Reversible-deactivation radical polymerization
RDRP – sometimes misleadingly called 'free' radical polymerization – is one of the most widely used polymerization processes since it can be applied The steady-state concentration of the growing polymer chains is 10−7 M by order of magnitude, and the average life time of an individual polymer radical before termination is about 5–10 s. A drawback of the conventional radical polymerization is the limited control of chain architecture, molecular weight distribution, and composition.
[4] [5] This had the effect of prolonging the lifetime of the growing polymer chains (see above) to values comparable with the duration of the experiment.
[1] Some important aspects of these are compared in the table: As the name suggests, the prerequisite of a successful RDRP is fast and reversible activation/deactivation of propagating chains.
In any RDRP processes, the radicals can propagate with the rate coefficient kp by addition of a few monomer units before the deactivation reaction occurs to regenerate the dormant species.
[7] The overall rate coefficient of chain breaking reactions besides the direct termination between two radicals is represented as ktx.
In all RDRP methods, the theoretical number average molecular weight of obtained polymers, Mn, can be defined by following equation:
A well controlled RDRP process requires: 1) the reversible deactivation process should be sufficiently fast; 2) the chain breaking reactions which cause the loss of chain end functionalities should be limited; 3) properly maintained radical concentration; 4) the initiator should have proper activity.
This method is very versatile but requires unconventional initiator systems that are sometimes poorly compatible with the polymerization media.
Given certain conditions a homolytic splitting of the C-O bond in alkoxylamines can occur and a stable 2-centre 3 electron N-O radical can be formed that is able to initiate a polymerization reaction.
Discovered in the late 1970s in the USSR it was found that cobalt porphyrins were able to reduce the molecular weight during polymerization of methacrylates.
Later investigations showed that the cobalt glyoxime complexes were as effective as the porphyrin catalysts and also less oxygen sensitive.
[8][9] Iodine-transfer polymerization (ITP, also called ITRP), developed by Tatemoto and coworkers in the 1970s[10] gives relatively low polydispersities for fluoroolefin polymers.
While it has received relatively little academic attention, this chemistry has served as the basis for several industrial patents and products and may be the most commercially successful form of living free radical polymerization.
Upon encountering an iodoperfluoroalkane, a growing poly(fluoroolefin) chain will abstract the iodine and terminate, leaving the now-created perfluoroalkyl radical to add further monomer.
As in RAFT processes, as long as the rate of initiation is kept low, the net result is the formation of a monodisperse molecular weight distribution.
Diphenyl diselenide and several benzylic selenides have been explored by Kwon et al. as photoiniferters in polymerization of styrene and methyl methacrylate.
However, their low transfer constants allow them to be used for block copolymer synthesis but give limited control over the molecular weight distribution.
[18] Telluride-mediated polymerization or TERP first appeared to mainly operate under a reversible chain transfer mechanism by homolytic substitution under thermal initiation.
The importance of X to chain transfer increases in the series O [20][21] Yamago has also published a patent indicating that bismuth alkyls can also control radical polymerizations via a similar mechanism.
