Living radical polymerization block copolymer

Keywords Review, Copolymer, Controlled/living radical polymerization, Block, Graft, Gradient, Statistical... 

The C-S bond of the sulfide end groups can be relatively weak and susceptible to thermal and photo- or radical-induced homolysis. This means that certain disulfides [for example 7-9] may act as iniferters in living radical polymerization and they can be used as precursors to block copolymers (Sections 7.5.1 and 9.3.2). 

Further examples of micellar stabilization when micelles are composed of block copolymers formed by living radical polymerization are mentioned in Section 9.9.2. 

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. 

Many block and graft copolymer syntheses involving transformation reactions have been described. These involve preparation of polymeric species by a mechanism that leaves a terminal functionality that allows polymerization to be continued by another mechanism. Such processes are discussed in Section 7.6.2 for cases where one of the steps involves conventional radical polymerization. In this section, we consider cases where at least one of the steps involves living radical polymerization. Numerous examples of converting a preformed end-functional polymer to a macroinitiator for NMP or ATRP or a macro-RAFT agent have been reported.554 The overall process, when it involves RAFT polymerization, is shown in Scheme 9.60. 

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. 

Keywords Controlled Polymerization Living Radical Polymerization Iniferter Chain-End Structure Molecular Weight Control Block Copolymer Dithiocarbamate Disulfide Nitroxide Transition Metal Complex... 

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). 

The resulting polymers can further induce the radical polymerization of second monomers to give block copolymers. The polymerization with the alkoxyamines has developed to the recent living radical polymerization providing polymers with well-controlled molecular weight and molecular weight distribution, as will be described in Sect 6.1. 

Block copolymerization was carried out in the bulk polymerization of St using 18 as the polymeric iniferter. The block copolymer was isolated with 63-72 % yield by solvent extraction. In contrast with the polymerization of MMA with 6, the St polymerization with 18 as the polymeric iniferter does not proceed via the livingradical polymerization mechanism,because the co-chain end of the block copolymer 19 in Eq. (22) has the penta-substituted ethane structure, of which the C-C bond will dissociate less frequently than the C-C bond of hexa-substituted ethanes, e.g., the co-chain end of 18. This result agrees with the fact that the polymerization of St with 6 does not proceed through a living radical polymerization mechanism. Therefore, 18 is suitably used for the block copolymerization of 1,1-diubstituted ethylenes such as methacrylonitrile and alkyl methacrylates [83]. 

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] ... 

It was confirmed that the resulting polymers obtained from the St polymerization with 13 induced further photopolymerization of MMA to produce a block copolymer, and the yield and molecular weight increased as a function of the polymerization time, similar to the results for the polymerization of MMA with 13, indicating that this block copolymerization also proceeds via a living radical polymerization mechanism [64]. Similar results were also obtained for the photoblock copolymerization of VAc. Thus, various kinds of two- or three-component block copolymers were prepared [157,158]. 

As is expected from these results, it is very difficult to control the polymerization of monomers other than St, e.g., that of MMA, because of the too small dissociation energy of the chain end of poly(MMA). In fact, the polymerization of MMA in the presence of TEMPO yielded the polymer with constant Mn irrespective of conversion, and the Mw/Mn values are similar to those of conventional polymerizations [216]. The disproportionation of the propagating radical and TEMPO would also make the living radical polymerization of MMA difficult. In contrast, the controlled polymerization of MA, whose propagating radical is a secondary carbon radical,has recentlybeen reported [217]. Poly(MA) with a narrow molecular weight distribution and block copolymers were obtained. 

Nitroxide attached to macromolecules also induces the living radical polymerization of St. Yoshida and Sugita [252] prepared a polymeric stable radical by the reaction of the living end of the polytetrahydrofuran prepared by cationic polymerization with 4-hydroxy-TEMPO and studied the living radical polymerization of St with the nitroxide-bearing polytetrahydrofuran chain. The nitroxides attached to the dendrimer have been synthesized (Eq. 69) to yield block copolymers consisting of a dendrimer and a linear polymer [250,253]. 

The living radical polymerization of some derivatives of St was carried out. The polymerizations of 4-bromostyrene [254], 4-chloromethylstyrene [255, 256], and other derivatives [257] proceed by a living radical polymerization mechanism to give polymers with well-controlled structures and block copolymers with poly(St). The random copolymerization of St with other vinyl... 

Sawamoto et al. have revealed that the ruthenium complex induces the living radical polymerization of MMA [30,273-277]. For example, RuCl2(PPh)3 provided poly(MMA) with Mw/Mn 1.1 and the block copolymers. This system has a unique characteristic in that it is valid not only for MMA and other methacrylates, but also for acrylates and St derivatives. 

Living radical polymerizations have received considerable attention because they provide a convenient alternative for synthesizing block copolymers, polymers of narrow polydispersity and complex polymer structures (1-5). Because of their ability to initiate living free radical polymerizations, iniferters have been examined extensively after Otsu et al. (6) introduced them in 1982. In particular, dithiocarbamate derivatives have been studied more closely by several researchers. Lambrinos et al (7) have examined the molecular weight evolution during the polymerization of n-butyl acrylate using p-x ylylene bis(A,A-diethyl... 

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. 

Bes L, Angot S, Limer A, Haddleton DM. Sugar-coated amphiphilic block copolymer micelles from living radical polymerization recognition by immobilized lectins. Macromolecules 2003 36 2493-2499. 

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. 

Davis KA, Matyjaszewski K (2002) Statistical, gradient, block and graft copolymers by controlled/living radical polymerizations. Springer, Berlin Heidelberg New York... 

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. 

Living radical polymerizations in miniemulsions have also been conducted by de Brouwer et al. using reversible addition-fragmentation chain transfer (RAFT) and nonionic surfactants [98]. The polydispersity index was usually below 1.2. The living character is further exemplified by its transformation into block copolymers. 

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