Hydrogen atom transfer from polymer

Termination by disproportionation comes about when an atom, usually hydrogen, is transferred from one polymer radical to another ... 

Radical polymerization can lead to branched polymers by intramolecular hydrogen atom transfer, a process sometimes called backbiting. Removal of H through a six-membered transition state moves the growing radical atom five atoms back down the chain, and leads to butyl side-chains. A more stable secondary radical is produced and chain growth then occurs from that point. 

The transition states of the latter are therefore more sensitive to stereochemical and electronic influences, which also leads to a higher selectivity than in analogous electrochemical conversions. At some oxide electrodes, such as the Ni(OH)2 electrode [13], the oxidation occurs as inner sphere electron transfer by hydrogen atom transfer. Also at doped titanium anodes this seems to be partially the case [14]. It cannot be definitely excluded that also in some oxidations at platinum anodes, higher valency oxides at the surface act as inner sphere electron transfer agents. Electrochemical" inner sphere electron transfers are intentionally used in indirect electrochemical conversions where selective chemical oxidants or reductants are regenerated by electron transfer from the electrode [15]. They are also immobilized by attaching polymer-bound electrocatalysts as mediators to the electrode surface [16]. 

One of the most striking features of CCT is the exceptionally fast rate at which it takes place. The molecular weight of a polymer can be reduced from tens of thousands to several hundred utilizing concentrations of cobalt catalyst as low as 100—300 ppm or 10 3 mol/L. The efficiency of catalysis can be measured as the ratio between the chain-transfer coefficients of the catalyzed reaction versus the noncatalyzed reaction. The chain-transfer constant to monomer, Cm, in MMA polymerization is believed to be approximately 2 x 10 5.29 The chain-transfer constant to catalyst, Cc, is as high as 103 for porphyrins and 104 for cobaloximes. Hence, improved efficiency of the catalyzed relative to the uncatalyzed reaction, CJCu, is 104/10 5 or 109. This value for the catalyst efficiency is comparable to many enzymatically catalyzed reactions whose efficiencies are in the range of 109—1011.18 The rate of hydrogen atom transfer for cobaloximes, the most active class of CCT catalysts to date, is so high that it is considered to be controlled by diffusion.5-30 32 Indeed, kc in this case is comparable to the termination rate constant.33... 

R00, is formed in the propagation step by rapid reaction with oxygen. In the absence of an efficient antioxidant, the peroxy radical is converted to the hydroperoxide, ROOH, by hydrogen-atom abstraction from the polymer chain giving rise to another polymeric radical, R , and peroxy radical R00. Propagation via chain transfer is also promoted by homolysis of ROOH to RO and 0H in the absence or depletion of the antioxidant AH. 

Some papers report on the influence of aromatic compounds on the polymerization of vinyl compounds other than vinyl acetate. Mayo et al.48 found that bromo-benzene acts as a chain transfer agent in the polymerization of styrene, although no fragments of bromobenzene are incorporated into the polymer. They concluded that a complex is formed between the solvent molecule and either the propagating poly-styryl radical or hydrogen atom derived from the latter. 

The termination reactions that are more important in polymer production are combination (or coupling) and disproportionation. In termination by combination, two growing polymer chains react with the mutual destruction of growth activity (Equation 2.15), while in disproportionation a labile atom (usually hydrogen) is transferred from one polymer radical to another (Equation 2.16). 

The lowest path is consistent with polymer MD simulations where many of the ring opening events are proceeded by the addition of an H atom that was lost from the same or another polymer molecule. This reaction should be important at high temperatures where free H atoms are found. At lower temperatures, this mechanism should be less efficient as few H atoms will exist however, reactions where the H atom transfers from another molecule are still possible, but the first step is no longer as exothermic as in case of a free H atom. In case of PAH molecules in the ISM, this H addition erosion mechanism should be important in regions where there is a significant hydrogen atom concentration, such as in PDRs. 

In the disproportionation mode of termination, a transfer of (usually) a hydrogen atom occurs from one polymer radical to the other (see Eq. 2.10). This leads to the formation of one polymer with a saturated end group and another polymer with an unsaturated end group. 

Cobalt porphyrin complexes are involved in the chain transfer catalysis of the free-radical polymerization of acrylates. Chain transfer catalysis occurs by abstraction of a hydrogen atom from a grow ing polymer radical, in this case by Co(Por) to form Co(Por)H. The hydrogen atom is then transferred to a new monomer, which then initiates a new propagating polymer chain. The reaction steps are shown in Eqs. 12 (where R is the polymer chain. X is CN), (13), and (14)." ... 

Bartlett and co-workers concluded that addition to the nitro group also occurs. Price and Read found that several m-dinitrobenzene molecules were combined with the polymer for each fragment from the p-bromobenzoyl peroxide used as initiator in the retarded polymerization of styrene. They inferred that the radical corresponding to IV transfers its hydrogen atom to a molecule of styrene as follows ... 

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