Living radical polymerization block copolymer synthesis

The hindered carbon-centered radicals are most suited as mediators in the polymerization of 1,1-disubstituled monomers e.g. MMA,78,95 other methacrylates and MAA,06 and AMS97). Polymerizations of monosubstituted monomers are not thought to be living. Dead end polymerization is observed with S at polymerization temperatures <100°C.98 Monosubstituted monomers may be used in the second stage of AB block copolymer synthesis (formation of the B block).95 However the non-living nature of the polymerization limits the length of the B block that can be formed. Low dispersities are generally not achieved. 

Living polymerization processes immediately lend themselves to block copolymer synthesis and the advent of techniques for living radical polymerization has lead to a massive upsurge in the availability of block copolymers. Block copolymer synthesis forms a significant part of most reviews on living polymerization processes. This section focuses on NMP,106 A TRP,265,270 and RAFT.- 07 Each of these methods has been adapted to block copolymer synthesis and a substantial part of the literature on each technique relates to block synthesis. Four processes for block copolymer synthesis can be distinguished. 

A half-metallocene iron iodide carbonyl complex Fe(Cp)I(CO)2 was found to induce the living radical polymerization of methyl acrylate and f-bulyl acrylate with an iodide initiator (CH3)2C(C02Et)I and Al(Oi- Pr)3 to provide controlled molecular weights and rather low molecular weight distributions (Mw/Mn < 1.2) [79]. The living character of the polymerization was further tested with the synthesis of the PMA-fc-PS and PtBuA-fi-PS block copolymers. The procedure efficiently provided the desired block copolymers, albeit with low molecular weights. 

Such a two-component iniferter technique is also applied to the living radical polymerization of several DC photoiniferters for the design of block and graft copolymer synthesis (Sect. 5). 

Tetraethylthiuram disulfide (13) induces St polymerization by the photodissociation of its S-S bond to give the polymer with C-S bonds at both chain ends (15). The C-S bond further acts as a polymeric photoiniferter, resulting in living radical polymerization. Eventually, some di- or monosulfides, as well as 13, were also examined as photoiniferters and were found to induce polymerization via a living radical polymerization mechanism close to the model in Eq. (18), e.g., the polymerization of St with 35 and 36 [76,157]. These disulfides were used for block copolymer synthesis [75,157-161] ... 

While there have been several studies on the synthesis of block copolymers and on the molecular weight evolution during solution as well as bulk polymerizations (initiated by iniferters), there have been only a few studies of the rate behavior and kinetic parameters of bulk polymerizations initiated by iniferters. In this paper, the kinetics and rate behavior of a two-component initiation system that produces an in situ living radical polymerization are discussed. Also, a model that incorporates the effect of diffusion limitations on the kinetic constants is proposed and used to enhance understanding of the living radical polymerization mechanism. 

Huan, K. Bes, L. Haddleton, D. M. Khoshdel, E. Surfactant Properties of Poly(dimethylsiloxane)-Gontaining Block Copolymers from Living Radical Polymerization. In Synthesis and Properties of Silicones and Silicone-Modified Materials Clarson, S. J., Fitzgerald, J. J., Owen, M. J., Smith, S. D., Van Dyke, M. E., Eds. ACS Symposium Series 838 American Chemical Society Washington, DC, 2003 pp 260-272. 

To prepare block copolymers by ATRP, the initiation site for living radical polymerization can be introduced at the end of a polymer chain. In this context, terminally functionalized POs are useful for the synthesis of block copolymers. 

A series of at least 14 papers [200-208] have been published dealing with the synthesis of telechelic polymers or block copolymers from the radical polymerization of various vinyl monomers with substituted 1,1,2,2-tetraphenyl ethanes. These aromatic compounds, known for over a century [209], are efficient in radical polymerization [201,210], They behave as both initiators and terminating agents [200] that can be involved in living radical polymerization as illustrated in the following reaction ... 

