Living free-radical polymerization
The Controlled Free Radical Polymerization (CFRP ) or Living Free Radical Polymerization ( LFRP ) (controlled free radical polymerization or living free radical polymerization ) is a polymerization process in which through optimization of the reaction conditions, the advantage of the free radical polymerization with the advantages living polymerization can be linked.
This polymerizations and copolymerizations are possible,
- Which are less sensitive to contamination,
- In which a high degree of selection can be used of monomers
- That take place under relatively mild reaction conditions,
- The very uniform polymers with narrow molecular weight distribution deliver
- And the controlled polymer architectures allow.
Living polymerizations
After Szwarc a living polymerization is defined as a chain reaction without transfer reactions or termination reactions.
The following conditions must be met:
These reaction conditions allow the selective synthesis of block copolymers:
The process of living polymerization is possible only in some anionic and cationic polymerizations group transfer polymerizations. And that only a small number of monomers and with a great effort with respect to preparation and reaction conditions, mainly because of the high sensitivity to dirt, water, air, etc.
Free radical polymerization
The free radical polymerization met only a few conditions of a living polymerization:
By suitable adjustment of the reaction conditions it is possible to generate electricity by Free Radical Polymerization a " Living" Free Radical Polymerization Controlled, wherein the reaction never met all the requirements of a living polymerization. " Live " here indicated only that similar reaction products ( rel. uniform molmassenverteilte polymers, block copolymers, ... ) can be obtained. Szwarzs definition of alive but never referring to the properties of the product, which is evident from the reaction. So you can for example get very broad molecular weight distributions with a real living anionic polymerization.
CFRP
For this purpose the chain termination reaction of the free radical polymerization is reduced as much as possible. These termination reactions are there, first reduce the number of active species and, secondly, lead to the broadening of the molecular weight distribution.
This suppression of termination reactions can be achieved by kinetic control of the polymerization:
Dilution
The rate of chain termination reactions is given by:
Kstop therein the velocity coefficient and [P *] is the concentration of polymer radicals.
The concentration of radicals P * flows square in the rate of termination reactions. The further lowering the concentration of these radicals, the less likely are breaks in the chain.
By dilution with much solvent can achieve this effect, however, this will reduce the rate of reaction of the desired reactions. In addition, it falls to much dirty solvent. Accordingly, the diluent should not be too large.
" Radical buffer "
Another way to reduce [P *] is the use of special reagents which are capable of forming free radicals and radicals absorb again.
These reagents are added as a dormant species in non- radical form of the polymerization mixture. Part of the dormant species decays directly and forms a free radical active species.
The active species can be converted into the dormant species by absorbing polymer radicals again, so that an equilibrium is formed of several reactions, which are released or absorbed radicals.
This has the result that a constant concentration of [P *] is adjusted:
- Are produced in the polymerization too many radicals P *, these are captured by the active species of the Spezialreagenzes.
- Formed in the polymerization too much sleeping species, decays again a part of it, so that [P *] is more or less constant.
This equilibrium concentration of P * can be specifically controlled by the amount of added Spezialreagentien.
Most important method
The special reagents can be very different, which are most effective and therefore the most common:
- Stable Free Radical Polymerization ( SFRP )
- Reversible Addition Fragmentation Chain Transfer Polymerization ( RAFT)
Applications
The CFRP is explored only in recent years, but it is being used increasingly in industrial / industrial area - mainly because it must take place no extensive mechanical changes.
It enables:
- Selective preparation of catalysts and catalyst support surfaces
- Targeted control and functionalization of polymer properties for medicine, engineering and plastics in household use.