Termination monomer

Depending on the termination reaction of the vinyl monomer, termination by disproportionation or termination by combination occurs. As a result, AB or ABA block copolymers might be obtained. 

Monomers that participate in step growth polymerization may contain more or fewer than two functional groups. Difunctional monomers create linear polymers. Trifiinctional or polyfunctional monomers introduce branches which may lead to crosslinking when they are present in sufficiently high concentrations. Monofunctional monomers terminate polymerization by capping off the reactive end of the chain. Figure 2.12 illustrates the effect of functionality on molecular structure. 

Assuming monomer termination as the main termination reaction, L6hr et al. (64) estimated the termination rate constant from the molecular weight distribution of the polymers. No direct evidence for the termination mechanism assumed was given, however. 

Up to now it has not been possible to prove a mechanism which is able to explain all these effects. Neither intramolecular termination alone, nor a combination of intramolecular termination and monomer termination fit the data. However, they can be fitted by a hypothetical kinetic scheme which assumes that a slow reaction takes place between the living end and a deactivating species which is produced in the initial stage of the reaction. The dependence of the rate of termination on the initial monomer concentration can only be explained on the basis of a side reaction occuring in the initiation process. To verify this model, however, the deactivating species has to be identified by analyzing the oligomers produced in the initiation step. 

These reactions have transition states different from those of propagation and transfer and can occur in the absence of monomer. Termination via ion collapse may be hindered in the ion-counterion-monomer complex by the presence of monomer, that is, the ion-counterion pair is in fact separated by complexed monomer. Reactions (17>—(19) may occur only in the uncomplexed ion-counterion pair. In that case ... 

Another approach is to transform the time variable in the original equations where chain length is discrete (57). Monomer termination permits linearization in this particular case. 

Batch Reactors. One of the classic works in this area is by Gee and Melville (21), based on the PSSA for chain reaction with termination. Realistic mechanisms of termination, disproportionation, and combination, are treated with a variety of initiation kinetics, and analytical solutions are obtained. Liu and Amundson (37) solved the simultaneous differential equations for batch and transient stirred tank reactors by using digital computer without the PSSA. The degree of polymerization was limited to 100 the kinetic constants used were not typical and led to radical lifetimes of hours and to the conclusion that the PSSA is not accurate in the early stages of polymerization. In 1962 Liu and Amundson used the generating function approach and obtained a complex iterated integral which was later termed inconvenient for computation (37). The example treated was monomer termination. 

To achieve optimal properties in an AB block copolymer, it is important to control the molecular weight of the blocks, and minimize the amount of A homopolymer produced on addition of the second monomer. Termination reactions do occur in these systems [20], but the rate is fairly slow, particularly at temperatures below about 100 °C. In a practical sense, protic impurities present a much greater challenge. In a two-reactor system it is common practice to prepare the first block in one reactor, titrate out impurities in the B monomer charge in a second reactor by adding small increments of butyllithium to a solution of the B monomer until the first sign of color or exotherm, and then the transfer poly(A)Li solution to the second reactor. 

The type of block copolymer produced in this S3m-thesis is dependent on the termination reactions of both monomers. If both monomers terminate by disproportionation diblock copolymers result. If one reacts by combination and one by disproportionation triblock copol3nners are formed and if both monomers terminate by combination multiblock copolymers result. This process is imavoidably accompanied by the formation of homopolymers. So the central question of this synthesis is How high can the fraction of block co-pol3raiers be ... 

The particular features of anionic polymerization that made the polymer chains living were discussed above. The main requirement for a living polymerization is the absence of any process for spontaneous termination so that the degree of polymerization is controlled by the ratio of monomer to initiator concentrations. The molar-mass of the polymer therefore increases linearly with monomer conversion. On exhaustion of the monomer, the initiation centres remain, so chains may be re-initiated by addition of further monomer. Termination or chain transfer is controlled by the delibemte addition of a reagent to remove the living end. The resulting polymers will also have very narrow molar-mass distributions since rapid initiation ensures that all chains are initiated at the same time. 

C20H22Y20 whh only head-to-tail bonding between successive pairs of monomers. These molecules are oligomers of ten VDY monomers, terminated by an extra H atom at each end. 

Another approaeh to the fabrieation of LB films from preformed polymers is to form a hydrophobic main chain by reacting monomers terminated by a vinyl group [102. 103. 104. 105 and 106]. The side groups studied also included perfluorinated hydroearbon chains, which tilt with respect to the normal to the plane of the film, whereas the analogous ordinary hydroearbon chains do not [105]. 

This effect can be explained by a side reaction consisting of monomer termination by reaction of a living end with the carbonyl group ... 

Figure 1. Schematic representation of addition polymerization. I and I initiator, M monomer, terminator, 0-1 chain transfer agent active end, and O inactive end.

Oligosaccharide Man" Composition and Ratio of the Monomers Terminal Residue... 

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