Radical polymerization technique

The structure-property relationship of graft copolymers based on an elastomeric backbone poly(ethyl acry-late)-g-polystyrene was studied by Peiffer and Rabeony [321. The copolymer was prepared by the free radical polymerization technique and, it was found that the improvement in properties depends upon factors such as the number of grafts/chain, graft molecular weight, etc. It was shown that mutually grafted copolymers produce a variety of compatibilized ternary component blends. 

Ejaz, M., Yamamoto, S., Ohno, K., Tsujii, Y. and Fukuda, T. (1998) Controlled graft polymerization of methyl methacrylate on silicon substrate by the combined use of the Langmuir-Blodgett and atom transfer radical polymerization techniques. Macromolecules, 31, 5934-5936. 

Stalmbach, U., deBoer, B Videlot, C van Hutten, P. F. and Hadziioannou, G. (2000) Semiconducting diblock copolymers synthesized by means of controlled radical polymerization techniques. J. Am. Chem. Soc., 122, 5464-5472. 

The controlled radical polymerization techniques opened up a new era in polymer synthesis, and further growth and developments are certain. However, the control of the molecular characteristics and the variety of macro-molecular architectures reported by these methods cannot be compared with those obtained by other living polymerization techniques such as anionic polymerization. 

Finally, the use of stable free radical polymerization techniques in supercritical C02 represents an exciting new topic of research. Work in this area by Odell and Hamer involves the use of reversibly terminating stable free radicals generated by systems such as benzoyl peroxide or AIBN and 2,2,6,6-tetramethyl-l-piperidinyloxy free radical (TEMPO) [94], In these experiments, styrene was polymerized at a temperature of 125 °C and a pressure of 240-275 bar C02. When the concentration of monomer was low (10% by volume) the low conversion of PS which was produced had a Mn of about 3000 g/mol and a narrow MWD (PDI < 1.3). NMR analysis showed that the precipitated PS chains are primarily TEMPO capped, and the polymer could be isolated and then subsequently extended by the addition of more styrene under an inert argon blanket. The authors also demonstrated that the chains could be extended... 

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. 

Within the short period of time since its discovery, ATRP has developed remarkably fast to become the most employed controlled radical polymerization technique... 

The principle free radical polymerization techniques are bulk, solution, suspension, and emulsion. Tables 6.5 and 6.6 briefly describe these techniques. 

Husseman M, Malmstrom EE, McNamara M, Mate M, Mecerreyes D, Benoit DG, Hedrick JL, Mansky P, Huang E, RusseU TP, Hawker CJ (1999) Controlled synthesis of polymer brushes by Living free radical polymerization techniques. Macromolecules 32 1424-1431... 

Fig. 4 Chemoenzymatic strategies (a—c) for the synthesis of block copolymers employing enzymatic ROP and radical polymerization techniques...

Initially, the polymerization of macromonomers was achieved by free radical polymerization reactions, which allowed only a limited control of the final properties. With the advent of ROMP and new free radical polymerization techniques, such as atom transfer radical polymerization (ATRP) the control of final properties became more facile (16). ATRP and ROMP techniques can be combined for the synthesis of macroinitiators (17). 

The need to better control surface-initiated polymerization recently led to the development of controlled radical polymerization techniques. The trick is to keep the concentration of free radicals low in order to decrease the number of side reactions. This is achieved by introducing a dormant species in equilibrium with the active free radical. Important reactions are the living radical polymerization with 2,2,4,4-methylpiperidine N-oxide (TEMPO) [439], reversible addition fragment chain transfer (RAFT) which utilizes so-called iniferters (a word formed from initiator, chain transfer and terminator) [440], and atom transfer radical polymerization (ATRP) [441-443]. The latter forms radicals by added metal complexes as copper halogenides which exhibit reversible reduction-oxidation processes. 

Today, the majority of all polymeric materials is produced using the free-radical polymerization technique [11-17]. Unfortunately, however, in conventional free-radical copolymerization, control of the incorporation of monomer species into a copolymer chain is practically impossible. Furthermore, in this process, the propagating macroradicals usually attach monomeric units in a random way, governed by the relative reactivities of polymerizing comonomers. This lack of control confines the versatility of the free-radical process, because the microscopic polymer properties, such as chemical composition distribution and tacticity are key parameters that determine the macroscopic behavior of the resultant product. 

