Monomers and Reaction Conditions

ATRP has been widely used for the polymerization of methacrylates. However, a very wide range of monomers, including most of those amenable to conventional radical polymerization, has been used in ATRP. ATRP has also been used in cyclopolymerization e.g. of 160 ) and ring opening polymerization or copolymerization (e.g. of 161 and 162 ).  

Tlie selection of reaction conditions for ATRP is dependent on many factors including the particular monomer, initiator and catalyst. 

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The mode of termination varies with monomer and reaction conditions. While styrene macroradicals typically terminate by coupling, methyl methacrylate macroradicals terminate by coupling at temperatures below 60°C, but by disproportionation at higher temperatures. 

The radiolysis of olefinic monomers results in the formation of cations, anions, and free radicals as described above. It is then possible for these species to initiate chain polymerizations. Whether a polymerization is initiated by the radicals, cations, or anions depends on the monomer and reaction conditions. Most radiation polymerizations are radical polymerizations, especially at higher temperatures where ionic species are not stable and dissociate to yield radicals. Radiolytic initiation can also be achieved using initiators, like those used in thermally initiated and photoinitiated polymerizations, which undergo decomposition on irradiation. 

A variety of behaviors are observed for the polymerization rate versus conversion depending on the relative rates of initiation, propagation, and termination, which are in turn dependent on the monomer and reaction conditions (Fig. 4-2). Irrespective of the particular behavior observed, three intervals (I, II, III) can be discerned in all emulsion polymerizations based on the particle number N (the concentration of polymer particles in units of number of... 

There are very few reported copolymerizations between cyclic monomers and carbon-carbon double-bond monomers. Such copolymerizations would require a careful selection of the monomers and reaction conditions to closely match the reactivities of the different monomers and propagating centers. The almost complete absence of successes indicates that the required balancing of reactivities is nearly impossible to achieve. There are a few reports of copolymerizations between carbon-carbon double-bond monomers and cyclic ethers or acetals [Higashimura et al., 1967 Inoue and Aida, 1984 Yamashita et al., 1966],... 

The isoselective polymerization of a racemic mixture of monomers can proceed in two ways depending on initiator, monomer, and reaction conditions. Racemate-forming enantiomer-differentiating polymerization involves both the R and 5 monomers polymerizing at the same rate hut without any cross-propagation [Hatada et al., 2002]. A racemic monomer mixture polymerizes to a racemic mixture of all-5 and all-5 polymer molecules [Pino, 1965 ... 

Table 3 Properties of several monomers and reaction conditions...

The synthetic sequence to methylene-bridged poly(phenylene)s 71 represents the first successful employment of the stepwise process to ladder-type macromolecules involving backbone formation and subsequent polymer-analogous cyclization. As shown, however, such a procedure needs carefully tailored monomers and reaction conditions in order to obtain structurally defined materials. The following examples demonstrate that the synthesis of structurally defined double-stranded poly(phenylene)s 71 (LPPP) via a non-concerted process is not just a single achievement, but a versatile new synthetic route to ladder polymers. By replacing the dialkyl-phenylenediboronic acid monomer 68 by an iV-protected diamino-phenylenediboronic acid 83, the open-chain intermediates 84 formed after the initial aryl-aryl cross-coupling can te cyclized to an almost planar ladder-type polymer of structure 85, as shown recently by Tour and coworkers [107]. 

Some characteristics of initiators used for thermal initiation arc summarized in Table 3.1. These provide some general guidelines for initiator selection. In general, initiators which afford carbon-ccntcrcd radicals e.g. dialkyldiazcncs, aliphatic diacyl peroxides) have lower efficiencies for initiation of polymerization than those that produce oxygen-centered radicals. Exact values of efficiency depend on the particular initiators, monomers, and reaction conditions. Further details of initiator chemistry are summarized in Sections 3.3.1 (azo-compounds) and 3.3.2 (peroxides) as indicated in Table 3,1. In these sections, we detail the factors which influence the rate of decomposition i.e. initiator structure, solvent, complexing agents), the nature of the radicals formed, the susceptibility of the initiator to induced decomposition, and the importance of transfer to initiator and other side reactions of the initiator or initiation system. The reactions of radicals produced from the initiator arc given detailed treatment in Section 3.4. 

There are two important methods for PLA synthesis direct polycondensation of lactic acid and ROP of lactic acid cyclic dimer, known as lactide. In direct condensation, solvent is used and higher reaction times are required. The resulting polymer is a material of low to intermediate molecular weight. ROP of the lactide needs catalyst but results in PLA with controlled molecular weight. Depending on the monomer and reaction conditions, it is possible to control the ratio and sequence of d- and L-lactic add units in the final polymer [74,75],... 

In summary, the numerous variations in substituents, length, branching, and cross-linking make it possible to produce a variety of properties for each type of addition polymer. Chemists and chemical engineers can fine-tune the properties of a polymer to match desired properties. Appropriate selection of monomer and reaction conditions accounts for the widespread use of these giant molecules. 

In comparison to other controlled radical polymerization techniques, RAFT is more tolerant to a wide range of monomers and reaction conditions. 

While this approach proved to be an efficient and attractive means for a wide range of monomers and reaction conditions, side reactions and deviations from ideality due to incompatibility between certain monomer types and control agents and experimental conditions are reported [73]. 

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