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Atom Transfer Radical Polymerization

In a carefully controlled living polymerization both termination and transfer reactions are absent and all chain growth sites are instantaneously initiated so that all chains grow simultaneously. This type of polymerization allows for precisely controlled molecular weight (MW), and narrow molecular weight distribution (MWD). Until recently, this method was limited to ionic polymerization. However, ionic chain growth reactions are more difficult to control than free-radical reactions and are very sensitive to trace amounts of moisture, carbon dioxide, acids, bases and many other impurities, and thus are only used on an industrial scale if the monomer in question does not polymerize by a free-radical mechanism or when polymers with regular structure and narrow molecular weight distribution are required. Furthermore, only a limited number of vinyl monomers can be polymerized via an ionic mechanism, and copolymerization is more challenging due to the large difference in the reactivity of even structurally similar monomers.1 On the other hand, the free-radical polymerization is suitable for a large number of vinyl monomers and many can be copolymerized due to the non-specific nature of the free radicals towards vinyl monomers. However, well defined block copolymers are difficult to grow and the MWD is very broad.

For many years a controlled "living" radical polymerization was considered impossible owing to chain transfer and chain termination reactions that compete with chain growth. Typically, chain propagation is much faster than initiation and termination which leads to a large radical concentration and hard to control chain growth. However, recently, new chain growth methods have been developed2-4 that involve equilibria between growing chain radicals and their dormant counterparts. Or in other words, chain growth radicals are converted to a dormant state after a few monomer additions and reactivated again at a later point in time. These methods have been coined atom transfer radical polymerization (ATRP).

In ATRP, reversible activation/deactivation of the growth center is mediated by a redox-active transition metal complex, MtzXlLm. Typical metal complexes include halides of many transition metals such as Mo, Re, Ru, Rh, Fe, Ni, Pd, and Cu with triphenylphosphine (PPh3) or bipyridine ligands. The main purpose of the ligand is to solubilize the transition metal salt in the organic solvent (or bulk monomer). The ligands also affect the rate constant of activation (kact) and deactivation (kdeac). Typical ATRP initiators are functional alkyl halides. The metal complex abstracts a halogen atom from the initiator molecule and thereby generates a free radical (R·). In this process, the complex changes its oxidation state from z to z+1:

Mtz-Xl / Lm + R-X ↔ R· + Mtz+1-Xl+1 / Lm

where X is a halogen atom (Cl, Br), L a ligand, and Mt a metal in oxidation state z.
In a second step, the functional initiator (R·) reacts with a monomer (M) and starts propagation:

R· + M → P1·

P1· + n M → Pn+1·

 
Atom Transfer Radical Polymerization (ATRP)

 

Typical functional ATRP initiators (RX) are 2-bromopropanitrile, ethyl 2-bromo-propionate, tosyl chloride among many other halides. In general, the efficiency increases with decreasing bond strength (R-Cl > R-Br > R-I). Thus, alkyl iodides are the most efficient initiators. However, alkyl iodides are light sensitive and tend to form metal iodide complexes. Therefore, bromo- and chloro-compounds are mainly used as initiators. In general tertiary alkyl halides are more reactive than secondary, and secondary are more reactive than primary alkyl halides. The initiators, when reacting with monomers, not only start the propagation process but also introduce a functionality at the chain end whereas difunctional initiators create a functionality at the chain center.

A large number of monomers containing polar functional groups have been successfully polymerized by ATRP. This includes 2-hydroxyethyl (meth)acrylate, acrylonitrile, glycidyl (meth)acrylate, and (meth)acrylamides among many other monomers.

To achieve a narrow polydispersity, the growing polymer chains should be frequently deactivated because a long activation/deactivation cycle will increase the likelihood for chain termination via bimolecular radical recombination:

Pn· + Pm· → Pn + Pm

Pn· + Pm· → Pm+n

The radicals become dormant when abstracting a halogen atom from the transition metal complex. In this process, the complex changes its oxidation state from z+1 to z:

Pn· + Mtz+1-Xl+1 / Lm ↔ Mtz-Xl / Lm + Pn-X

The dormant species in the ATRP equilibrium reaction can be not only a free polymer chain but a polymer attached to a functional colloidal particle or a biomolecule etc.

References & Notes

  1. K. Matyjaszewski and N.V. Tsarevsky, Nature Chemistry, Vol. 1, 276-288 (2009)

  2. J.S. Wang and K. Matyjaszewski, J. Am. Chem. Soc. 117, 5614-5615 (1995)

  3. J.-S. Wang, K. Matyjaszewski, Macromolecules, 28, 7901-7910 (1995)

  4. K. Matyjaszewski, and J. Xia, Chem. Rev. 101, 2921-2990 (2001)

  5. T.G. Ribelli, F. Lorandi, M. Fantin and K. Matyjaszewski, Macromol. Rapid Commun. 40,
    1800616 (2019)

Revised July 17, 2019

  • Summary

    Atom Transfer Radical Polymerization

    is a type of living chain growth polymerization in which the reversible activation/deactivation of the free radical growth centers is mediated by redox-active transition metal complexes.

  • ATRP allows for precisely controlled molecular weight, nearly mono-disperse molecular weight distribution, as well as controlled topology and functionality.

  • The polydispersity depends on the rate of activation, propagation and deactivation. The fewer monomers propagate during one activation-deactivation cycle, the narrower the molecular weight distribution becomes.

  • Many functional vinyl monomers with polar side groups such as (meth)acrylates, acrylonitriles, and acrylamides readily undergo ATRP polymerization.

  • Typical functional ATRP initiators are alkyl halides with various other functional groups. In general, the efficiency increases with decreasing bond strength.

  • Typical redox-active metal complexes include halides of many transition metals such as copper, ruthenium and iron with triphenylphosphin, or bipyridine ligands.

  • Copper is the most widely used transition metal due to its low cost, versatility, robustness and ready availability.

  • The ligands solubilize the transition metal salt in the organic solvent or bulk monomer and increase / decrease the rate constant of activation and deactivation.

  • ATRP is one of the most versatile polymerization methods. It allows for the synthesis of polymers with novel and well-defined architectures including star, block, brush, comb, random and gradient copolymers. It also can be used to create polymers with a broad range of functionalities.

  • Functionalities can be incorporated into ATRP polymers in three possible ways:

    a) Direct polymerization of functional monomers creates functionalities along the polymer backbone.

    b) Monofunctional ATRP initiators introduce functionality at the chain ends.

    c) Difunctional initiators generate functionality at the chain center.