Investigations were mainly devoted to the synthesis of telechelic polymers and copolymers rather than to living radical polymerization. In particular, from 1960, Imoto et al. [234] started surveys on the synthesis of block copolymers from this method. Thus, polystyrene-i>-poly(vinyl alcohol) diblock copolymer... 

One of the most distinguishable characteristics of the metal-catalyzed living radical polymerization is that it affords polymers with controlled molecular weights and narrow MWDs from a wide variety of monomers under mild conditions even in the presence of a protic compound such as water. This permits the synthesis of a vast number of polymers with controlled structures such as end-functionalized polymers, block copolymers, star polymers, etc., where they are widely varied in comparison with those obtained by other living polymerizations. This is primarily due to the tolerance to various functional groups and the polymerizability/controllability of various vinyl monomers as mentioned above. 

When coupled with living radical systems, living ringopening metathesis polymerization (ROMP) also permits the synthesis of other types of block copolymers (Figure 23) such as B-102 to B-108.67,396,397 A molybdenum carbene or ROMP intermediate is converted into a benzyl bromide-type terminal by quenching the ROMP with /> (b r omo me thy 1) b en z al d e hy d e by a retro-Wittig reaction.396 The macroinitiator thus obtained induced living radical polymerizations of styrene and MA with copper catalysts to afford B-102 to B-105. 

Such living conditions are found principally In anlonlcally Initiated systems and Involve common monomers such as styrene, ormethylstyrene, butadiene and Isoprene (1,22). They are far less common In catlonlcally Initiated systems, there being virtually no established example Involving vinyl monomers, but some cyclic monomers such as tetrahydrofuran (THF) and the oxetanes may be polymerized under carefully specified conditions to yield living polymers ( ). Although living free radical systems have also been described In which radicals have been preserved on surfaces. In emulsion, or by precipitation before termination occurs, these are special conditions not easily adapted for clean block copolymer synthesis. 

Bouix M, Gouzi J, Charleux B, Vairon JP, Guinot P. Synthesis of amphiphilic polyelectrolyte block copolymers using living radical polymerization. Application as stabilizers in emulsion polymerization. Macromol Rapid Commun 1998 19 209-213. 

Controlled/ Living radical polymerization (CRP) of vinyl acetate (VAc) via nitroxide-mediated polymerization (NMP), organocobalt-mediated polymerization, iodine degenerative transfer polymerization (DT), reversible radical addition-fragmentation chain transfer polymerization (RAFT), and atom transfer radical polymerization (ATRP) is summarized and compared with the ATRP of VAc catalyzed by copper halide/2,2 6 ,2 -terpyridine. The new copper catalyst provides the first example of ATRP of VAc with clear mechanism and the facile synthesis of poly(vinyl acetate) and its block copolymers. 

VAc has been successfully polymerized via controlled/ living radical polymerization techniques including nitroxide-mediated polymerization, organometallic-mediated polymerization, iodine-degenerative transfer polymerization, reversible radical addition-fragmentation chain transfer polymerization, and atom transfer radical polymerization. These methods can be used to prepare well-defined various polymer architectures based on PVAc and poly(vinyl alcohol). The copper halide/t is an active ATRP catalyst for VAc, providing a facile synthesis of PVAc and its block copolymers. Further developments of this catalyst will be the improvements of catalytic efficiency and polymerization control. 

Ma, Z. and Lacroix-Desmazes, P., Synthesis of hydrophiUc/C02-philic poly(ethylene oxide)-i-poly(l,l,2,2-tetrahydroperfluorodecyl acrylate) block copolymers via controlled living radical polymerizations and their properties in hquid and supercritical CO2 (experimental data by P. Lacroix-Desmazes), J. Polym. Sci. Part A Polym. Chem., 42, 2405, 2004. 

Georges, M.K., Hamer, G.K., Listigovers, N.A., 1998. Block copolymer synthesis by a nitroxide-mediated living free radical polymerization process. Macromolecules 31 (25), 9087-9089. 

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