Fragment of initiator

Both termination mechanisms have been shown to occur experimentally, the method being to examine the polymer molecules formed for fragments of initiator. In such a way polystyrene has been found to terminate mainly by combination and poly(methyl methacrylate) entirely by disproportionation at temperatures above 60 °C. 

The reaction sequence is assumed to be launched by the fragmentation of initially formed aziridinylmethyl radicals to give a N-allylaminyl radical, which undergoes a twofold 5-exo-trig cyclization. 

Figure 1 Reaction scheme for the free-radical polymerisation (I is the initiator, R the fragment of initiator, M the monomer and AH the chain transfer agent).

In the case of more water-soluble monomers and (amphiphilic) macromonomers, the Smith-Ewart [16] expression does not satisfactorily describe the particle nucleation. The HUFT [9,10] theory, however, satisfactorily describes the polymerization behavior or the particle nucleation of such unsaturated hydrophilic and amphiphilic monomers. The HUFT approach implies that primary particles are formed in the aqueous phase by precipitation of oligomer radicals above a critical chain length. The basic principals of the HUFT theory is that formation of primary particles will take place up to a point where the rate of formation of radicals in the aqueous phase is equal to the rate of disappearance of radicals by capture of radicals by particles already formed. Stabilization of primary particles in emulsifier-free emulsion polymerization may be achieved if the monomer (or macromonomer) contains surface active groups. Besides, the charged radical fragments of initiator increases the colloidal stability of the polymer particles. 

Instability of the polymer is responsible for the primary step in decomposition and is attributed either to fragments of initiator or to branched chains or to terminal double bonds. The appearance of branching is the result of reactions of chain transfer through the polymer, while that of unsaturated terminal groups results from reaction of disproportionation and chain transfer through the monomer. During thermal and thermo-oxidative dehydrochlorination of PVC, allyl activation of the chlorine atoms next to the double bonds occurs. In this volume, Klemchuk describes the kinetics of PVC degradation based on experiments with allylic chloride as a model substance. He observed that thermal stabilizers replace the allylic chlorine at a faster ratio than the decomposition rate of the allylic chloride. 

Chain transfer was not observed. With lower-molecular-weight polymers (DP = 5 to 20), derived from OXL, 4-methyl- and 4-ethylOXL. 13C-NMR spectroscopy revealed the expected end-groups, namely fragments of initiating and terminating species17). 

The transfer of rotational motion from parent to products may be illustrated by the simple example of a planar fragmentation of initially non rotating H2O to OH and H. Although there is no initial rotation in the parent molecule there is bending zero point motion in the coordinate y. If one H-atom were not present, the other H-atom could rotate freely around the 0-atom. However, due to the presence of the H-atom, this rotational motion is hindered. If the H is eliminated without any force in the exit channel, the rotation becomes free and shows up in the products. This is the type of effect described by the matrix element . 

Tandem MS, tandem mass spectrometry (MS/MS, MS ) Experiments that permit one to exanune ions in the gas phase via mass analysis of fragmentation of initially formed ions. 

Borsig, E., et al. (1969). Reinitiation reactions of poly(methyl methacrylate) with labile bound fragments of initiator. Angew. Makromol. Chem., 9 89 95. 

Each initiator molecule decomposes into two free radicals. Each free radical starts a polymer chain, and the chain termination process is such that there is one fragment of initiator in each molecule of polymer that leaves the reactor. What is the average number of monomer units in each molecule of polymer ... 

Scheme 16.1. Radical elimination to yield even-electron products from initially formed radical products in 157-nm UVPD (a, b). Generation of side-chain loss fragments through secondary fragmentation of initially formed radical products (c-e). (Reproduced from Ref. 154 with permission from Elsevier Science.)...

Inhibitors act by removing free radicals from the system. Depending on the relative reactivities of the inhibitor and the radical concerned, the free radicals may be removed either as fragments of initiator decomposition or as veiy short chain radicals. 

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