Copolymerization complex formation

Despite numerous efforts, there is no generally accepted theory explaining the causes of stereoregulation in acryflc and methacryflc anionic polymerizations. Complex formation with the cation of the initiator (146) and enoflzation of the active chain end are among the more popular hypotheses (147). Unlike free-radical polymerizations, copolymerizations between acrylates and methacrylates are not observed in anionic polymerizations however, good copolymerizations within each class are reported (148). 

The first quantitative model, which appeared in 1971, also accounted for possible charge-transfer complex formation (45). Deviation from the terminal model for bulk polymerization was shown to be due to antepenultimate effects (46). Mote recent work with numerical computation and C-nmr spectroscopy data on SAN sequence distributions indicates that the penultimate model is the most appropriate for bulk SAN copolymerization (47,48). A kinetic model for azeotropic SAN copolymerization in toluene has been developed that successfully predicts conversion, rate, and average molecular weight for conversions up to 50% (49). 

Chemical Properties. Higher a-olefins are exceedingly reactive because their double bond provides the reactive site for catalytic activation as well as numerous radical and ionic reactions. These olefins also participate in additional reactions, such as oxidations, hydrogenation, double-bond isomerization, complex formation with transition-metal derivatives, polymerization, and copolymerization with other olefins in the presence of Ziegler-Natta, metallocene, and cationic catalysts. All olefins readily form peroxides by exposure to air. 

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 often less compelling. One of the most studied systems is S-.V1 Al I copolymerization/8 75 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 7.6.65.-... 

Currently this model is one of the most commonly used in the theory of free-radical copolymerization. The formation of a donor-acceptor complex Ma... iVlbetween monomers Ma and in some systems is responsible for a number of peculiarities absent in the case of the ideal model. Such peculiarities are due to the fact that besides the single monomer addition to a propagating radical, a possibility also exists of monomer addition in pairs as a complex. Here the role of kinetically independent elements is played by ultimate units Ma of growing chains as well as by free (M ) and complex-bound (M ) monomers, whose constants of the rate of addition to the macroradical with a-th ultimate unit will be... 

However, in many papers on polymerization and copolymerization of organotin monomers, the role of complex formation in elementary acts of polymer formation has been either ignored or considered inadequately. 

Both the cross-propagation and complex mechanisms may be operative in alternating copolymerizations with the relative importance of each depending on the particular reaction system. The tendency toward alternation, with or without added Lewis, acid, is temperature-and concentration-dependent. Alternation decreases with increasing temperature and decreasing total monomer concentration since the extent of complex formation decreases. When the alternation tendency is less than absolute because of high reaction temperature, low monomer concentration, absence of a Lewis acid, or an imbalance in the coordinating abilities of the two monomers, copolymerization proceeds simultaneously by the two mechanisms. The quantitative aspects of this situation are considered in Sec. 6-5. 

In addition to ionized acrylate groups being introduced into PAA chains by partial neutralization, other kinds of structure defects , such as sulfonated groups and non-ionic groups, e.g. isopropylamide [-CONHCH(CH3)2] and hydroxyethyl (-CH2CH2OH), were incorporated into PAA by copolymerization or condensation reactions, and their influence on complex formation was examined. Viscometry and potentiometry studies [42] on the complex formation of PEO with PAA-copol-... 

A yellow color developed when CPT and S02 were mixed, and the equilibrium constant of the molecular complex formation was measured as K = 0.0353 ( 40°C, in hexane) (2). This complex might be the intermediate in this alternating copolymerization, and it might participate both in the initiation and propagation steps of this spontaneous copolymerization. 

Complex formation between RNA and water soluble copolymers obtained in the copolymerization of methacryloyloxyethyl-type monomers containing nu-cleobases with water soluble monomers was also studied. Mixing curves between copolymers and between copolymers and RNA are shown in Figs. 14-16. The interaction between poly(MAOFU-co-AAm), poly(MAOT-eo-AAm), or poly-(MAOA-cn-AAm) with RNA was observed, as shown in Fig. 14. The overall stoichiometry of the complexes was about 1 1 and the hypochromicity was about 2% for the copolymer-RNA system under the conditions used. The observed interaction was not as strong as for the poly(VAd)-RNA system and poly(MAOA)-poly(MAOT) system [64], since the solubilizer, AAm, in the... 

