Controlled/Living Free-Radical Polymerization

Living polymerizations are chain-growth reactions where the propagating centers on the growing chains do not terminate and do not undergo chain transfer. Such polymerizations are noted for preparations of polymers with controlled molecular weights, desired end groups and low polydispersities. In addition, the preparations of polymers with predetermined molecular weights and narrow molecular weight distributions require fast initiations and fast exchanges between sites of variable activities and variable lifetimes. Such chain-growth reactions, ionic in nature, are discussed in Chap. 4. In typical homogeneous free radical polymerizations, however, bimolecular terminations between two growing radicals cannot be avoided and, therefore, typical living free radical polymerization cannot be fully realized. Also, in conventional free radical polymerizations, the initiations are slow, while high-molecular-weight polymers form shortly after the start of the reactions. As the reactions progress, polydispersities increase, while the molecular weights actually decrease. It is possible, however, to adjust conditions of some radical polymerizations in such a way that polymers with controlled molecular weights and relatively low polydispersities form [220, 221]. These are not true living polymerization as such because termination reactions do occur. They possess, however, some characteristics that are similar to living polymerizations and are referred to by many as controlled/“living” polymerizations. Such reactions yield polymers with controlled molecular weights, exhibit increase in molecular weight with conversion, yield narrow molecular weight distributions, and can be used to form copolymers. Some examples of such polymerizations include nitroxyl radical-mediated polymerizations of styrene [222–225], atom transfer polymerizations controlled by ruthenium-(II)/aluminum [226, 227] or by copper/bipyridine complexes [228], Co (II)-mediated polymerizations of methacrylates and acrylates [229], and polymerization of styrene using a degenerative transfer method [230], as well as others. Some features are unusual for radical processes and the radical nature of some of these reactions might be questioned, as for instance, polymerizations catalyzed by transition metals. Evidence has been presented, however, that strongly indicates radical nature in at least in atom transfer polymerizations [231]. The evidence, however, is not unambiguous.

Some initial attempts at producing “living” polymerizations made use of iniferters. This term appears to come from the word inifer, a bifunctional compound that brings about both initiation and chain transfer. “Living” cationic polymerizations make use of inifers to form block copolymers. This is discussed in Chap. 9. The term iniferter was proposed by Otsu and Yoshida in 1982 [232]. Iniferters used in controlled/living free-radical polymerizations are sulfur-centered free radicals that can be generated from sulfur-containing molecules such as dithiocarbamates. The radicals react reversibly with growing polymeric chain ends, thereby controlling the concentration of the radical species. Many of these sulfur centered radicals, however, can also initiate new polymer chains. This can lead to uncontrolled growth. To overcome these difficulties, other approaches were also developed [233]. Deactivation of growing radicals with stable radicals can be carried out with the aid of various nitroxyl radicals, protected phenoxy radicals, dithiocarbamate, trityl, and benzhydryl derivatives. Growing radicals can also be deactivated with nonradicals in the presence of organometallic compounds that form stabilized hyper-coordinated radicals. The polymerizations with the aid of reversible degradative chain transferring are unique in that they requires very rapid and “clean” chain transfers without side reactions. The enhanced control of polymerization process relies on reduction in the ratio of the rate of termination to that of propagation, due to low instantaneous concentration of growing radicals. This means that initiation and propagation reactions must proceed at similar rates due to application of the initiators resembling polymer end groups in their dormant state. Also, in these polymerization reactions, there must be a low proportion of chains marked by uncontrolled termination and/or transfer due to relatively low molecular weights. Homogenous controlled/“living” free radical polymerizations are based, therefore, on the reversible deactivations of growing radicals. Early, Matyjaszewski divided such polymerizations into three classes [240, 241]. These were:

1. Deactivations of growing radicals with stable radicals by reversible formations of dormant covalent species, followed by homolytic cleavages:

1. Reversible deactivations of growing radicals with “nonradical” species by formation of dormant persistent radicals:

1. Reversible degenerative transfers based on thermodynamically neutral exchange reactions between growing radicals and transfer agents:

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مواضيع ذات صلة

Special Types of Controlled/Living Polymerizations

Combinations of Click Chemistry and ATP as Well as ATP and RAFT Polymerizations

Reversible Addition-Fragmentation Chain Transfer Polymerization

Nitroxide-Mediated Radical Polymerizations

Atom Transfer Radical Polymerizations

Steric Control in Free-Radical Polymerization

The rmal Polymerization

Inhibition and Retardation

The Termination Reaction

Ring Forming Polymerization

Polymerization of Monomers with Multiple Double Bonds

Methods for Measuring Molecular Weights of Polymers