Copolymerization complex participation model

The complex participation model has been tested in the radical copolymerizations of 1,1-diphenylethylene-methyl acrylate, styrene-P-cyanoacrolein, vinyl acetate-hexafluoroace-tone, A-vinylcarbazole diethyl fumarate, A-vinylcarbazole funiaronitrile, maleic anhydride-vinyl acetate, styrene-maleic anhydride [Burke et al., 1994a,b, 1995 Cais et al., 1979 Coote and Davis, 2002 Coote et al., 1998 Dodgson and Ebdon, 1977 Fujimori and Craven, 1986 Georgiev and Zubov, 1978 Litt, 1971 Lift and Seiner, 1971 Yoshimura et al., 1978]. 

The sequence distributions expected for the different models have been described [Hill et al., 1982, 1983 Howell et al., 1970 Tirrell, 1986] (Sec. 6-5a). Sequence distributions obtained by 13C NMR are sometimes more useful than composition data for discriminating between different copolymerization models. For example, while composition data for the radical copolymerization of styrene-acrylonitrile are consistent with either the penultimate or complex participation model, sequence distributions show the penultimate model to give the best fit. 

Several studies on the reactivities of small radicals with donor-acceptor monomer pairs have been carried out to provide insight into the mechanism of copolymerizations of donor-acceptor pairs. Tirrell and coworkers " reported on the reaction of n-butyl radicals with mixtures of N-phcnylmalcimidc and various donor monomers e.g. S, 2-chloroethyl vinyl ether),. lenkins and coworkers have examined the reaction of t-butoxy radicals with mixtures of AN and VAc. Both groups have examined the S-AN system (see also Section 7.3.1.2). In each of these donor-acceptor systems only simple (one monomer) adducts are observed. Incorporation of monomers as pairs is not an important pathway i.e. the complex participation model is not applicable). Furthermore, the product mixtures can be predicted on the basis of what is observed in single monomer experiments. The reactivity of the individual monomers (towards initiating radicals) is unaffected by the presence of the other monomer i.e. the complex dissociation model is not applicable). Unless propagating species are shown to behave differently, these results suggest that neither the complex participation nor complex dissociation models apply in these systems. 

In these models, the complex formed by the monomer pair competes with the individual monomer molecules for the propagation reaction with the radicals. There are two variations of this approach in the complex participation model, the pair of monomers form a complex and are added to the chain radical [106-109]. On the other hand, in the complex dissociation model, the complex participates in the propagation process, but dissociates upon reaction and only one of the monomers is added to the chain [101, 103]. Although there is ample experimental evidence for the existence of such complexes in these copolymerizations (such as the bright colors associated with them) [76], it is questionable whether the complexes actually participate in the propagation step [76]. Additionally, for several years, Hall and Padias have accumulated experimental and theoretical evidence that refutes the validity of the models based on complex participation [76, 77]. Both the complex participation and the penultimate models were combined in the so-called comppen model [110]. 

Many analytical techniques have been utilized to analyze the SAN microstructure including LALLS [38], CNMR [19,31,39-44], infrared spectroscopy [45-49], ultraviolet spectroscopy [50-52], pyrolysis GC [8,27,53], pyrolysis mass spectroscopy [54,55], fluorescence [20,56], GPC-IR [57,58], and GPC-UV [52]. Since the terminal model allows the calculation of sequence distribution, the calculated and measured sequence distributions can be compared. This comparison generally shows deviation of the measured sequence distribution vs that predicted using the terminal model. Ham [59] was the first to notice the deviation and explained the deviation based upon penultimate effects. Since that time several other researchers have also notic deviation of their data from the terminal model and have applied more elaborate copolymerization models (Scheme 4) to explain the mechanism of SAN copolymerization. The penultimate [60,61] and complex participation models [33,62,63] have both been evaluated and give a better fit to the SAN system than the terminal model. 

The various copolymerization models that appear in the literature (terminal, penultimate, complex dissociation, complex participation, etc.) should not be considered as alternative descriptions. They are approximations made through necessity to reduce complexity. They should, at best, be considered as a subset of some overall scheme for copolymerization. Any unified theory, if such is possible, would have to take into account all of the factors mentioned above. The models used to describe copolymerization reaction mechanisms arc normally chosen to be the simplest possible model capable of explaining a given set of experimental data. They do not necessarily provide, nor are they meant to be, a complete description of the mechanism. Much of the impetus for model development and drive for understanding of the mechanism of copolymerization conies from the need to predict composition and rates. Developments in models have followed the development and application of analytical techniques that demonstrate the inadequacy of an earlier model. 

The ability to determine which copolymerization model best describes the behavior of a particular comonomer pair depends on the quality of the experimental data. There are many reports in the literature where different workers conclude that a different model describes the same comonomer pair. This occurs when the accuracy and precision of the composition data are insufficient to easily discriminate between the different models or composition data are not obtained over a wide range of experimental conditions (feed composition, monomer concentration, temperature). There are comonomer pairs where the behavior is not sufficiently extreme in terms of depropagation or complex participation or penultimate effect such that even with the best composition data it may not be possible to conclude that only one model fits the composition data [Hill et al., 1985 Moad et al., 1989]. 

While there is clear evidence for complex formation between certain electron donor and electron acceptor monomers, the evidence for participation of such complexes in copolymerization is oflen less compelling. One of the most studied systems is S-MAH copolymerization." However, the models have been applied to many copolymerizations of donor-acceptor pairs. Acceptor monomers have substituents such as carboxy, anhydride, ester, amide, imide or nitrile on the double bond. Donor monomers have substituents such as alkyl, vinyl, aryl, ether, sulfide and silane. A partial list of donor and acceptor monomers is provided in Table 7.6. ... 

It has been suggested that CT complexes may actually participate in the polymerization. For example, Butler and Campus (12) presented evidence for participation of the CT complex of MA and divinyl ether (DVE) in a terpolymerization study using fumaroni-trile, DVE and MA. Recently, a computer program (13) was published for evaluating the CT copolymerization modeTwhen operating in competition with the terminal model. 

Models Involving the Participation of Complexes As mentioned in Section 3.1, several copolymerizations that lead to alternating copolymers have been analyzed by the use of the concept of chain transfer complexes. Some of these systems are styrene-maleic anhydride [101-103], allyl acetate-maleic anhydride [104], and norbornene-maleic anhydride [105]. 

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