Nitroxide mediated polymerization is a controlled radical polymerization technique that uses stable nitroxide radicals. It allows high control over polymer architecture and properties. The principle involves reversible trapping of propagating radicals by nitroxide to form alkoxyamines. This prevents irreversible radical termination. Nitroxide mediated polymerization can be used to synthesize homopolymers, random/block copolymers, and functionalized polymers with applications in coatings, adhesives, and biomaterials.

1. NITROXIDE MEDIATED
POLYMERIZATION
GOPI PRAMANIK
19POL206
M TECH (POLYMER ENGG & TECHNOLOGY)

2. INTRODUCTION
• Nitroxide mediated polymerization allows for a high degree of control over the
synthesis of tailored molecular architectures by making use of a stable free radical .
• Various stable radicals such as nitroxide, triazolinyl , trityl, and dithiocarbamate
used as the mediating or persistent radical (deactivator) for SFRP.
• SFRP with Nitroxide called as Nitroxide Mediated Polymerization, which more
efficient than others.
• As per IUPAC it is called ‘‘aminoxyl-mediated radical polymerization’’ (AMRP).
• NMP was only discovered in 1985 by the Commonwealth Scientific and Industrial
Research Organization (CSIRO) team, and was widely advertised by the seminal
work of Georges in 1993.

3. PRINCIPLE
• It basically involves reversible trapping and release of the propagating radical by the nitroxide.
• In the trapped form, the end-group structure is that of an alkoxyamine in which the C-O bond is
weak and dissociates to regenerate the chain radical and nitroxide.
• The chain grows in very short bursts of propagation when it is released from the alkoxyamine.
Once dissociated from the alkoxyamine end group, the free nitroxide (being small) can diffuse
rapidly and so there is a dynamic equilibrium in which a chain radical is likely to be recaptured
by a different nitroxide molecule.

4. Some Nitroxide and alkoxyamine used in NMP
• The chemical structure of TEMPO and acyclic nitroxides used widely in NMP.
• The structure of TIPNO and SG1 is the hydrogen atom attached to one of the carbon atoms bonded to the nitrogen
atom, because this changes the reactivity and makes NMP possible with styrene and other monomers (principally
acrylates, acrylamides and dienes) at lower temperatures (typically 110–120 °C).

5. ALKOXYAMINE INITIATORS
• The initiating material for NMP.
• An alkoxyamine can essentially be viewed as an alcohol bound to a secondary amine by an N-O single bond.
• The utility of this functional group is that under certain conditions, homolyses of the C-O bond can occur,
yielding a stable radical in the form of a 2-center 3-electron N-O system and a carbon radical which serves as an
initiator for radical polymerization.
• For the purposes of NMP, the R groups attached to the nitrogen are always bulky, sterically hindering groups and
the R group in the O- position forms a stable radical, generally is benzylic for polymerization to occur
successfully.
• NMP allows for excellent control of chain length and structure, as well as a relative lack of true termination that
allows polymerization to continue as long as there is available monomer. Because of this it is said to be “living".

6. PERSISTENT RADICAL EFFECT
• In the case of a nitroxide-mediated polymerization reaction, the persistent radical is the nitroxide species
and the transient radical is always the carbon radical.
• This leads to repeated coupling of the nitroxide to the growing end of the polymer chain, which would
ordinarily be considered a termination step, but is in this case reversible.
• Because of the high rate of coupling of the nitroxide to the growing chain end, there is little coupling of
two active growing chains, which would be an irreversible terminating step limiting the chain length.
• The nitroxide binds and un-binds to the growing chain, protecting it from termination steps.
• Because of the PRE, it can be assumed that at any given time, almost all of the growing chains are “capped”
by a mediating nitroxide, meaning that they dissociate and grow at very similar rates, creating a largely
uniform chain length and structure.

8. INITIATION STAGE
• NMP can be performed in bulk, solution, or dispersed media. Whatever the conditions,
only three types of initiations are possible: monocomponent initiating system,
bicomponent initiating system, and in situ polymerization.
• It must be noted that the monocomponent initiating system in bulk polymerization has
been the most thoroughly investigated because it is the simplest system, and the most
suitable to test.

9. NITROXIDE CHOICE
• An effective polymerization (fast rate of chain growth, consistent chain length) results
from a nitroxide with a fast C-O homolysis and relatively few side reactions.
• A more polar solvent lends itself better to C-O homolysis, so polar solvents which cannot
bind to a labile nitroxide are the most effective for NMP.
• It is generally agreed that the structural factor that has the greatest effect on the ability of a
nitroxide to mediate a radical polymerization is steric bulk.
• Greater steric bulk on the nitroxide leads to greater strain on the alkoxyamine, leading to
the most easily broken bond, the C-O single bond, cleaving homolytically.

