Living radical copolymerization reactivity ratios

VF.s do not readily enter into copolymerization by simple cationic polymerization techniques instead, they can be mixed randomly or in blocks with the aid of living polymerization methods. Reactivity ratios must be taken into account if random copolymers, instead of mixtures of homopolymers, are to be obtained by standard cationic polymerization. VEs can also copolymcrizc by ftcc-radical initiation with a variety of comonomers. 

Although, there are reports on differences in reactivity ratios observed for conventional radical copolymeri/.ation vs living radical copolymerization or RAKT " ), most research suggests that reactivity ratios are identical and any discrepancies in composition should be attributed to other factors. 

Klumperman and coworkers [259] observed that while it is lately quite common to treat living radical copolymerization as being completely analogous to its radical counterpart, small deviatiOTis in the copolymerization behavior do occur. They interpret the deviations on the basis of the reactions being specific to controlled/living radical polymerization, such as activation—deactivation equilibrium in ATRP. They observed that reactivity ratios obtained from atom transfer radical copolymerization data, interpreted according to the conventional terminal model deviate from the true reactivity ratios of the propagating radicals. 

The homopolymerization of MMA with the soluble catalyst was found to exhibit the characteristic of living polymerization at the initial stage of polymerization ( 5 h) giving poly(MMA) with a narrow molecular weight distribution (Mw/IVln = 1.2, Mn = 2400), at 25 °C. To elucidate the mechanism of the MMA polymerization, the copolymerization of MMA with styrene was carried out. The observed reactivity ratios (rs = 0.5, rMMA = 0.4) indicated that the living polymerization of MMA occurred via a radical intermediate. 

Narrow distribution in the backbone length as well as in the chemical composition or the branch frequency may be expected from a living-type copolymerization between a macromonomer and a comonomer provided the reactivity ratios are close to unity. This appears to have been accomplished to some extent with anionic copolymerizations with MMA of methacrylate-ended PMMA, 29, and poly(dimethylsiloxane) macromonomers, 30, which were prepared by living GTP and anionic polymerization, respectively [50,51]. Recent application [8] of nitroxide (TEMPO)-mediated living free radical process to copolymerizations of styrene with some macromonomers such as PE-acrylate, la, PEO-methacr-ylate, 27b, polylactide-methacrylate, 28, and poly(e-caprolactone)-methacrylate, 31, may be a promising approach to this end. 

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. 

In comparing observed reactivity ratios between various polymerization systems, it is important to take into account the possible effect of molecular weight on copolymer composition. " In conventional radical copolymerization, the specificity shown in the initiation and termination steps can have a significant effect on the composition of low molecular weight copolymers (usually <10 units). These effects are discussed in Section 7,4.5. In a living polymerization molecular weights are low at low conversion and increase with conversion. In these... 

According to the literature (Brandrup et al., 1999), reactivity ratios for AA (1)/VA (2) copolymerization are r = 2.0 and r2 = 0.1. This means that from a reactivity standpoint, AA-radical ends will want to react with AA monomer, while VA-radical ends will also want to react with AA monomer. Thus, if there exists a 50/50 mole ratio of AA to VA in the reactor fluid, AA will tend to add into a growing copolymer chain at 20 times faster than VA the reactivity of AA monomer would normally result in AA-rich and AA-poor chains, due to chain termination. This implies a very active AA monomer, which is the reason why it was well known that VA/AA copolymers with AA contents greater than 10 wt% cannot be produced efficiently. If the introduction of AA in the reactor is controlled, then the reaction of VA will allow the overall control of the propagation rate while keeping polymer radicals live. This will result in the possibility of producing relatively high AA-content copolymers, as opposed to mixtures of AA- and VA-rich polymers with... 

Controlled radical polymerization allows the synthesis of block copolymers from addition monomers without the kinetic constraint of the reactivity ratios. Thus, for CRP, it is perhaps the CSD rather than the CCD that is important in characterizing the polymer. Zargar and Schork [6, 8] and Ye and Schork [9,10] have modeled the various chemistries for CRP. Their models include the CSD. Information about the CCD comes naturally from their analysis of the CSD. Mathematical modeling of CRP copolymerization would take (to a first approximation) the same form as the living (ionic) polymerization analysis given earlier. 

Despite the recent theoretical analysis reported above, the reactivity ratios were determined experimentally for various systems and did not show significant differences with those found in classical radical copolymerization. In those systems, the polymerizations were essentially carried out in controlled/ living conditions either because both comonomers were easily controlled or because the composition was such that the major comonomer exhibited controlled NMP with the selected nitr-oxide. Many examples are based on styrene with a variety of methacrylate and acrylate comonomers, mosdy in the presence of TEMPO or SGI. 

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