Free Radical Polymerization Techniques

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. 

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... 

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... 

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). 

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. 

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]. 

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... 

Graft copolymers represent a valuable class of polymeric materials. They are composed of a main polymer chain to which one or more side chains are connected through covalent bonds. The branches are usually randomly distributed along the backbone. Synthetic methods initially developed for their preparation led to the formation of rather ill-defined polymers. These techniques were based mainly on free radical polymerization techniques because of their simplicity. More elaborate techniques were developed later to produce more homogeneous and well characterized graft copolymers. 

A controlled free radical polymerization technique was also used, namely, atom transfer radical polymerization (ATRP), to synthesize ATE on the basis of ABA triblock copolymers (Fig. 11.3b) (Cui et al., 2004). The triblock copolymer was designed to have a rubbery midblock of poly(n-butyl acrylate) (PnBA) and two end blocks of poly 6-[4-(4-methoxyphenylazo)phenoxy]hexyl methacrylate) (PAzoMA) that is azo-SCLCP. For synthesis, a dibromo initiator, namely, l,l -biphenyl-4,4 -bis(2-bromoisobutyrate), can first be used to prepare the dibromo PnBA macroinitiator, which is then used to polymerize the azobenzene methacrylate monomer to yield the two end blocks of PAzoMA. This ATE is different from azo-SCLCP-grafted SBS. It is a thermoplastic elastomer, in which... 

Some special reaction with a particularly designed route can be used to synthesize azo BCs. For instance, a series of poly(vinyl ether)-based azo LCBCs were synthesized by using living cationic polymerization and free-radical polymerization techniques (Serhatli and Serhatli, 1998). As shown in Scheme 12.8, 4.4 -azobis(4-cyano pentanol) (ACP) was used to couple quantitatively two well-defined polymers of LC living poly(vinyl ether), initiated by the methyl trifluor-omethane sulfonate/tetrahydrothiophene system. Then the ACP in the main chain was thermally decomposed to produce polymeric radical, which was used to initiate the polymerization of MMA or styrene to obtain PMMA-based or PS-based azo BCs (AB or ABA types). 

Scheme 12.8 Preparation of azo BCs by using living cationic polymerization and free-radical polymerization techniques. Source Reproduced from Serhatli and Serhatli, 1998.

The emergence of controlled living radical polymerization techniques has opened a novel era for the synthesis of a vast variety of diblock copolymers consisting of blocks that could not be linked together by using the ionic polymerization techniques. The control of chain transfer reactions and the suppression of unwanted termination are now possible in free radical polymerization techniques through ATRP, RAFT, and NMRP. 

The reverse mode in which controlled free radical polymerization techniques are employed as the first step is also possible. The most widely employed controlled radical polymerization methods for this particular transformation are ATRP and NMRP. This is mainly because of the fact that hydroxyl and amino groups, potential initiating sites for the AROP of certain monomers, are compatible with the ATRP and NMRP of vinyl monomers. Table 4 summarizes the examples of this transformation approach. 

In a conceptually similar methodology, ATRP was replaced with another controlled free radical polymerization technique, NMRP, using a hydroxyl-functionalized alkoxyamine derived from the SGI nitroxide. Sequential NMRP of nBA and AROP of CL yielded a partially degradable segmented polymeric structure, PBA-I7-PCL (Scheme 49). 

Similar to anionic polymerization, cationic polymerization has been combined with controlled free radical polymerization techniques such as NMRP, ATRP, and RAFT for the synthesis of well-defined block copolymers. 

This is not possible in any other free-radical polymerization technique (bulk, solution, suspension). 

Polymers PAM was synthesized by conventional free radical polymerization techniques. 

Free radical polymerization processes are used to produce approximately 50% of polymer products worldwide and are therefore of great industrial importance [1]. However, many product properties carmot be controlled precisely using conventional free radical polymerization techniques due to the fundamental reaction mechanism. Whereas general bulk properties of polymers can be controlled to some extent with conventional processes, structural control at the molecular level carmot be achieved. 

Over the past 15 years, new free radical polymerization techniques have been developed which allow significantly improved control over polymer structure at the molecular level. By using these techniques, customized polymeric materials can be produced which are not possible using conventional methods of the past. These new techniques are typically termed living or controlled free radical polymerization. There is some debate over the semantic use of these terms [2,3], but the term living radical polymerization (LRP) will be used here for simplicity. 

NMP is another controlled free radical polymerization technique that can be used to polymerize styrene [98-100]. Weimer et al. investigated surface-initiated PS-MMT nanocomposites using MMT modified with a nitroxyl-mediated living... 

Polypropylene-g-PS copolymers were synthesized by combination of metallocene and TEMPO living free-radical polymerization techniques (41). The backbone was synthesized by copolymerization of propylene and a TEMPO-fimctionalized derivative containing a a-double bond. The TEMPO groups were then used for the pol5mierization of styrene by hving free-radical pol5onerization (Fig. 7). 

The evolution of the living free-radical polymerization techniques (mainly TEMPO (2,2,6,6-tetramethylpiperidinyl-l-oxy) and ATRP methods) very soon led to the synthesis of macromonomers. These methods combine the advantages of the free-radical polymerization with those of the living polymerization techniques, despite the fact that control over the functionalization reaction is not always comparable to the anionic polymerization methods (91,92). 

The scavenger molecule is by itself a radical and reacts with any other radicals in the system to generate nonreactive products. Because of the stability of the radical compounds employed for such inhibition/retardation reactions, the generated bond is very weak and may homolytically cleave at elevated temperatures to give back the radical reactants. This reaction behavior is exploited in the living free radical polymerization technique using nitroxides as mediators (434,435). 

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