Back-biting termination

Polymerisations of 8,9, and 10 were rapid but stopped at limited conversions owing to a termination process involving an intramolecular back-biting reaction, the tertiary amino groups on the polymer backbone being more... 

Other reactions may be taken into consideration, with an effect on polymer structure, namely the formation of short- and long-chain branches. A complete list of reactions in S-PVC polymerization may be found in Kiparissides et al. [5]. On the above basis kinetic equations may be written. To keep it simple the chain transfer, back-biting and inhibition reactions are disregarded, while termination is considered to occur only by disproportionation. The elementary reaction rates for initiator decomposition and free radicals generation are as follows ... 

Termination and Transfer Processes Macrocyclization. End-Biting and Back-Biting... 

The anionic ROP of cyclotrisiloxane in THF initiated with lithium trialkylsilanolate or butyl lithium is our system of choice for the controlled synthesis of functionalized polysiloxanes. The polymerization proceeds in a selective way. The undesired reactions of back biting, chain transfer and terminal unit exchange may be eliminated. The initiation is fast and complete, and the quenching of polymerization by triorganochlorosilanes is fast and clean. The polymerization may be performed at low temperatures, which increases its selectivity [4, 5]. In addition, the SiOLi/THF system tolerates many functional groups in the monomer. 

It cannot be excluded that the back biting depicted in Scheme 10 could be responsible for the observed absence of methyl proton.389 Scheme 10 would leave most of the dead polymer molecules without methyl end groups. The chemical shifts of the vinylic protons in the resulting oligo-acrylamides are at 5.75 and 6.25 ppm, which is similar to the methacrylate end group of polyBA which has been terminated by back biting. In oligo-acrylates produced by CCT, the vinylic protons have resonances at 6.8 and 5.85 ppm.383 The mechanism of Scheme 11 in acrylate polymerization requires additional study. 

Transfer and Termination Involving the Polymer Backbone. The polymer chain contains the same heteroatora as the parent monomer. Therefore, the nucleophilic attack of a monomer molecule on the strained onium ion can be replaced by the attack of a polymer chain. The polymer chain can react with active species of a foreign macromolecule or of its own macromolecule (back biting). 

The principal termination reaction in the anionic polymerization of MMA is a unimolecular back-biting reaction with the penultimate ester group to form a six-membered ring, P-keto ester group at the chain end as shown in Equation 7.25. The rate of this back-biting reaction decreases with increasing size of the counterion [118]. 

Liquid and solid NMR study of these nanoparticles reveals that the signal of the a-carbon atom (6.2 ppm) disappears for octylsilane stabihzed nanoparticles. Note that the ]3-carbon (27.3 ppm) is also subjected to the influence of its proximity to ruthenium. In addition, the methyl group carbon signal at 14.1 ppm is also notably enlarged, especially for Si/Ru=0.2. This suggests that the octyl chain back-bites the ruthenium surface via its terminal CH3 . Other resonances present quite the same chemical shifts and the same broadness as in free octylsilane (Fig. 18.17). 

Undesirable chain transfer can also occur with impurities present in the reactor or from the feed stock. The most important chain transfer reactions which occur during the free radical synthesis of LDPE are shown in Figure 4. These reactions form the ethyl, 2-ethylhexyl, and butyl branches as well as the vinyl termination on the PE chains. These are the eharacteristic structural features which distinguish LDPE from LLDPE, and HDPE. These reactions all begin with the back-biting step, first proposed by Roedel at Du Pont [18]. This step forms a secondary radical which results in formation of a butyl group once the secondary radical reacts with an ethylene molecule. 

The other ring-chain equilibrium constant is provided by the equilibrium monomer (r-lactide) concentration [M]e that is affected by the back-biting reaction of the hydroxyl terminal. As mentioned in the preceding section, the [M]e are lower than 1.0 wt% below 120 °C and higher than 5 wt% above 180 °C in the ROP of L-lactide. High evacuation, needed to remove condensed water, is likely to result in the removal of L-lactide from the system... 

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