Polymer Chain Termination

Antioxidants markedly retard the rate of autoxidation throughout the useful life of the polymer. Chain-terminating antioxidants have a reactive —NH or —OH functional group and include compounds such as secondary aryl amines or hindered phenols. They function by transfer of hydrogen to free radicals, principally to peroxy radicals. Butylated hydroxytoluene is a widely used example. 

Chain growth continues until the propagating polymer chain terminates. 

Finally where both reactivity ratios take the value of zero, the monomers do not react at all, with growing polymer chains terminated in their own kind of monomer unit. This results in alternating copolymerisation. A few typical monomer reactivity ratios are given in Table 2.2. 

Radiochemical experiments show that the number of polymer chains terminated by allyl groups are a minor fraction of the total, and the majority of chains derive from the realkylated hydrides. In the presence of hydrogen it is evident from Table III that chain transfer reactions dominate and some saturated polymer is formed ... 

It should be mentioned that donor substitution of the phenylene backbone of the salphen ligand was shown to have a decreasing effect on activity [103], which explains the overall lower productivity compared with halogen-substituted chromium salphens. However, experiments clearly proved an increased activity upon dimerization. Whereas the monomeric complex m = 4) converts about 30% of p-BL in 24 h, producing a molecular weight of 25,000 g/mol, the corresponding dimer yields up to 99% conversion with > 100,000 g/mol. Moreover, the smaller polydispersity (PD < 2) shows the better polymerization control, which is attributed to the decreased rate of polymer chain termination. This behavior is caused by the stabilization of the coordinated chain end by the neighboring metal center, as recently reported for dual-site copolymerizations of CO2 with epoxides [104-106]. The polymeric products feature an atactic microstructure since the... 

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. 

Give an example of coordination polymerisation involving nucleophilic attack on the growing polymer chain terminated by the coordinating monomer. Write down the scheme. 

Anionic deactivation processes can also be used to introduce quantitatively into a polymer chain terminal functions which can be utilized for the synthesis of macromonomers. 

As far as the fate of 6.6 is concerned, there is a third possibility. The polymer chain may remain attached to the metal atom that is, the metal-alkyl bond remains intact. The product of the overall reaction in this case is 6.6. Polymerization of this type is often called living polymerization. Note that in this case there is no polymer chain termination step, and the catalytic cycle is not completed. In other words, although a single Ti4+ ion may be responsible for the polymerization of thousands of ethylene molecules, in the strictest sense of the term it is not a true catalytic reaction. In the event 6.6 is converted to 6.7 either by reaction with H2 or by /8-elimination, 6.7 further reacts with ethylene. Insertion of ethylene into the Ti-H bond regenerates 6.2 and completes the catalytic cycle. 

The mechanism of this polymerization has been discussed by Cundy et al. (9). The first step is apparently the insertion of a low-valent metal into the strained C—Si bond to give a silametallacyclopentane. Metallacycles 4 and 5 have in fact been isolated from the reactions of Fe2(CO)9 and CpMn(CO)3 with 1, respectively (10, 11). Complex 4 when treated with phosphines gives polymer and LFe(CO>4. If the metallacycle resulting from insertion (6) is unstable, repeated insertions (oxidative additions) and reductive eliminations lead to polymer. Chain termination results from reductive elimination of =SiH [Eq. (13)]. 

Many examples of such eliminations have now been seen for the f-block and for d metals. This type of /3-aIkyl elimination is recognized as an important chain transfer step in Ziegler-Natta and metallocene polymerization catalysis. When it occurs the polymer chain terminates in a C=C bond (equation 2) and in certain cases the aUcene chain end can undergo reinsertion and get back into the polymer growth... 

The rate of allyl acetate pol5mierization is proportional to the first power of initiator concentration which is in accord with the kinetics for polymer chain termination resulting solely by transfer to monomer with the formation of a radical incapable of initiating new chains. Dibutyl... 

Catalytic chain transfer in acrylate polymerizations is problematic due to the propensity of acrylates to form stable Co—C bonds between the CCT agent and the propagating radical of both the monomer and its oligomers.268,269,373 It has even been possible to observe a growing polymer chain terminated with a Co—C bond directly by MALDI.374 This bond is stronger than that in the case of styrene. These complications have a direct impact on the use of CCT in acrylic polymerizations. 

The surface of FPS is characterized by the presence of at least three dominant types of groups hydrophobic hydrocarbon radicals R bonded to silicon atoms, often containing electron-donating or electron-accepting groups residual silanol groups, =Si-OH, where the polymer chain terminates oxygen atoms of siloxane bonds, =Si-0-Si=. Such a variety of the surface character allows for interactions with different sorbates ... 

Besides H NMR, C NMR also indicates that PECHG prepared using EG does not contain any EG units (H0-CH>-CH -0 61.5 ppm) at the polymer chain terminal position. 

The data obtained for the broadening of the product MWD in the course of the reaction, combined with the theoretical calculations of the effect of various elementary reactions on MWD, made it possible to conclude that the reaction taking place in the system is that of transfer with polymer chain termination. The rate constant of this reaction is (8 + 3) x 10-s 1 mol-1 s l. 

Since 1-propanol is monofunctional and reacts with "matched dissociation isocyanate" (at internal urethane chain positions) as well as "unmatched isocyanate" (at polymer chain terminal positions) its effects include cleavage of some polyurethane chains in the process of generating more urethane groups. As a result, polymer DP, melt viscosity, and torque drop until the shortstop is consumed, or escapes the mixture by volatilizaton. Figure 9 shovjs that the more 1-propanol used to shortstop the polymerizations, the more pronounced the polymer reversion was. 

It is often observed that the measured molecular, weight of a polymer product made by free-radical chain polymerization is lower than the molecular weights predicted from Eq. (6.102) for termination by either coupling [Eq. (6.103)] or disproportionation [Eq. (6.104)]. Such an effect, when the mode of termination is known to be disproportionation, can be due to a growing polymer chain terminating prematurely by transfer of its radical center to other species, present in the reaction mixture. These are referred to as chain transfer reactions and may be generally written as... 

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