Reversible chain transfer catalyzed process

RTCP involves a reversible chain transfer (RT) process with a catalyst (Scheme 4a) that improves the dispersity control, as well as the mentioned small contribution of DT (Scheme Id). The catalyst can be, e.g., Af-iodosuccinimide (NIS) (Scheme 4a) [31], and works as a deactivator. Polymer (which is originally supplied by the conventional radical initiator) reacts with NIS to produce N-succinimide radical (NS ). NS works as an activator of Polymer-I to generate Polymer and NIS again. This cycle allows for frequent reversible activation of Polymer-I. This process is a reversible chain transfer of NIS that catalytically activates Polymer-I. Therefore, the polymerization was termed reversible chain transfer catalyzed polymerization (RTCP). Regarding the components used, RTCP is similar to initiators for continuous activator regeneration (ICAR)-ATRP [65]. Both systems use a monomer, a dormant species (alkyl iodide or alkyl bromide), a conventional radical initiator, and a deactivator [NIS or copper (II)] to regenerate a highly reactive activator [NS or copper (I)]. 

Actinide, lanthanide, and yttrium-based catalyst systems showing characteristics of reversible chain transfer in ethylene polymerization are summarized in Table 3. Samsel and Eisenberg claimed to observe the characteristics in ethylene polymerization with several metallocenes of actinides, such as the bis(pentamethylcyclopentadienyl) thorium complex 5 in combination with aluminum alkyl reagents. These systems catalyze the production of aluminum alkyl chain growth products at lower temperatures than those required by the uncatalyzed Ziegler process. The systems were limited to production of low-molecular-weight PE oligomers. 

These are hemoproteins that catalyze electron transfer through the reversible change in oxidation state of the heme iron.637 They are involved in the respiratory chain, and in a wide range of other processes such as photosynthesis and the nitrogen cycle. Over 50 cytochromes have been studied, notably cytochrome c, which is one of the best studied biological molecules. Bacteria in particular produce a wide range of cytochromes which currently are attracting much attention. 

The reversible phosphorylation of proteins is one of the most widespread posttranslational modifications, mediating responses to internal and external signals in a variety of cellular processes [1-3]. In eukaryotes 30-70 % of all proteins are phosphorylated on tyrosine (Tyr), threonine (Thr), and/or serine (Ser) residues [4-6]. Protein phosphorylation is catalyzed by protein kinases, which transfer the y-phosphate of ATP to a hydroxyl side chain, resulting in the formation of a phosphate monoester. Protein phosphatases hydrolyze these phosphate monoesters and make protein phosphorylation a reversible modification [4]. 

A subset of this process is the transition metal catalyzed, reversible cleavage of the covalent bond in the dormant species via a redox process (Scheme 2). Since the key step in controlling the polymerization is transfer of an atom (or group) between a dormant chain and a transition metal catalyst in a lower oxidation state forming an active chain end and a transition metal deactivator in a higher oxidation state, this process was named atom transfer radieal polymerization (ATRP) [27-32]. 

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