Polymer formation chain termination

Inherent viscosities of the polymer (Table) show that 10 and that polymerization at higher temperatures reduces the molecular weight (Figure 5). Infrared spectral analysis of the polyethylenes obtained in the Table showed no H(R)C=CR2 groups that would result from a-olefin formation then incorporation into growing polymer then chain termination via... 

Problem 1.3 Styrene monomer containing 0.02% (by wt.) benzoyl peroxide initiator was reacted until all the initiator was consumed. If at this stage 22% of the monomer remained unreacted, calculate the average degree of polymerization of the polymer formed. Assume 100% efficiency of the initiator (i.e., all initiator molecules are actually consumed in polymer formation) and termination of chain radicals by coupling alone. 

In the polymerization of common vinyl monomers, such as styrene and methyl methacrylate, most of the polymer molecules are produced by the termination reaction it is often possible to deduce from the end groups whether the termination has occurred by coupling or by disproportionation. As mentioned above, stable polymer formation by termination in the polymerization of vinyl acetate is not important. Most of the stable polymer molecules are produced by the chain transfer to monomer, polymer, and solvent. As already described, chain transfer consists of the dehydrogenation,... 

The addition polymerization of a vinyl monomer CH2=CHX involves three distinctly different steps. First, the reactive center must be initiated by a suitable reaction to produce a free radical or an anion or cation reaction site. Next, this reactive entity adds consecutive monomer units to propagate the polymer chain. Finally, the active site is capped off, terminating the polymer formation. If one assumes that the polymer produced is truly a high molecular weight substance, the lack of uniformity at the two ends of the chain—arising in one case from the initiation, and in the other from the termination-can be neglected. Accordingly, the overall reaction can be written... 

In addition to this reaction, quinones and other alkyl radical acceptors retard polymer oxidation by the reaction with alkyl radicals (see earlier). As a result, effectiveness of these inhibitors increases with the formation of hydroperoxide groups in PP. In addition, the inhibiting capacity of these antioxidants grows with hydroperoxide accumulation. The results illustrating the efficiency of the antioxidants with cyclic chain termination mechanisms in PP containing hydroperoxide groups is presented in Table 19.12. The polyatomic phenols producing quinones also possess the ability to terminate several chains. 

The polymer formation in the radical polymerization of vinyl monomers initiated by a usual initiator R-R is expressed by Eqs. (4) and (5) if termination proceeds via combination and disproportionation and no chain transfer reaction occurs. 

It is rare for a polymerization reaction to proceed to completion because the viscosity of a polymer increases during chain growth - this is why monomers are usually liquid, while the polymers produced are hard solids. The last step during polymer formation is a termination reaction known as radical annihilation , in which two radicals meet and then spin-pair to form a covalent bond. 

The reaction sequence for a typical vinyl polymer has four steps. In the first step, a free radical must be produced from the initiator such as those shown in Figs. 2.18 and 2.19. These radical formation reactions are typically first order in rate and are promoted by the elevated temperature of the reaction. For some free radical initiators, light can also promote the reaction. Then a sequence of events in the reaction mixture occurs, including initiation of a chain, followed by propagation, and finally termination of the chain. Termination of the chain will be discussed later. The schematic steps to produce an addition polymer from bulk or solvent polymerization are detailed in Fig. 2.19. The radical produced from the initiator reacts with the monomer in Step 2 to produce a new free radical by opening the double bond of a... 

Then, they depend also on the viscosity of the system. Specific diffusion control is characteristic of fast reactions like fluorescence quenching. In polymer formation, specific diffusion control is responsible for the acceleration of chain polymerization due to the retardation of the termination by recombination of two macroradicals (Trommsdorff effect). Step reactions are usually too slow to exhibit a dependence on translational diffusion also, the temperature dependence of their rate constants is of the Arrhenius type. 

The stereo-regularity of a polymer can be reduced by a variety of defects. For example, regio-errors arise when a 2,1-monomer insertion occurs instead of a 1,2-in-sertion the latter is usually 10 -10 times faster. Misinsertions can lead to chain termination and formation of butenyl end groups, they can be incorporated into... 

Transfer of a P-proton from the propagating carbocation is the most important chain-breaking reaction. It occurs readily because much of the positive charge of the cationic propagating center resides not on carbon, but on the P-hydrogens because of hyperconjugation. Monomer, counterion or any other basic species in the reaction mixture can abstract a P-proton. Chain transfer to monomer involves transfer of a P-proton to monomer with the formation of terminal unsaturation in the polymer. 

The polymerization proceeds via a radical chain-reaction mechanism, judging from some features of the polymerization initiation by irradiation or upon heating, no formation of oligomers, and polymer formation irrespective of the medium or atmosphere. The propagating radicals are readily detected by ESR spectroscopy during polymerization in the crystalline state (Fig. 2), because termination between the propagating radicals occurs less frequently in the solid state [50]. 

The kinetics of template polymerization depends, in the first place, on the type of polyreaction involved in polymer formation. The polycondensation process description is based on the Flory s assumptions which lead to a simple (in most cases of the second order), classic equation. The kinetics of addition polymerization is based on a well known scheme, in which classical rate equations are applied to the elementary processes (initiation, propagation, and termination), according to the general concept of chain reactions. 

The termination of the growing polymeric chain may occur through several different processes, mostly by chain transfer. Either the process of chain transfer with the monomer, or the reaction of dissociation to hydride, leads to the formation of terminal vinylidenic groups, whose presence was noticed in the olefin polymers, obtained with the previously described catalysts (22). 

It should be recognized from the results that cationic polymerizations are usually initiated at low temperatures in order to suppress chain-terminating reactions and also to keep the reaction from becoming explosive in nature. These low temperatures thus favor high molecular weight polymer formation. 

Photooxidation of Eosin with periodate ion has been used to initiate the polymerization of acrylonitrile in aqueous solution [187]. Addition of acrylonitrile to a periodate solution shifts the absorption maximum from 220 to 280 nm. This spectral change is interpreted as being due to complex formation between the monomer and oxidizing agent. The rate of photopolymerization increases linearly with the absorbed light intensity and monomer concentration. The observed intensity dependence indicates the main chain terminator is not produced photochemically. Polymer is not formed when the concentration of periodate ion is lower than 0.5 mM and the rate of polymerization is independent of its concentration for higher values. 

Overberger and Jarovitzky (37) proposed the formation of cationic propagating species through diproportionation of the titanium aluminum complex into RjAlClJ and a positive titanium species. Propagation of this ion pair, before temporary chain termination by alkyl migration, produces the same possibility for steric control by the end of the polymer chain. ... 

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