Living radical polymerization characteristics

The unexpected good control in the incorporation of borane groups to polyolefin by metallocene catalysis and the subsequent radical chain extension by the incorporated borane groups prompted us to examine this free radical polymerization mechanism in greater details. Several relatively stable borane-based radical initiators were discovered, which exhibited living radical polymerization characteristics, with a linear relationship between polymer molecular weight and monomer conversion [27] and producing block copolymers by sequential monomer addition [28]. This stable radical... 

Polymerization of acrylates and methacrylates [32] was studied at temperatures of 25—40°C. At these temperatures, it is reasonable that the C—O bond of the growing end of polymer chain, formed by radical recombination would be weakened to allow a terminal alkoxyl group and would, thus, not exhibit living radical polymerization characteristics. Solomon et al. 

For this book, we have decided to entitle this chapter Living Radical Polymerization and use the term throughout, it is a chapter describing various approaches to living radical polymerization. We do not intend to imply that termination is absent from all or, indeed, any of the polymerizations described, only that the polymerizations display at least some of the observable characteristics normally associated with living polymerization. 

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. 

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. 

Abstract. Water-soluble polymers of acrylamide and acrylic acid with high extent ( 90%) of Ceo consumption are obtained by technique of low-temperature radiation living radical polymerization. In absorption spectra of these copolymers one can see gradually descended unstructured absorption in the range 240-700 mn, characteristic for fullerene covalent-bound or its klasters. The way of radiation initiation is used to obtain the products of high purity, because it is not necessary to embed into the system any initiators or catalyst. Latter is very important in the case of synthesis of polymers for medical purposes. Also at radiation initiation a rate of initiation reaction does not depend on the temperature and the sterilization of products takes place simultaneously. 

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. 

A mixture of two monomers that can be homopo-lymerized by a metal catalyst can be copolymerized as in conventional radical systems. In fact, various pairs of methacrylates, acrylates, and styrenes have been copolymerized by the metal catalysts in random or statistical fashion, and the copolymerizations appear to also have the characteristics of a living process. The monomer reactivity ratio and sequence distributions of the comonomer units, as discussed already, seem very similar to those in the conventional free radical systems, although the detailed analysis should be awaited as described above. Apart from the mechanistic study (section II.F.3), the metal-catalyzed systems afford random or statistical copolymers of controlled molecular weights and sharp MWDs, where, because of the living nature, there are almost no differences in composition distribution in each copolymer chain in a single sample, in sharp contrast to conventional random copolymers, in which there is a considerable compositional distribution from chain to chain. Figure 26 shows the random copolymers thus prepared by the metal-catalyzed living radical polymerizations. 

Controlled/ living radical polymerization (CLRP) processes are well-established synthetic routes for the production of well-defined, low-molecular weight-dispersity polymers [99]. The types of CLRP processes (initiator-transfer agent-terminator (INIFERTER), atom transfer radical polymerization (ATRP), nitroxide-mediated radical (NMRP) polymerization, reversible addition-fragmentation transfer (RAFT)) and their characteristics are described in Section 3.8 of Chapter 3 and in Section 14.8 of Chapter 14. 

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]. 

Use these results to demonstrate that the reaction has the characteristics of a living radical polymerization, and identify the type of CRP. 

Heterobifiinctional block copolymers of PEO and poly (N-isopropylacrylamide) (PNIPAM) were synthesized by RAFT polymerization of N-isopropylacrylamide (NIPAM) using a PEO-based trithiocarbonate macromolecular CTA. The polymerization exhibited all the expected characteristics of living radical polymerization and allowed the synthesis of copolymers with different lengths of the PNIPAM block. 

Among the iniferters used, some compounds containing i, i r-diethyl-dithiocarbamate groups were found to be excellent photoiniferters of living radical polymerization. Otsu et al. [55] summarized these ideas and discussed some characteristics of the living radical polymerization with tetraethylthiuram disulfide (TD), benzyl JV,JV-diethyl-dithiocarbamate (BDC), / -xylene bis(iV,JV-diethyl-dithiocarbamate (XDC), and tetrakis(AT,iV-diethyl-dithiocarbamayl) benzene (DDC) as photoiniferters. 

Some of the more remarkable examples of this form of topologically controlled radical polymerization were reported by Percec et cii.231 234 Dendron maeromonomers were observed to self-assemble at a concentration above 0.20 mol/L in benzene to form spherical micellar aggregates where the polymerizable double bonds are concentrated inside. The polymerization of the aggregates initiated by AIBN showed some living characteristics. Diversities were narrow and molecular weights were dictated by the size of the aggregate. The shape of the resultant macroniolecules, as observed by atomic force microscopy (ATM), was found to depend on Xn. With A, <20, the polymer remained spherical. On the other hand, with X>20, the polymer became cylindrical.231,232... 

The first reports of ATRP (Atom Transfer Radical Polymerization), which clearly displayed the characteristics of living polymerization, appeared in 1995 from the Laboratories of Sawamoto,2 Matyjaszewski266 and Percec.267 The literature on ATRP is now so vast that a comprehensive review cannot be... 

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