Polymerization reversible activation

It can also be noted that reversible chain transfer, in RAFT and similar polymerizations, and reversible activation-deactivation, in NMP and ATRP,... 

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. 

Polymerization can be started using an alkoxyamine as initiator such that, ideally, no reactions other than the reversible activation of dormant species and the addition of monomer to carbon-centered radicals take place. The alkoxyamine consists of a small radical species, capable of reacting with monomer, trapped by a nitroxide. Upon decomposition of the alkoxyamine in the presence of monomer, polymeric dormant species will form and grow in chain length over time. Otherwise, polymerization can be started using a conventional free-radical initiator and a nitroxide. The alkoxyamine will then be formed in situ when an initiator molecule decomposes, and, after adding a monomer unit or two, is trapped by a nitroxide. 

Irreversible adsorption on active-sites (less than 90% yield) For basic compounds use end-capped, base-deactivated, sterically protected, high coverage, or polymeric reversed-phase for acidic compounds use endcapped or pol3mieric packing acidify mobile phase. 

A strong deactivating effect of hydrogen has been noted in ethylene polymerization with vanadium-based catalysts such as M gC 12/VC 14—AIB u 3.409 This deactivation was reversible, activity being restored when hydrogen was removed. It was proposed that the deactivating effect of hydrogen arose from the reaction of V-H species with the... 

The metal-catalyzed living radical polymerization thus proceeds (or at least is mostly considered to proceed) via reversible activation of carbon—halogen terminals by the metal complex, where the metal center undergoes redox reactions via interaction with the halogens at the polymer terminal, as shown in Scheme 3. The reaction is usually initiated by the... 

One of the most important components in the metal-catalyzed living radical polymerization is the transition-metal complex. As a catalyst, the complex induces reversible activation (homolytic cleavage) of... 

Butadiene polymerization studies with HMTT/CH2Li catalysts have given results which are directly contrary to those expected from the Hay mechanism. Activity at 0.5 HMTT/CH2Li was double that at 1 1 ratio whereas the reverse should have been obtained if the tetramine solvated lithium compound were the active species. Our lithium catalyst studies suggest that all of the known TMED complexes are active for butadiene polymerization with activity increasing roughly in the order (RLi)4 TMED < (RLi)2 TMED < RLi TMED << RLi(TMED)2. The equilibrium to form RLi(TMED )2 is believed to be unfavorable except when R" is a highly delocalized carbanion. 

VAc has been successfully polymerized via controlled/ living radical polymerization techniques including nitroxide-mediated polymerization, organometallic-mediated polymerization, iodine-degenerative transfer polymerization, reversible radical addition-fragmentation chain transfer polymerization, and atom transfer radical polymerization. These methods can be used to prepare well-defined various polymer architectures based on PVAc and poly(vinyl alcohol). The copper halide/t is an active ATRP catalyst for VAc, providing a facile synthesis of PVAc and its block copolymers. Further developments of this catalyst will be the improvements of catalytic efficiency and polymerization control. 

Living radical polymerization (LRP) has attracted growing attention as a powerful synthetic tool for well-defined polymers 1,2). The basic concept of LRP is the reversible activation of the dormant species Polymer-X to the propagating radical Polymer (Scheme la) 1-3). A number of activation-deactivation cycles are requisite for good control of chain length distribution. 

The significant difference between conventional radical polymerizations and all of the LRP techniques developed so far is the establishment of a rapid dynamic equilibrium between a very small amount of chain-growing free radicals and a large excess of the dormant species. The general scheme for this reversible activation process is represented schematically below ... 

In propylene polymerization, the activating effect of hydrogen, i.e., an increase in initial polymerization rate as well as in overall activity, is observed [83-90] (Fig. 9). This activation is reversible and the polymerization rate decreases after the removal of hydrogen from the reaction zone [89, 90]. The degree of increase in the activity and change of the polymerization rate with time, in comparison with polymerization without hydrogen, depend on the catalyst nature and the hydrogen concentration. 

Goto et al. [279] developed a process that they describe as reversible living chain transfer radical polymerization [278], where they us Ge, Sn, P, and N compounds iodides in the iodide mediated polymerizations." In this process, a compound such as GeLt is a chain transferring agent and the polymer-iodide is catalytically activated via a RFT process. They proposed that the new reversible activation process be referred to as RTCP [279]. The process can be illustrated by them as follows [279] ... 

Scheme 1.14 Polymerization of lactide hy reversible activation and deactivation of SIMes.

Scheme 1.15 Polymerization of lactide by reversible activation of triazolylidene carbenes.

General scheme of reversible activation in controlled radical polymerization. 

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