Mode of Chain Termination

The number-average degree of polymerization DF of the polymer product formed by bimolecular termination is related to the kinetic chain length. Thus if the propagating radicals terminate by coupling or combination [Eq. (6.13)], the resulting dead polymer molecule will be made of two 

In this case, there will be an initiator fragment at each end of the polymer molecule. On the other hand, if the termination occurs by disproportionation [Eq. (6.14)], the dead polymer molecules formed will be made of one kinetic chain each, that is. 

In this case, the polymer molecule will have an initiator fragment only at one end. So, if termination takes place by both coupling and disproportionation, the number of initiator fragments per polymer molecule will be between 1 and 2. 

Problem 6.14 For a radical chain polymerization with bimolecular termination, the polymer produced has on the average 1.60 initiator fragments per polymer molecule. Calculate the relative extents of termination by disproportionation and by coupling, assuming that no chain transfer reactions occur. Derive rst a general relation for this calculation. 

Total number of initiator fragments Total number of polymer molecules n 2 

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The mode of chain termination affects the type of block copolymer formed. For example, if a MAI (based essentially on the first monomer A) possessing one central azo bond is decomposed in the presence of monomer B, the growing chain Bn can terminate either by disproportionation or combination, leading to AB and ABA type copolymers, respectively. 

Analysis of radioactively labelled end-groups Absolute single determination wide range of mol. wt. Restricted to certain types of polymer requires knowledge of mode of chain termination 1-2... 

This reaction may account in part for the oligomers obtained in the polymerization of pro-pene, 1-butene, and other 1-alkenes where the propagation reaction is not highly favorable (due to the low stability of the propagating carbocation). Unreactive 1-alkenes and 2-alkenes have been used to control polymer molecular weight in cationic polymerization of reactive monomers, presumably by hydride transfer to the unreactive monomer. The importance of hydride ion transfer from monomer is not established for the more reactive monomers. For example, hydride transfer by monomer is less likely a mode of chain termination compared to proton transfer to monomer for isobutylene polymerization since the tertiary carbocation formed by proton transfer is more stable than the allyl carbocation formed by hydride transfer. Similar considerations apply to the polymerizations of other reactive monomers. Hydride transfer is not a possibility for those monomers without easily transferable hydrogens, such as A-vinylcarbazole, styrene, vinyl ethers, and coumarone. 

As we have seen in previous sections, the radical chain polymerization involves several possible modes of chain termination — disproportionation, coupling, and various chain transfer reactions. These contribute to the complexity of molecular... 

Once again, however, there are superposed complexities due to various competing modes of chain termination and especially to the existence of a mechanism for the branching of chains, namely ... 

Experimental confirmation that the polymerization of VCM is subject to chain transfer to the monomer and to existing polymer is given in Cotman et al, [56] and Abdel-Alim and Hamielec [80]. Disproportionation seems to be the dominant mode of chain termination [80]. 

The determination of the microstructure of vinyl polymers is not merely a characterisation tool. Each polymer molecule is unique, and each polymer chain is a record of the history of its formation, including mis-insertions, rearrangements, the incorporation of co-monomers, and the mode of its termination. NMR analysis of polymers can therefore be used to provide detailed mechanistic and kinetic information. This approach has been applied particularly successfully to the microstructure, i. e. the sequence distribution of monomer insertions, of polypropylene, giving rise to a wealth of studies far too numerous to cover here. Progress in this area has recently been summarised in two excellent and very comprehensive review articles [122, 123[. Here we will cover only the most fundamental aspects of stereoselective polymerisations. 

Much effort has been devoted during the last 30 years toward understanding the mechanisms operative in the coordination catalysis of ethylene and a-olefin polymerization using Ziegler-Natta systems (metal halide and aluminum alkyl, sometimes with Lewis base modifiers). Aspects of the complex heterogeneous reactions have been elucidated (jL- ) but the intimate mechanistic detail - for example the role of inhibitors and promoters, kinetics and thermodynamics of chain growth, modes of chain transfer and termination - comes primarily from studies of homogeneous catalysts ... 

The polymerization was found to proceed smoothly to high conversions. The time dependence of logarithmic initial-to-current monomer concentration ratio ln(mo/m) is linear (Figure 2, curve 1), thus indicating the absence of chain termination processes, as case inherent in polymerization proceeding in the living mode. MW of the obtained polymers increases linearly with the conversion (Figure 2). The polydispersity indexes somewhat decrease with the conversion, a fact that is also typical of controlled radical polymerization. GPC... 

The kinetic treatment of these polymerization reactions depends upon the establishment of stationary state equations for the rates of formation and disappearance of all the transitory intermediates. The form of the expressions derived for the rate of the main reaction depends largely upon the mode of chain ending, and the constants entering into the formulae are those characterizing initiation, propagation, and termination respectively. Special means may be employed for the study of some of these constants in isolation, whereby rather complicated relations can be unravelled. For example, the reaction may be excited photoohemically, in which case the rate of initiation is calculable from the number of quanta of light which are absorbed. This method can be applied with ease to those polymerization reactions which are started by radicals formed, for example, in the photolysis of aldehyde ... 

The lifetime of the excited state of oxygen (Of) in solutions of NaLS and CTAB studied by laser photolysis, has been found to be 53 5 s, longer than values previously measured in D2O [80]. Lifetimes measured in nonionic surfactant solutions (e.g. Brij 35, Igepal CO 630 and Igepal CO 660) were found to be considerably shorter (21 to 26 /is) probably due to the loss of electronic excitation and to vibrational modes of the terminal hydroxyl groups of the polyoxyethylene chains of these surfactants [81]. 

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