Controlled radical polymerization

The controlled radical polymerization ( English controlled radical polymerization , CRP) or according to IUPAC reversible-deactivation radical polymerization is (RDRP) a polymerization process, in which the benefits of through optimization of the reaction conditions, free radical polymerization with the advantages of living polymerization are linked.

This enables polymerizations and copolymerizations ,

• that are not very sensitive to dirt ,
• in which a wide range of monomers can be used,
• which take place under relatively mild reaction conditions,
• which provide very uniform polymers with a narrow molecular weight distribution,
• and which enable controllable polymer architectures.

Living polymerizations

According to Michael Szwarc , a living polymerization is defined as a chain reaction without transfer reactions or termination reactions .

To do this, the following conditions must be met:

1. The number of active centers remains constant during the polymerization.
2. The kinetics of chain growth is first order with respect to the concentrations of monomer and active species.

These reaction conditions enable the targeted construction of block copolymers:

1. First, homopolymer scaffolds are created that carry two or more active ends.
2. Another monomer can then be added, which binds in blocks to the active ends. When this reaction is complete, new heteropolymeric scaffolds are obtained with two or more active ends.
3. At this point, the polymerization can be terminated by special reagents, or further monomers can be added, etc.

The living polymerization process is only possible with some anionic polymerizations and cationic group transfer polymerizations . And that only with a small number of monomers and with greater effort in terms of preparation and reaction conditions, especially because of the high sensitivity to contamination, water , air, etc.

Free radical polymerization

Free radical polymerization only fulfills a few conditions of living polymerization :

1. Initiation is much faster than chain growth.
2. The number of active centers, in this case: the radicals, remains constant only at low conversions ; at higher conversions, termination reactions occur.
3. The kinetics of chain growth is first order with regard to the concentrations of monomers and the macro radicals P *.
4. The molar mass distribution does not correspond to the Poisson distribution , but to the Schulz-Flory distribution , i.e. H. the fluctuation range of the molar masses is undesirably high.

By suitably adapting the reaction conditions, it is possible to generate a controlled radical polymerization from the free radical polymerization , the reaction never fulfilling all the requirements of a living polymerization. Strictly speaking, the term "living" is not correct for this type of reaction and only indicates that similar reaction products (relatively uniformly molar mass distributed polymers, block copolymers ...) can be obtained. For example, very broad molecular weight distributions can also be obtained with a real living anionic polymerization.

RDRP

For this purpose, the chain termination reaction of free radical polymerization is reduced as much as possible. It is these termination reactions that firstly reduce the number of active species and secondly lead to the broadening of the molar mass distribution.

This suppression of the termination reactions is achieved by kinetic control of the polymerization:

dilution

The speed of the chain termination reactions is given by:

${\displaystyle v_{stop}=k_{stop}\cdot [P*]^{2}}$

Here k _{stop is} the rate coefficient and [ P *] the concentration of the polymer radicals.

The concentration of the radicals P * has a quadratic effect on the speed of the termination reactions. The further one lowers the concentration of these radicals, the less likely it is to break the chain.

This effect can be achieved by diluting with a large amount of solvent , but this also reduces the reaction rate of the desired reactions. In addition, there is a lot of dirty solvent. Accordingly, the dilution must not be too great.

"Radical buffer"

Another way to lower [ P *] is to use special reagents that are able to form radicals and capture radicals again.

These reagents are added to the polymerization mixture as dormant species in non-radical form. Part of the dormant species decays directly and forms a radical active species .

The active species can be converted back into the dormant species with the uptake of polymer radicals , so that a balance of several reactions is formed in which radicals are released or absorbed.

As a result, a constant concentration [P *] is established:

• If too many radicals P * are produced during the polymerization , these will be caught by the active species of the special reagent.
• If too many dormant species are formed during the polymerization , part of them decays again, so that [ P *] is more or less constant.

This equilibrium concentration of P * can be specifically controlled by the amount of special reagents added.

Main procedures

The special reagents can be very different, the most effective and therefore the most common are:

• Atom Transfer Radical Polymerization ( ATRP )

Initiator: organohalide RX

Radical buffer: transition metal - complex compound MX _n L _x

• Stable Free Radical Polymerization ( SFRP )

Radical buffer: stable radicals based on linear or cyclic nitroxides, e.g. B. SPEED

• Reversible Addition Fragmentation Chain Transfer Polymerization ( RAFT )

Initiator: peroxo or azo compound, more rarely photoinitiators or gamma radiation

Controlling species: substituted dithioester, xanthate or trithiocarbonate

Note: The RAFT process differs from the other methods in that control is not achieved via a reduction in [P *], but via a degenerative chain transfer mechanism

Applications

The CFRP has only been researched for a few years, but it is increasingly being used in large-scale / industrial areas - mainly because it does not require extensive machine changes.

It enables:

• targeted production of catalysts and catalyst carrier surfaces
• Targeted control and functionalization of polymer properties for medicine , technology and plastics in household use.

Individual evidence

1. ^ Aubrey D. Jenkins, Richard G. Jones, Graeme Moad: Terminology for reversible-deactivation radical polymerization previously called "controlled" radical or "living" radical polymerization (IUPAC Recommendations 2010) . In: Pure and Applied Chemistry . tape 82 , no. 2 , November 18, 2009, p. 483 , doi : 10.1351 / PAC-REP-08-04-03 .
2. ↑ M. Szwarz, Nature 1956 , 178 , 1168-1169.