Evaluate advantages and disadvantages iodine transfer radical polymerization technique in comparison with other methods of controlled radical polymerization. Give example.
Iodine-transfer polymerizationgives relatively low polydispersities for fluoroolefin polymers.
The mechanism of ITP involves thermal decomposition of the radical initiator (AIBN), generating the initiating radical In•. This radical adds to the monomer M to form the species P1•, which can propagate to Pm•. By exchange of iodine from the transfer agent R-I to the propagating radical Pm• a new radical R• is formed and Pm• becomes dormant. This species can propagate with monomer M to Pn•. During the polymerization exchange between the different polymer chains and the transfer agent occurs, which is typical for a degenerative transfer process.
Typically, iodine transfer polymerization uses a mono- or diiodo-perfluoroalkane as the initial chain transfer agent. This fluoroalkane may be partially substituted with hydrogen or chlorine. The energy of the iodine-perfluoroalkane bond is low and, in contrast to iodo-hydrocarbon bonds, its polarization small. Therefore, the iodine is easily abstracted in the presence of free radicals. 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. But the iodine-terminated poly(fluoroolefin) itself acts as a chain transfer agent. 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.
Use of conventional hydrocarbon monomers with iodoperfluoroalkane chain transfer agents has been described. The resulting molecular weight distributions have not been narrow since the energetics of an iodine-hydrocarbon bond are considerably different from that of an iodine-fluorocarbon bond and abstraction of the iodine from the terminated polymer difficult. The use of hydrocarbon iodides has also been described, but again the resulting molecular weight distributions were not narrow.
Although use of living free radical processes in emulsion polymerization has been characterized as difficult, all examples of iodine-transfer polymerization have involved emulsion polymerization. Extremely high molecular weights have been claimed.
Evaluate advantages and disadvantages of reversible addition-fragmentation chain transfer process in comparison with other methods of controlled radical polymerization. Give example.
The development of addition–fragmentation chain transfer agents and related ring-opening monomers highlighting recent innovation in these areas. The major part of this review deals with reagents that give reversible addition–fragmentation chain transfer (RAFT). These reagents include dithioesters, trithiocarbonates, dithiocarbamates and xanthates. The RAFT process is a versatile method for conferring living characteristics on radical polymerizations providing unprecedented control over molecular weight, molecular weight distribution, composition and architecture. It is suitable for most monomers polymerizable by radical polymerization and is robust under a wide range of reaction conditions. It provides a route to functional polymers, cyclopolymers, gradient copolymers, block polymers and star polymers.
|
|
|
Addition–fragmentation chain transfer
Addition–fragmentation transfer agents and mechanisms whereby these reagents provide addition–fragmentation chain transfer during polymerization are shown in Scheme 1. Unsaturated compounds of general structure 1 or 4 can act as transfer agents by a two-step addition–fragmentation mechanism. In these compounds C 
X should be a double bond that is reactive towards radical addition. X is most often CH2or S. Z is a group chosen to give the transfer agent an appropriate reactivity towards propagating radicals and convey appropriate stability to the intermediate radicals (2 or 5, respectively). Examples of A are CH2, CH2 CHCH2, O or S. R is a homolytic leaving group and R 
should be capable of efficiently reinitiating polymerization. In all known examples of transfer agents 4, B is O. Since functionality can be introduced to the products 3 or 6 in either or both the transfer (typically from Z) and reinitiation (from R) steps, these reagents offer a route to a variety of end-functional polymers including telechelics.
Дата добавления: 2018-02-15; просмотров: 957; Мы поможем в написании вашей работы! |
Мы поможем в написании ваших работ!