Monomer Complex Formation

There are many reports on polymerization of different monomers showing different degrees of kinetic complexity where the observed effects are well explained and understood on the basis of equilibrium complex formation between the initiator used and the monomer [45-47]. 

Problem 6.30 Formation of complex (C) between initiator (I) and monomer (M) leads to nonideal kinetic behavior in many cases. A simplified scheme of chain initiation based on the concept of equilibrium complex formation between initiator and monomer may be given as 

Equation (P6.30.4) describes a change in reaction order with respect to monomer from 1.5 to 1 with increasing [M]. Equation (P6.30.4) may be transformed as follows  

Equation (P6.30.5) permits a plot of [M] / R vs. [M] to give a straight line such that the quotient of the slope and the intercept is equal to K. 

Initiators under ideal conditions would contribute only to chain initiation by dissociation [Eq. (6.3)] into primary radicals (R ). But in certain systems they also contribute to chain termination, partly or exclusively, giving rise to significant deviations from the ideal kinetics (see Problem 6.31). The degradative chain transfer to initiator (I) may be written as 

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Katz, L. E., and Hayes, K. F. (1995). Surface complexation modeling. 1. Strategy for modeling monomer complex formation at moderate surface coverage. J. Colloid Interface Sci. 170, 477-490. 

The heat of each stage (qi for the w-complex formation and monomer insertion) will change to the new values ... 

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.-... 

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. 

Another system showing a similar behaviour results from complex formation between monomer and the growing polymer, i.e.,... 

Apparently the complex formation with a 71-acceptor is suitable for characterization of the donor ability of the entire -system of the monomers. Simultaneously, it can be derived that the EDA-complex formation is only insignificantly influenced by steric effects. Because the above named variation in structure does not disturb the planarity of the center of the monomer double bond, the interaction of the 71-systems from both donor and coplanar acceptor cannot be limited by steric effects. 

The validity of this statement is confirmed by the rates of IC1 additions (see Table 12). Because for these additions the formation of a cationic intermediate by direct attack of the electrophile on the double bond is rate determining, their order of rates is comparable to those of polymerizations. It is therefore understandable that the polymerization rates correlate much better with the reactivities of the monomers during an electrophilic addition of cationogenic agents (such as IC1) than with the relatively unspecific EDA complex formation. 

If the nucleophilicity of the anion is decreased, then an increase of its stability proceeds the excessive olefine can compete with the anion as a donor for the carbenium ion, and therefore the formation of chain molecules can be induced. The increase of stability named above is made possible by specific interactions with the solvent as well as complex formations with a suitable acceptor 112). Especially suitable acceptors are Lewis acids. These acids have a double function during cationic polymerizations in an environment which is not entirely water-free. They react with the remaining water to build a complex acid, which due to its increased acidity can form the important first monomer cation by protonation of the monomer. The Lewis acids stabilize the strong nucleophilic anion OH by forming the complex anion (MtXn(OH))- so that the chain propagation dominates rather than the chain termination. 

The formation of monomer and dimer of (salen)Co AIX3 complex can be confirmed by Al NMR. Monomer complex la show Al NMR chemical shift on 5=43.1 ppm line width =30.2 Hz and dimer complex lb 5=37.7 ppm line width =12.7 Hz. Further instrumental evidence may be viewed by UV-Vis spectrophotometer. The new synthesized complex showed absorption band at 370 nm. The characteristic absorption band of the precatalyst Co(salen) at 420 nm disappeared (Figure 1). It has long been known that oxygen atoms of the metal complexes of the SchifT bases are able to coordinate to the transition and group 13 metals to form bi- and trinuclear complex [9]. On these proofs the possible structure is shown in Scheme 1. 

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. 

Complex formation in binary systems under the influence of organotin monomers (organotin carboxylate and epoxide monomers) was studied by IR and NMR techniques 2,3,18 -23). 

The reaction proceeds at the stage of pseudo-cyclocopolymerization involving complex formation between the growing macroradical and the monomer which is responsible for the alternation of monomer units along the macromolecular chain ... 

There are many possibilities to use these complex formations in fluorescence sensing. If the excimer is not formed, we observe emission of the monomer only, and upon its formation there appears characteristic emission of the excimer. We just need to make a sensor, in which its free and target-bound forms differ in the ability of reporter dye to form excimers and the fluorescence spectra will report on the sensing event. Since we will observe transition between two spectroscopic forms, the analyte binding will result in increase in intensity of one of the forms and decrease of the other form with the observation of isoemissive point [22]. 

FIGURE 5-12 A processive clamp model for the ATPase cycle of the ABC transporter Mdllp. ATP binding (step I) on NBD domains of both monomers induces formation of the dimer (step 2). After ATP hydrolysis by the first NBD (step 3), either the P, is released first (step 4), followed by hydrolysis of the second ATP step 5) and release of the second P, step 8), or the second ATP is hydrolyzed first step 6) and then both phosphates are set free steps 7 and 8). After both ATPs are hydrolyzed to ADP and both phosphates are released, the dimeric complex dissociates step 9) and ADP step 10) is released. The hydrolysis cycle can then start again with ATP binding. (With permission from Fig. 7 of reference [34].)... 

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