Unsaturated chain ends

Studies on model compounds also suggest that unsaturated chain-end groups should not have an important influence on the thermal stability of PVC [21]. In conclu-... 

Stames el al.I7 have provided support for the above mechanism (Scheme 6.29) by determining the unsaturated chain ends (112) in low conversion PVAc by l3C NMR. They were able to distinguish (112) from chain ends that might have been formed if transfer involved abstraction of a vinylic hydrogen. The number of unsaturated chain ends (112) was found to equate with the number of -CH OAc ends suggesting that most chains arc formed by transfer to monomer. Stames et a . 13 also found an isotope effect k kD of 2.0 for the abstraction reaction with CTTpCHOiCCD as monomer. This result is consistent with the mechanism shown in Scheme 6.28 but is contrary to an earlier finding.174... 

Studies with model compounds show that secondary benzoate esters eliminate benzoic acid to form unsaturated chain ends as shown in Scheme 8.2.15... 

Unstable structures are known to arise by chain termination. Mechanisms for radical-radical termination in MMA polymerization have been discussed in Sections 5.2.2.1.2 and 5.2.2.2.2 and these are summarized in Scheme 8.5. It is established that both disproportionation and combination occur to substantial extents. The head-to-head linkages 1 and the unsaturated chain ends 2 both constitute weak links in PMMA.26 2 "33 The presence of these groups account for... 

There are other sources of unsaturated chain ends in PMMA formed by radical polymerization ... 

However, the presence of unsaturated chain ends can have other consequences for polymer properties ... 

Addition of TEMPO post-polymerization to a methacrylate polymerization provides an unsaturated chain end (Scheme 9.52)i07 sw presumably by disproportionation of the PMMA propagating radical with the nitroxide. For polymers based on monosubstituted monomers (PS,1 0" PBA59,[Pg.534]

In certain situations, termination occurs by disproportionation. This termination process involves chain transfer to a hydrogen atom from one chain end to the free radical chain end of another growing chain, resulting in one of the dead polymer chains having an unsaturated chain end (Equations 6.19 and 6.20). 

In curve B, the first and second peaks correspond to the unsaturated chain end and to the saturated chain end of the polymer respectively. In curve C, one observes that the peak corresponding to polymer chains terminated through disproportion have disappeared this points out the fact that the cellulosic macromolecules could be crosslinked by PMMA chains. 

When the substituent R stabilizes radicals as in (A) and (C), chain scission is more likely than termination by coupling. Radicals (C) then propagate the depolymerization process with volatilization of polypropylene and polystyrene at a temperature at which these polymers would not give significant amounts of volatile products when heated alone. Moreover, unsaturated chain ends such as (B) would also initiate the volatilization process because of the thermal instability of carbon-carbon bonds in P position to a double bond (Equation 4.23). 

The above reaction (Equation 4.25) would reduce the rate of volatilization of polypropylene by reducing the concentration of unsaturated chain ends which act as initiating structures.56 As an alternative or in addition, a stabilization mechanism based on the reported formation of metallic bismuth can be proposed.53 57 Stabilization of polypropylene by metal compounds is in agreement with activity of several metal compounds as radical catalyst/inhibitor depending on metal concentration and/or temperature of the system.58... 

A number of other unsaturated electrophilic compounds were used by Milkovich 18 as deactivators for living polystyrene or living polydienes. A characterization of the macromonomers obtained showed that the reaction of the living polymer with compounds such as maleic anhydride, vinyl chloroacetate, or 2-chloroethylvinyl ether yields the following unsaturated chain ends (in some cases the addition of 1,1-diphenylethylene is necessary) ... 

Release of the unsaturated chain end of a polyolefin can occur by fi-H transfer to the metal or to a monomer molecule (see Appendix 1 for backgound material). A metal-alkyl species, i.e. the starting unit for a new polymer chain, arises from the metal-hydride species formed in the first case by insertion of an olefin, or it can be formed directly by f-H transfer to a monomer (Figure 20). While the results are thus identical, the two reaction paths differ in their respective kinetics In the first case, the rate-limiting p-H transfer is independent of the olefin concentration, while the rate of p-H transfer to a monomer requires the formation of an olefin-containing reaction complex and will thus increase linearly with olefin concentration. 

Figure 21 Chain growth termination by fi-H transfer and displacement of unsaturated chain end from nickel centre by a monomer, hindered by bulky substituents R.

Discussion Point DP6 Alternative reaction sequences for the release of unsaturated chain ends from a catalyst centre are represented in Figure 20. Determine for each of these sequences how the termination rate Vt depends on the monomer concentration. Write reaction equations for several alternative chain-release and re-start reactions mentioned in Section 7.4.4. Can you think of polymerization systems for which increased ethylene concentrations might lead to polymers with reduced molar mass ... 

The unsaturated chain end-group, C1CH2-CH=CH—, is identified by H NMR the C1CH2 proton signal resonates at 4.05 4.10 ppm. Its formation is explained by the following reaction scheme as a head-to-head addition occurs, the radical isomerizes to a secondary radical, which releases a chlorine radical to form the allylic end-group.139... 

Poly-a-methylstyrene and polymethylmethacrylate degradations show strong similarities. Thus, both polymers depolymerize at relatively low temperatures. Almost pure monomer is obtained as volatile product Depolymerization of polymethylmethacrylate, however, is initiated at unsaturated chain ends below 250°C, whereas poly-a-methylstyrene undergoes mainly random-chain scission. 

The concentration of these unsaturated chain ends increases and, since they are unstable at 220°C, the rate of volatilization is accelerated. Since depolymerization cannot pass through acrylonitrile, the chain length of the residue is the average distance between adjacent acrylonitrile units. 

The fact that the decrease in molecular weight in the photodegradation of polymethylvinylketone does not continue at the same rate throughout the irradiation has been ascribed to the occurrence of a competing reaction that opposes the main chain scission process. This is assumed to be the formation of new polymer—polymer linkages by mutual recombination of macro-radicals resulting from the addition of CHj or CH3—CO to unsaturated chain ends formed in the Norrish type II reaction [55]. Evidence for the presence of such macro-radicals is found in the production of graft copolymers when solutions of polymethylvinylketone in various monomers (acrylonitrile, methyl methacrylate, vinyl acetate) are irradiated [57]. 

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