Living radical polymerization mediating agent

Controlled/ living radical polymerization (CLRP) processes are well-established synthetic routes for the production of well-defined, low-molecular weight-dispersity polymers [99]. The types of CLRP processes (initiator-transfer agent-terminator (INIFERTER), atom transfer radical polymerization (ATRP), nitroxide-mediated radical (NMRP) polymerization, reversible addition-fragmentation transfer (RAFT)) and their characteristics are described in Section 3.8 of Chapter 3 and in Section 14.8 of Chapter 14. 

Initiators for the controlled living radical polymerization could also be introduced to silica particles. Nitroxide-mediated polymerization (NMP) conducted with styrene in miniemulsion led to the generation of core-shell particles, with styrene grafted to the central silica particle [131]. PBA could be polymerized from 20 nm silica beads by attaching an ATRP agent to the silica surface and subsequent miniemulsion polymerization [132]. Confining the polymerization to miniemulsion droplets could avoid gel formation, which was observed in the bulk reaction. Due to the limited monomer diffusion, only 25-35% of conversion could be obtained in bulk. 

Nitroxide mediated free radical polymerization is a living or controlled polymerization process. It can be used to initiated or terminate polymerization reactions as needed (1). The use of Phosphino, aryloxy, silyl, boryl and seleno mediating agents are described (2). 

Using NMP [114, 115] or reversible addition-fragmentation chain transfer (RAFT) [ 119,120,127], agents with ammonium groups for the ion exchange allowed the attachment of initiators on the clay surface for controlled radical polymerizations (NMP, RAFT). Samakande et al. investigated the kinetics of RAFT-mediated living polymerization of styrene [120] and styrene/BA [119] mixtures in miniemulsion. 

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] ... 

To approach truly living conditions in free radical polymerization, the extent of the termination reactions has to be reduced as much as possible. The natural way to establish such conditions is the reduction of the concentration of active chains, which, in turn, results in the decrease of polymer productivity. To contrast such decrease, the segregation of the radical chains, which is present in heterogeneous processes such as emulsion polymerization, can be exploited. This allows reducing terminations while preserving the overall concentration of radical chains. However, the appUcation of RAFT mediated polymerizations to emulsion systems has also been rather problematic, mainly with respect to the transport of RAFT agent through the aqueous phase. Attempts to carry out RAFT polymerizations in ab-initio emulsion processes... 

However, all synthetic approaches involving ATRP rely on a metal catalyst. Full metal-free and thus greener approaches to block copolymers were realized by the combination of Upase ROP with nitroxide-mediated living free radical polymerization [44]. With this system it was also possible to successfully perform a one-pot chemoenzymatic cascade polymerization from a mixture containing a dual initiator, CL and styrene (Fig. 12). Moreover, it was shown that this approach is compatible with the stereoselective polymerization of 4-methylcaprolactone for the synthesis of chiral block copolymers. A metal-free synthesis of block copolymers using a radical chain transfer agent as a dual initiator in enzymatic ROP to yield poly(CL-f -styrene) was also reported recently [119]. 

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