Controlled radical polymerization (CRP) is an attractive tool, because of the resultant controllability of polymerization, and because of it being a versatile method to synthesize of well-defined polymer hybrids. The three main radical polymerization techniques, ATRP, NMP, and RAFT polymerization, have thus been employed. Other techniques, such as the oxidation of borane groups, have also been studied. In general, using CRP techniques, block copolymers can be synthesized from terminally functionalized PO as PO macroinitiator, and block copolymers can be prepared from functionalized PO produced by the copolymerization of olefins with functional monomers. 

Nitroxide mediated polymerization (NMP) is another type of controlled radical polymerization technique used to synthesize polymer hybrids. It relies on the reversible trapping of growing macro-radicals by nitroxide to form dormant species in which the C-ON covalent bond is thermally cleavaged (Fig. 19). At a polymerization temperature, the equilibrium between dormant and active species is strongly shifted to the dormant side, which Emits the irreversible chain termination reaction. 

As mentioned earlier, polymerization techniques can also be used in the presence of nanotubes for preparation of polymer/CNT nanocomposite materials. In these, in-situ radical polymerization techniques of polymerization in the presence of CNT filler under or without applied ultrasound. Both new factors (presence of CNT and ultrasound) can affect reaction kinetics, stability of suspension or the size of prepared particles. For example, ultrasound waves can open C=C bond of monomer, which starts polymerization initiation. Thus vinyl monomers (styrene, methyl methacrylate or vinyl acetate) can be polymerized without addition of initiator, only by application of ultrasound. This is called sonochemical polymerization method (15,33,34). 

Radical polymerization techniques that are for example used for the crosslinking of methacrylate-modified polysaccharides [9] are not included in this chapter. Prepolymer crosslinking through radical polymerization techniques as well as non-covalent microgel formation has recently been reviewed in a number of excellent review articles [2, 4, 10-14] and is thus not discussed within this chapter. 

There is significant interest in enzymes as they have proven to be powerful and environment-friendly natural catalysts for the polymerization of water-soluble monomers that can function under milder reaction conditions than those used in traditional free radical polymerization techniques. Hence, the combination of SCCO2 and water as reaction medium is a significant advancement made by Villarroya et al. 

Nitroxide-Mediated Controlled Radical Polymerization (NMCRP) was first discovered by Solomon et al., who patented their discovery in 1985 [205]. This opened up new pathways in the field of free-radical polymerization. Polymer architectures, which were the domain of the anionic polymer chemist, became accessible to the free-radical polymer chemist. However, it was not until the work of Georges et al. [206] was published in 1993, that the world of polymer chemistry became aware of the possibihties of this new class of free-radical polymerization. This was the beginning of what is today one of the leading topics in free-radical polymer chemistry Controlled or Living Free Radical Polymerization. This initiated the search for new Controlled or Living Free Radical Polymerization techniques, and soon afterwards other methods (which will be discussed later) were developed. 

A conventional free-radical initiator is added (contrary to some other controlled free-radical polymerization techniques) that generates radicals, which can add either to the monomer or the S=C moiety of the RAFT agent (step 1). In most cases the addition of small carbon-centered radicals to the RAFT agent is rapid and is not rate determining. Therefore, step (1) involves polymeric radical addition to 1 to form an intermediate radical species 2 that will fragment back to the original polymeric radical species or fragment to a dormant species 3... 

In addition to blending with SPMI copolymers, PMI can be incorporated into ABS using mass, emulsion [46-50] or suspension [42] free radical polymerization techniques. The high heat ABS resin can be completely produced by mass polymerization, or mass polymerized PMI-SAN can be blended with (emulsion polymerized) SAN-grafted rubber concentrates and/or conventional mass ABS. Sumitomo Naugatuck determined an empirical relation for the compatibility of SAN/SAN-PMI blends based on the polar monomers in each component [51]. Figure 15.4 shows that the miscibility window with SANs becomes wider with increasing PMI level in the terpolymer [52]. 

S. Hohnberg, P. Holmlund, C.E. Wilen, T. KaUio, G. Sundhohn, and F. Sundhohn. Synthesis of proton-conducting membranes by the utilization of preirradiation grafting and atom transfer radical polymerization techniques. Journal of Polymer Science Part A Polymer Chemistry 40, 591-600 2002. 

The first example of the use of a controlled radical polymerization technique for the construction of polymers with side chain peptide moieties was... 

The nature of free-radical polymerization has traditionally hindered attempts to produce an ideal living free radical polymerization technique. It is very difficult to prevent chain transfer and termination reactions in free-radical polymerizations and although several methods have afforded polymers with very low polydispersities < 1.1), these approaches are often referred... 

Fig. 47. Illustration of the growth of polymer chains using the MM method and various radical polymerization techniques. Reprinted with permission from [320]. Copyright (2001) American Chemical Society.

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