Models involving monomer complexes

Mechanisms for copolymerization involving complexes between the monomers were first proposed to explain the high degree of alternation observed in some copolymerizations. They have also been put forward, usually as alternatives to the penultimate model, to explain anomalous (not consistent with the terminal model) composition data in certain copolymerizations.65 74 

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.- 

Common features of polymerizations involving such monomer pairs are  

However, these observations are not proof of the role of a donor-acceptor complex in the copolymcrization mechanism. Even with the availability of sequence information it is often not possible to discriminate between the complex model, the penultimate model (Section 7.3.1.2) and other, higher order, models.28 A further problem in analyzing the kinetics of these copolyincrizations is that many donor-acceptor systems also give spontaneous initiation (Section 3.3.6.3). 

Equilibrium constants for complex formation (A ) have been measured for many donor-acceptor pairs. Donor-acceptor interaction can lead to formation of highly colored charge-transfer complexes and the appearance of new absorption bands in the UV-visible spectrum may be observed. More often spectroscopic evidence for complex formation takes the font) of small chemical shift differences in NMR spectra or shifts in the positions of the UV absorption maxima. In analyzing these systems it is important to take into account that some solvents might also interact with donor or acceptor monomers. 

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The second model involves the reversible formation of a complex between the iodine and the monomer, and in this context it is unimportant whether the iodine forms an n-complex, e.g., with the O-atom of a vinyl ether, or a 7t-complex with a double-bond or an aromatic ring of the monomer (styrene, N-vinylcarbazole) the only important point is that the complexed monomer is very much more reactive with the propagating ester group than the uncomplexed monomer, and that its formation is an equilibrium reaction. [Very detailed studies of the reactions of iodine with styrene [38] and with butyl vinyl ether (nBVE) [39, 40] have been reported in which the formation of various complexes is discussed.]... 

Another model used to describe deviations from the terminal model involves the participation of a comonomer complex (Sec. 6-3b-3) [Cais et al., 1979 Coote and Davis, 2002 Coote et al., 1998 Seiner and Litt, 1971]. The comonomer complex competes with each of the individual monomers in propagation. The monomer complex participation model involves eight... 

A variation of the complex participation model, referred to as the monomer complex dissociation model, involves disruption of the complex during reaction with a propagating chain end [Hill et al., 1983 Karad and Schneider, 1978]. Reaction of the propagating center with... 

The nature of interactions involved between the template and the monomers [hydrogen bonding, electrostatic and 11-11 staking] can be studied by the characterization of template-monomer complex model. The functional groups taking part in the complex formation and site of attachment can be visualized by the clear picture of molecular complexes. Moreover, the mole ratio of template monomer can also be optimized using computational methods. 

How is the binding specificity of the heterodimer achieved compared with the specificity of Mat a2 alone The crystal structure rules out the simple model that the contacts made between the Mat a2 homeodomain and DNA are altered as a result of heterodimerization. The contacts between the Mat o2 homeodomain and DNA in the heterodimer complex are virtually indistinguishable from those seen in the structure of the Mat o2 monomer bound to DNA. However, there are at least two significant factors that may account for the increased specificity of the heterodimer. First, the Mat al homeodomain makes significant contacts with the DNA, and the heterodimeric complex will therefore bind more tightly to sites that provide the contacts required by both partners. Second, site-directed mutagenesis experiments have shown that the protein-protein interactions involving the... 

Models for emulsion polymerization reactors vary greatly in their complexity. The level of sophistication needed depends upon the intended use of the model. One could distinguish between two levels of complexity. The first type of model simply involves reactor material and energy balances, and is used to predict the temperature, pressure and monomer concentrations in the reactor. Second level models cannot only predict the above quantities but also polymer properties such as particle size, molecular weight distribution (MWD) and branching frequency. In latex reactor systems, the level one balances are strongly coupled with the particle population balances, thereby making approximate level one models of limited value (1). 

Models (Hi) and (iv). Strictly, the only way of finding out definitely whether there is any complexation between the growing cation and the monomer or the polymer, or both, is to investigate whether (and if so, how) the apparent kp+ depends on monomer concentration [16, 17]. We have such evidence only for ACN and styrene and for these the value of kp does not depend on m. This is in accord with the prediction [15,17] that in a highly polar solvent the complexation of Pn+ by a Jt-donor monomer or its polymer is likely to be negligible. The likely behaviour of the w-donor vinyl ethers and their polymers is less clear, but a consideration of the dipole moments and concentrations involved makes it extremely unlikely that these monomers or their polymers could compete successfully for a place in the solvation shell of the growing cations. 

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