Reversible addition-fragmentation chain-transfer polymerization

4 Reversible Addition-Fragmentation Chain Transfer Polymerization 

It was reported that RAFT-mediated pol5mieiization reactions typically contain anomalies, such as an inhibition period and rate retardation. The rate retardations or reductions in the polymerization rates apparently occur in the presence of RAFT agents, and are not observed when RAFT agents are absent. Examples are dithiobenzoate-mediated polymerization reactions [275]. 

The effectiveness RAFT agents were investigated by Moad and coworkers [277]. These RAFT agents, such as thiocarbonylthio compounds, depend in effectiveness on the nature of the group, Z and R (shown below) that modify the reactivity of the thiocarbonyl group toward free radical addition. R is the free radical leaving group [277]  

These RAFT agents are based on the structure [S=C(Ph)S-R] [277] They found that the effectiveness of these agents also depends upon the nature of the monomer and on the polymerization conditions. For the polymerization of styrene, methyl methacrylate, butyl acrylate, and methyl acrylate at 60°C, the effectiveness of R decreases in the following order [277]  

In addition, among the above compounds, only when R=C(CH3)2CN or C(CH3)2Ph did these thiocarbonylthio compounds yield polymers with narrower polydispersities in batch polymerizations. Also, only these compounds allowed molecular weight control that may be expected from a living polymerization. The reaction mechanism was proposed by Moad and coworkers as follows [277]  

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Fijten MWM, Meier MAR, Hoogenboom R, Schubert US (2004) Automated parallel inves-tigations/optimizations of the reversible addition-fragmentation chain transfer polymerization of methyl methacrylate. J Polym Sci Part A Polym Chem 42 5775-5783... 

Paulus RM, Fijten MWM, de la Mar MJ, Hoogenboom R, Schubert US (2005) Reversible addition-fragmentation chain transfer polymerization on different synthesizer platforms. QSAR Comb Sci 24 863-867... 

Hawker et al. 2001 Hawker and Wooley 2005). Recent developments in living radical polymerization allow the preparation of structurally well-defined block copolymers with low polydispersity. These polymerization methods include atom transfer free radical polymerization (Coessens et al. 2001), nitroxide-mediated polymerization (Hawker et al. 2001), and reversible addition fragmentation chain transfer polymerization (Chiefari et al. 1998). In addition to their ease of use, these approaches are generally more tolerant of various functionalities than anionic polymerization. However, direct polymerization of functional monomers is still problematic because of changes in the polymerization parameters upon monomer modification. As an alternative, functionalities can be incorporated into well-defined polymer backbones after polymerization by coupling a side chain modifier with tethered reactive sites (Shenhar et al. 2004 Carroll et al. 2005 Malkoch et al. 2005). The modification step requires a clean (i.e., free from side products) and quantitative reaction so that each site has the desired chemical structures. Otherwise it affords poor reproducibility of performance between different batches. 

Titirici MM, Sellergren B. Thin molecularly imprinted polymer films via reversible addition-fragmentation chain transfer polymerization. Chem Mater 2006 18 1773-1779. 

Spherical gold nanoparticles coated with poly(N-isopropylacrylamide) (PNIPAM) grafts have been synthesized by controlled radical polymerization. The polymerization of N-isopropylacrylamide was initiated from the surface of the nanoparticles modified with 4-cyanopentanoic acid dithiobenzoate for reversible addition-fragmentation chain-transfer polymerization. The mean diameter of the Au core was 3.2 nm, as observed by means of high-resolution transmission electron microscopy [90]. 

Reversible Addition-Fragmentation Chain Transfer Polymerization. 99... 

RAFT reversible addition-fragmentation chain transfer (polymerization)... 

Economically produced block copolymers containing acrylonitrile or methacrylo-nitrile as the principal component have been prepared that are heat resistant, weath-erable, and oil and flame resistant. These materials were prepared using reversible addition-fragmentation chain transfer polymerization. 

Scheme 28 Reversible addition-fragmentation chain transfer polymerization (RAFT) or macromolecular design by interchange of xanthates mechanism...

Figure 13.3 Overview showing the use of bifunctional initiators for block-copolymer synthesis. Two examples using either reversible addition fragmentation chain transfer polymerization or atom transfer radical polymerization combined with eROP are shown.

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. 

STENZEL-ROSENBAUM M., DAVIS T.P., CHEN V., FANE A.G., Star-polymer synthesis via radical reversible addition-fragmentation chain-transfer polymerization. J. Polym Sci, Part A Polym Chem. (2001), 39 (9), 1353-65. 

BARNER L., QUINN J.F., BARNER-KOWOLLIK C.H., VANA P., DAVIS T.P., Reversible addition-fragmentation chain transfer polymerization initiated with y-radiation at ambient temperature an overview. European Polymer Journal, (2003), 39, 449-459. 

The development of controlled radical polymerization (CRP) methods,(1,2) including atom transfer radical polymerization (ATRP),(3-6) nitroxide-mediated radical polymerization,(7) and reversible addition fragmentation chain transfer polymerization,(8,9) has led to the synthesis of an unprecedented number of novel, previously inaccessible polymeric materials. Well-defined polymers, i.e., polymers with predetermined molecular weight, narrow molecular weight distribution, and high degree of chain end functionalization, prepared by... 

In the field of living radical polymerization, MALDI-TOF has been highly useful for characterization of polymers prepared by nitroxide-mediated radical polymerization (NMRP) [35, 36], atom transfer radical polymerization (ATRP) [37], and reversible addition-fragmentation chain transfer polymerization (RAFT) [38, 39]. The modern MALDI-TOF-MS permits fast and accurate determination of a variety of polymer characteristics [40]. 

Besides the ATRP method, other controlled radical polymerization techniques such as reversible addition/fragmentation chain transfer polymerization (RAFT) (Zhang et al., 2007) and nitroxide-mediated polymerization (NMP) (Yoshida and Ohta, 2005), have also been explored to synthesize azo BCs. 

Stenzel, M.H. Davis, T.P. Star polymer synthesis using trithiocarbonate functional P-cyclodextrin cores (reversible addition-fragmentation chain-transfer polymerization). J. Polym. Sci. A 2002,40 (24), 4498-4512. 

Russum JP, Jones CW, SchorkFJ (2004) Continuous reversible addition-fragmentation chain transfer polymerization in miniemulsion utilizing a multi-tube reaction system. Macromol Rapid Commun 25 1064—1068... 

A. B. Lowe, B. S. Sumerlin, M. S. Donovan, and C. L. McCormick, Facile preparation of transition metal nanoparticles stabilized by well-defined (co)poiymers synthesized via aqueous reversible addition-fragmentation chain transfer polymerization, J. Am. Chem. Soc., 124, 11562-11563 (2002). 

M. C. lovu, C. R. Craley, M. Jeffries-EL, A. B. Krankowski, R. Zhang, T. Kowalewski, R. D. McCullough, Conducting Regioregular Polythiophene Block Copolymer Nanofibrils Synthesized by Reversible Addition Fragmentation Chain Transfer Polymerization (RAFT) and Nitroxide Mediated Polymerization (NMP). Macromolecules 2007,40,4733-4735. 

The living radical polymerization (LRP) approach was first introduced in the 1980s. LRP is a type of polymerization in which a chain can only propagate and not undergo irreversible termination or chain transfer. Hence, LRP is an ideal system to produce monodisperse polymers of known molecular weights, architectures and compositions. Reversible addition-fragmentation chain transfer polymerization (RAFT), atom transfer radical... 

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