Nitroxide-mediated Radical Polymerization (NMP)

Nitroxide-mediated radical polymerization (NMP) is a controlled radical polymerization (CRP) technique that relies on the reversible capture of the propagating species by nitroxides with the formation of dormant chains (alkoxyamines). It is very attractive because of its metal-free property and effectiveness in the polymerization of a broad range of monomers with various functionalities. Its living character has opened up enormous possibilities for polymers with well controlled stereochemistry, very low dispersity, as well as predictable molecular weight and composition in a wide range of applications.

Mechanism

NMP can be conducted either through a unimolecular process or a bimolecular process.

General illustration of NMP mechanism. Fig. 1 General illustration of NMP mechanism. (Polymer Reviews, 2011, 51(2): 104-137)

Alkoxyamines Initiators for NMP

The initiating materials that work on the NMP process are a family of compounds referred to as alkoxyamines, providing both the reactive initiating radical and the stable mediating nitroxide radical. The development of stable, readily functionalized alkoxyamines initiators, which mimic the growing chain end for living free radical systems, is a crucial process in NMP. To date, a number of synthetic routes to alkoxyamines have been developed. As a rule, they mainly rely on the generation of carbon-centered radicals that are trapped by a preformed nitroxide.

a. Initially, the synthetic approaches to alkoxyamines involving the reaction of BPO with an excess of styrene to give a benzylic radical followed by trapping of the radical intermediate with TEMPO, as shown in Fig. 2.

Approach to alkoxyamines involving BPO, TEMPO and styrene monomer. Fig. 2 Approach to alkoxyamines involving BPO, TEMPO and styrene monomer. (Chemical reviews, 2008, 108(3): 1104-1126)

b. In addition, alkoxyamines could also be prepared by thermal decomposition of azo-initiators, i.e., azobisisobutyronitrile (AIBN), in the presence of a nitroxide.

Approach to alkoxyamines through azo-initiators. Fig. 3 Approach to alkoxyamines through azo-initiators. (Chemical reviews, 2008, 108(3): 1104-1126)

c. Alternative preparation approach to alkoxyamines could be conducted by organic halides and a nitroxide (TEMPO) in the presence of a copper complex.

Approach to alkoxyamines in the presence of a copper complex. Fig. 4 Approach to alkoxyamines in the presence of a copper complex. (Chemical reviews, 2008, 108(3): 1104-1126)

d. Finally, polyfunctional alkoxyamines (di- or tri-alkoxyamines), could be prepared on a basis of DEPN and poly-alkyl halides by atom transfer radical addition.

Approach to polyfunctional alkoxyamines. Fig. 5 Approach to polyfunctional alkoxyamines. (Chemical reviews, 2008, 108(3): 1104-1126)

Applications

NMP offers clear advantages in allowing the polymerization of a broad range of functional monomers (excepting methacrylates) into a range of random, gradient, block, and branched copolymer architectures and it has proven to be readily adapted to industrially relevant methods including polymerization in dispersed systems and continuous reactors.

References:

  1. Hawker, C. J.; Bosman, A. W., et.al. New polymer synthesis by nitroxide mediated living radical polymerizations, Chem. Rev. 2001, 101, 3661–3688.
  2. Grubbs R B. Nitroxide-mediated radical polymerization: limitations and versatility. Polymer Reviews, 2011, 51(2): 104-137.
  3. Sciannamea V, Jérôme R, et.al. In-situ nitroxide-mediated radical polymerization (NMP) processes: their understanding and optimization. Chemical reviews, 2008, 108(3): 1104-1126.

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