A further consequence of the ease of complex formation may be a more rapid copolymerization, although the complex may form an adduct or await the input of sufficient energy to open. 

Moreover, a whole set of monomers with bulky and polar substitutors is known, the copolymerization of which cannot, be described by the classic scheme (2.1). In this case, in order to calculate the copolymer composition, molecular structure and composition distribution, one should use a penultimate model or the model of complex formation. 

It is known that interactions between polynucleotides also depend on temperature the complex formation is favored at lower temperatures39. For the free-radical copolymerization of MAOA with MAOT, the relative rate is increased as the polymerization temperature is lowered37. In the case of template polymerization, however, a reverse temperature dependency has been observed (Fig. 12) the relative conversion tends to increase with rising temperatures. 

In addition to the formation of active centres and participation in elementary processes, the discussion of which forms the main topic of this volume, monomers very often react with some component(s) of the polymerizing medium under complex formation. This reaction is very important. Complex formation lowers the effective monomer concentration, and changes in the polymerization rate usually occur. When the complex is much more active than the monomer, it may react preferentially with the active centre. This, of course, changes the addition mechanism and kinetics. When the monomer and complex also compete, the macrokinetics need not necessarily change. Usually, however, the mechanism of the whole process is greatly complicated, and a kind of copolymerization occurs. 

Henrici-Olive and Olive were the first to put forward the hypothesis that complexes are sometimes formed between the active centre and the monomer and or/solvent [45], As only the complex with monomer is capable of propagation, part of the centres is inhibited and the polymerization rate is reduced. This theory was found to be valid with styrene [46], but not with MMA [47]. Burnett called attention to the important circumstance that radicals solvated in various ways may react differently, or at least at different rates [47]. His conclusions were based on kinetic studies of MMA polymerization in various halogenated aromatics. In the copolymerization of butyl vinyl ether with methacrylates, complex formation between the active centre and condensed aromatics prior to monomer addition was observed by Shaik-hudinov et al. [48], The growing polymer forms a stable donor-acceptor complex with naphthalene, described by the formula. 

When chains grow from more than two monomers, we speak of multicomponent copolymerization. In a simple case, the number of active centre types in the medium equals the number of copropagating monomers. Many systems are also known where the number of participating centres exceeds the number of monomers. In other cases, the monomers undergo complex formation so that their complexes take part as individual components in copropagation. Let us first pay attention to some cases generated by complications in nominally two-component systems. 

When donor—acceptor complexes are formed from the monomers, they can take part in copolymerization. When the equilibrium constants of complex formation are not extremely high, both complexes and monomers coexist and compete with active centres in the reaction. In addition, the reverse case may occur when one part of the active centres forms complexes with some component of the medium, the reactivity of the complexed centers is, of course, different from that of the free centres. The situation is formally similar to that of the preceding paragraph. 

If the complexed radical is inactive (k n = kx 2 = k22 = k21 = 0), Eq. (7.8) reduces to the ordinary Mayo-Lewis equation and no solvent effect on the reactivity ratio will be observed. Busfield et al.108) studied the solvent effect on the free radical copolymerization of vinyl acetate and methyl methacrylate. The methyl methacrylate content is unaffected by benzene and ethyl acetate. This result seems to be consistent with our assumption that the complexed radical is inactive in propagation. However, the solvent effect might not be observed in the case in which the reactivity of the complexed radical is proportional to that of the uncomplexed radical, because also in this case Eq. (7.8) reduces to the Mayo-Lewis form. It is difficult, therefore, to expect from the copolymerization experiment some evidence to support the concept of the complex formation. 

Currently this model is one of the most commonly used in the theory of free-radical copolymerization. The formation of a donor-acceptor complex Ma... 

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