Reversible Termination by Coupling

There are four principal mechanisms that have been put forward to achieve living free-radical polymerization (1) Polymerization with reversible termination by coupling, the best example in this class being the alkoxyamine-initiated or nitroxide-mediated polymerization, as first described by Solomon et al. (1985) (2) polymerization with reversible termination by hgand transfer to a metal complex (usually abbreviated as ATRP),(Wang and Matyjaszewski, 1995) (3) polymerization with reversible chain transfer (also termed degenerative chain transfer)-, and (4) reversible addition/ffagmentation chain transfer (RAFT). 

Free-radical polymerization mediated by stable nitroxyl radicals like TEMPO (2,2,6,6-tetramethylpiperidinyl-l-oxy) (Fig. 6.23) is a simple and robust method to synthesize homopolymers and random and block copolymers with narrow poly-dispersities and/or controlled chain architectures. A key to the success of this polymerization is believed to be the reversible combination of a polymer radical, P , with a stable nitroxyl radical, N, to form an adduct, P-N, which is a dormant species  

Problem 6.42 A TEMPO-mediated polymerization of styrene was carried out (Goto and 

Fukuda, 1997) as follows A mixture of benzoyl peroxide (0.072 mol L ), TEMPO (0.086 mol L ), and styrene, degassed and sealed off under vacuum, was heated at 90°C for 1 h. There was no formation of polymer as shown by GPC. Then the system was heated at 125°C for 5 h to yield a polymer (recovered by precipitation in excess methanol) with M = 1700 and = 1.11. Analysis showed that this polymer (PS) had a TEMPO molecule 

The concentration of styrene in the two systems and molecular weight of the polymer formed were measured periodically. Selected data derived from these measurements are given below. 

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