Free radical mechanism, chain molecular weight distribution

Polyamides, like other macromolecules, degrade as a result of mechanical stress either in the melt phase, in solution, or in the soHd state (124). Degradation in the fluid state is usually detected via a change in viscosity or molecular weight distribution (125). However, in the soHd state it is possible to observe the free radicals formed as a result of polymer chains breaking under the appHed stress. If the polymer is protected from oxygen, then alkyl radicals can be observed (126). However, if the sample is exposed to air then the radicals react with oxygen in a manner similar to thermo- and photooxidation. These reactions lead to the formation of microcracks, embrittlement, and fracture, which can eventually result in failure of the fiber, film, or plastic article. 

In processes based on reversible termination, like NMCRP and ATRP (Sect. 4.4.2), a species is added which minimizes bimolecular termination by reversible coupling. In NMCRP this species is a nitroxide. The mechanism of nitroxide-mediated CRP is based on the reversible activation of dormant polymer chains (Pn-T) as shown in Scheme 1. This additional reaction step in the free-radical polymerization provides the living character and controls the molecular weight distribution. 

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 kinetics of free radical polymerization and the molecular weight distribution of the polymer were already discussed in Section 1.6.2 of Chapter 1. To improve the chemical and mechanical properties of the polymer great efforts were undertaken a number of years ago to achieve narrow distributions. This is possible with anionic or cationic — so-called living — polymerization, in which chains can not terminate or transfer and grow at a rather uniform rate, thus yielding a polymer with a polydispersity close to one. This type of polymerization requires very special operating conditions and high purity of the feed, however. 

In most reports, the peptide-polymer-conjugates are prepared by using a polymeric macroinitiator for the polymerization of the polypeptide however, the sequence can also be reversed. Polypeptides can be prepared and used as macroinitiators for a polymerization. Particularly suited for this approach are controlled polymerization techniques because they usually allow good end-group control and adjustment of the molecular weight and the molecular weight distribution of the polymer block. There are different mechanisms for a controlled radical polymerization that can be used for this purpose stable free-radical polymerization (SFRP), ATRP, and reversible addition fragmentation chain transfer (RAFT) polymerization. 

Living free radical polymerizations were also carried out in miniemulsion systems via the reversible addition-fragmentation chain transfer mechanism [66]. The colloidal stability of miniemulsions is the key issue, and nonionic surfactants result in the best results. The polydispersity index of molecular weight distribution for the resultant miniemulsion polymer is generally smaller than 1.2. 

It has been generally accepted that the thermal decomposition of paraffinic hydrocarbons proceeds via a free radical chain mechanism [2], In order to explain the different product distributions obtained in terms of experimental conditions (temperature, pressure), two mechanisms were proposed. The first one was by Kossiakoff and Rice [3], This R-K model comes from the studies of low molecular weight alkanes at high temperature (> 600 °C) and atmospheric pressure. In these conditions, the unimolecular reactions are favoured. The alkyl radicals undergo successive decomposition by [3-scission, the main primary products are methane, ethane and 1-alkenes [4], The second one was proposed by Fabuss, Smith and Satterfield [5]. It is adapted to low temperature (< 450 °C) but high pressure (> 100 bar). In this case, the bimolecular reactions are favoured (radical addition, hydrogen abstraction). Thus, an equimolar distribution ofn-alkanes and 1-alkenes is obtained. 

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