10. POLYMERIZATION
• NMP has been employed for large number of preparations of macromolecular architectures. And composition allowing
a fine tuning of their physio-chemical properties.
• HOMOPOLYMERIZATION
NMP of styrene and styrene-based monomers such as styrenes para-substituted with F, Cl, Br, CH3, OCH3, CF3, CH2 Cl,
4- acetoxystyrene, 4-tert-butoxystyrene, or even para-(1-methylcyclohexyloxy) styrene has been successfully achieved.
Acrylonitrile and Acrylic Acid, Acrylamide, N-substituted and N,N-disubstituted Acrylamides, Dienes, Methacrylic
Esters, Cyclic ketene acetals.
• RANDOM & BLOCK COPOLYMERIZATION
The copolymers obtained when using the NMP technique exhibit very close compositions with obviously narrower PDIs.
A very interesting application of copolymerization consists in the control of various methacrylate derivatives using low
amounts of comonomer such as styrene or acrylonitrile.

11. • Aqueous miniemulsion polymerization:
Miniemulsion NMP can be broadly divided into two categories: (i) aqueous phase initiation and (ii) oil phase initiation. In
general, miniemulsion polymerization requires the addition of a hydrophobe, such as hexadecane, to suppress Ostwald
ripening. Interestingly, a nitroxide terminated macroinitiator (for instance polystyryl-TEMPO) can be used instead of
hexadecane. The addition of a small amount of high-molar mass polymers enhances droplet nucleation and stabilization.
Typically, both oil- and water-soluble initiators have been investigated in miniemulsion NMP in the presence of TEMPO
and SG1 nitroxides. Bicomponent systems were the first to be applied but improved systems were achieved using SG1-
based water-soluble alkoxyamines.
• Aqueous emulsion polymerization:
Implementation of NMP in emulsion has proved to be a significant challenge. The main problems associated to this system
were colloidal stability and high control/livingness. Indeed, to obtain successful emulsion NMP, several criteria have to be
fulfilled. Typically, emulsion polymerization mainly involves large droplets (<1 μm) of hydrophobic monomer(s) and a
large number of monomer-swollen micelles. Successful emulsion NMP implies that the polymerization takes place within
small monomer droplets generated via diffusion of the monomer droplets through the aqueous phase. In contrast to
miniemulsion, emulsion process requires the use of water-soluble radical initiators.

12. • Chain end functionalized polymers from NMP
α functionalization is obtained through the use of the corresponding functionalized alkoxyamine while ω
functionalization is achieved after chemical transformation of the nitroxide moiety attached to the polymer
chain end.
α-Functionalization of the polymer chains is obtained by using the commercially available or beforehand
prepared initiator. ω-Functionalization is by far more difficult to obtain since it usually requires at least one
chemical step after the polymerization process.

13. • Di-Block and Tri-Block copolymers:
Regarding the NMP technique, the most widely applied synthetic pathway for obtaining a block copolymer is
the sequential polymerization of different monomers.
A number of specific techniques have also been developed for the preparation of block copolymers such as
the coupling reaction or the use of a heterofunctional alkoxyamine.

14. Possible synthesis of block copolymer from a heterofunctional alkoxyamine (with • = functional group such as
OH, NH, SH, COOH, . . .).

15. DISADVANTAGE
• Not tolerant to all kind of functional groups, monomers
• Slow polymerization kinetics that require high temperatures and lengthy polymerization times,
• The inability to easily control the polymerization of methacrylate monomers due to side reactions and/or
slow recombination of the polymer radical with nitroxide
• Synthetic difficulties associated with nitroxide and alkoxyamine synthesis.

16. APPLICATION
• The development of controlled radical polymerization techniques has enabled extraordinary opportunities in
the synthesis of new materials such as block copolymers mainly designed for a broad range of applications
specialty such as detergents, cosmetics, health, paints, coatings, adhesives, electronics, transport, or energy.
• Its tolerance of many functional groups the absence of metal catalysts, as well as coordinating ligands, or
sulphur-containing moieties.
• synthesizing electroactive from ligand-functionalized monomers, and the presence of catalyst residues
might impair device performance.
• NMP is also well suited for the preparation of peptide–polymer conjugates.

17. REFERENCES
• H. Jeong, J. Kim, W. Son, and Y. Kim, 사지마비 장애인을 위한 근전도 기
반 입력 인터페이스 기술 및 그 응용 and Its Applications, vol. 125, no. 6.
1997.
• M. Chen-Wishart, 解剖列车中文第三版 Third Edition, no. January 2010.
2014.