formula S=C(Z)S-R, where Z is usually -Ph (phenyl) and R is chosen from a group of alkyl phenyls (e.g. -C(CH3)2Ph). 

The bifunctional initiator approach using reversible addition fragmentation chain-transfer polymerization (RAFT) as the free-radical controlling mechanism was soon to follow and block copolymers of styrene and caprolactone ensued [58]. In this case, a trithiocarbonate species having a terminal primary hydroxyl group provided the dual initiation (Figure 13.3). The resultant polymer was terminated with a trithiocarbonate reduction of the trithiocarbonate to a thiol allows synthesis of a-hydroxyl-co-thiol polymers which are of particular interest in biopolymer applications. 

While not related exclusively to block copolymer synthesis, the formation of many of the more complex architectures available through RAFT polymerization - including those based on a single monomer - shares the characteristics and caveats of linear block copolymer formation. One technique to obtain such structures (aldn to the triblock synthesis mentioned above) is the use of higher-level, multifunctional RAFT agents. A synthetic approach with a multifunctional core or a RAFT agent-functionalized polymer backbone allows... 

The RAFT-HDA concept relies heavily on the provision of diene-capped polymers, specifically those carrying the reactive cyclopentadienyl (Cp) group. Recently, a mild ambient-temperature approach towards Cp-capped polymers was introduced, whereby the use of nickelocene allowed the transformation of virtually all ATRP-prepared polymers into Cp-capped entities, in quantitative fashion [56]. Notably, the RAFT-HDA approach is not only suited to the preparation of block copolymers (or more complex structures) [57] rather, it has been exploited as a... 

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. 

In the case of AB diblock copolymers prepared by the RAFT technique, the order of monomer addition must be taken into account. A characteristic example of such a block copolymer synthesis is demonstrated in Scheme 19. Initially, a poly(N, N-dimethylacrylamide) (PDMA) macro-CTA was prepared, followed by the use of PDMA-CTA as an initiator to polymerize successfully the second monomer N,N-dimethyl vinyl benzy-lamide (DMVBA). The final diblock copolymer is not contaminated with homopolymer. It has been discovered that the reverse approach is impossible, probably due to the slow fragmentation of the intermediate radical or due to the slow initiation efficiency of the intermediate radical (styrenic macroradical). 

Most recently the core cross-linking approach was adopted for the preparation of novel nanoassemblies derived from pH-responsive AB diblock copolymers prepared via RAFT (222). Here the A block was the permanently hydrophilic DMA species with the timably hydrophilic/hydrophobic DMBVA forming the B block. In aqueous media, at high pH, these block copolymers form core-shell structures with the DMA residues forming the corona and the DMVBA chains in the core. Addition of a hydrophobic, difimctional, alkyl halide namely 1,4-bisbromomethylbenzene results in this species being sequestered into the hydrophobic core of the micelles where it reactions with the tertiary amine residues via the Menshutkin reaction to yield the core-cross-linked species. Successful core cross-linking was... 

One advantage of the RAFT process is its eompatibility with a wide range of monomers, including fnnctional monomers. Thus narrow polydispersity block copolymers have been prepared with monomers containing acid (e.g., acrylic acid), hydroxy (e.g., 2-hydroxyethyl methacrylate), and tertiary amino [e.g., 2-(dimethylamino) ethyl methacrylate] functionality (Chiefari et al., 1998). Linear block copolymers are the simplest polymeric architecmres achievable via RAFT process. There are two main routes for the synthesis of block copolymers by the RAFT process, viz., (i) sequential monomer addition (chain extension) and (ii) synthesis via macro-CTAs (by R- or Z-group approaches). These are schematically shown in Fig. 11.37. Linear block copolymers are the simplest polymeric architectures achievable via RAFT process. 

One of the major applications of living polymerization is the synthesis of block copolymers via sequential addition of monomers. In this approach, monomer A is rst polymerized quantitatively by the RAFT process, followed by direct addition of a second monomer for chain extension, or alternatively, the RAFT polymer of A from the rst step is puri ed and used as a macro-CTA to... 

Figure 12.21 Hetero-Diels-Alder (HDA) reaction between diene and electron-de cient dithioesters in the RAFT-HDA approach to make block copolymers.

Quemener, D., Davis, T.P., Bamer-Kowollik, C., and Stenzel, M.H. (2006) RAFT and click chemistry A versatile approach to well-defined block copolymers. Chemical Communications, 5051. 

SinnweU, S., Inglis, A.J., Davis, T.P. et al. (2008a) An atom-efficient conjugation approach to well-defined block copolymers using RAFT chemistry and hetero Diels-Alder cycloaddition. Chemical Communications, 2052. 

Fig. 22 Synthesis of peptide star-shaped polymers by the macromonomer approach Stages are synthesis of PBLG macromonomer with styrene end groups radical polymerization (SERF or RAFT) in the presence of DVB by crosslinking of block copolymers and deprotection of PBLG shell [132]...

Reversible addition-fragmentation chain transfer polymerization (RAFT) polymerization of methyl acrylate was combined with cationic polymerization of THF to synthesize comb copolymers. Asymmetric star block copolymers based on polystyrene (PS), PTHF, and PMMA were synthesized by a combination of CROP and redox polymerization methods. Miktoarm star polymers containing poly(THF) and polystyrene arms were also obtained by combining CROP and ATRP methods. Another approach for the synthesis of block copolymers... 

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