Conventional free radical termination reactions

A PP macromonomer with a methacryloyl end group was synthesized, and was used to prepare PMMA-g-PP graft copolymers by conventional free radical copolymerization [104]. Vinylidene-terminated PP (Mn = 1000) was converted into terminally hydroxylated PP (PP-OH) by a combination of the hydroboration reaction of the unsaturated group and oxidation reaction. Resulting PP-OH was reacted with methacryloylchloride to synthesize termi-... 

Although ATRP behaves differently from conventional free radical polymerization, the fundamental reactions involved are very similar and include initiation, propagation, transfer and termination (see Scheme 6). Since chain termination does not occur in a truly living polymerization, the living character of the chains in ATRP derives from the fact that chain propagation is first order with respect to radical concentration and irreversible bi-molecular termination is second order. As such, the concentration of the radicals is kept very low, the rate of bi-molecular termination is greatly reduced, and typically less than 10% of all of the chains will terminate. Unlike conventional free radical polymerization, where the rate is dictated by a steady state between the initiation and termination rates, the rate and concentration of propagating radicals in ATRP is controlled entirely by the equilibrium between activation and deactivation [255]. 

In conventional free radical polymerization, the initiation, propagation, and termination are kinetically coupled. Consequently, the increase of initiation rate increases the overall polymerization rate but reduces the degree of polymerization. In contrast to this situation (kinetically coupled initiation, propagation, and termination), the formation of chemically reactive species is not the initiation of a subsequent polymerization. Under such an activation/deactivation decoupled reaction system, the mechanism for how chemically reactive species are created and how these species react to form solid material deposition cannot be viewed in analogy to polymerization. 

Termination reactions cannot be eliminated in radical polymerizations because termination reactions involve the same active radical species as propagation therefore, eliminating the species that participates in termination would also result in no polymerization. Termination between active propagating species in cationic or anionic processes does not occur to the same extent because of electrostatic repulsions. Equation (1) represents the rate of polymerization, Rp, which is first order with respect to the concentration of monomer, M, and radicals, P, while Eq. (2) defines the rate of termination, Rt, which is second order with respect to the concentration of radicals. To grow polymer chains with a degree of polymerization of 1000, the rate of propagation must be at least 1000 times faster than the rate of termination (which under steady state condition is equal to the rate of initiation). This requires a very low concentration of radicals to minimize the influence of termination. However, termination eventually prevails and all the polymer chains produced in a conventional free radical process will be dead chains. Therefore they cannot be used in further reactions unless they contain some functional unit from the initiator or a chain transfer agent. 

In each of these mechanisms, the reverse reaction dominates the equihbriimi and keeps the overall concentration of the propagating radical (P ) low, typically [Pn ]/[Pn— X] < 10-5. If tijis reversible radical trapping process occurs frequently, it minimizes the irreversible termination reactions but also means that the polymer chains all have an equal chance to grow, resulting in polymers with a narrow molecular weight distribution. It also follows that, unlike conventional free-radical polymerizations, the polymer chain length will increase steadily with the reaction time, similar to living anionic polymerizations. 

The propagation and termination steps in the above reactions are claimed " to be related. As stated, an interaction and coupling between two diradicals is a propagation step. When such interactions result in disproportionations, however, they are termination steps. This means the charge-transfer mechanisms are different from conventional free-radical polymerizations. They involve not only interactions between growing polymer radicals and monomers, but also between polymer radicals and complexes. In addition, they involve interactions between the polymer radicals themselves. 

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