Special Types of Controlled/Living Polymerizations

It was reported that it is possible to employ persistent phosphorus-based radicals in controlled/living free-radical polymerization [283, 284]. Also, in cases of low stability of the hyper coordinated radicals, the ligand exchanges become facile and some organoaluminium, organoboron, and other compounds have been used successfully as transfer agents in polymerization of styrene, acrylics, and vinyl acetate . Chung and coworkers described a "living" radical initiator that is based on oxidation adducts of alkyl-9-borabicyclononanes, such as hexyl-9-borabicyclononane. The "living" radical polymerizations take place at room temperature. The initiators form in situ by reactions with oxygen:

The alkoxy radicals are very reactive and initiate radical polymerizations readily On the other hand, the borinate radicals are stabilized by the empty p-orbitals of boron through back-donating electron density and are too stable to initiate polymerizations. During the polymerization, the borinate radicals may form weak and reversible bonds with the growing chains.

Boroxyl mediated living radical polymerizations were subsequently described by Chung in a review article. The chemistry is centered on the living radical initiator, i.e., alkylperoxydialkylborane (C-O-O-BR) species, similar to the one shown above, that are formed by mono oxidation of an asymmetric trialkylborane with oxygen. In the presence of polar monomers (including acrylates and methacrylates), the C-O-O-BR., undergoes a spontaneous hemolytic cleavage at ambient tem- perature to form an active alkoxy radical and a stable boroxyl radical The alkoxyl radical is active in initiating the polymerization of vinyl monomers, and the stable boroxyl radical forms a reversible bond with the propagating radical site to prevent undesirable termination reactions. The living polymerizations were characterized by predictable polymer molecular weight, narrow molecular weight distributions, and by formation of telechelic polymers and block copolymers through sequen- tial monomer addition. Furthermore, this living radical initiator system benefits from two unique features of trialkyl borane. These are (a) easy incorporation into polymer chains (chain ends or side chains) and (b) in situ auto-transformation to living radical initiators.

Lacroix and coworkers reported a reverse iodine transfer polymerization (RITP), where elemental iodine is used as a control agent in living radical polymerization [288]. Styrene, butyl acrylate, methyl acrylate, and butyl a-fluoroacrylate were homopolymerized, using a radical catalyst and I₂ as a chain transfer agent. Methyl acrylate was also copolymerized with vinylidene chloride using this process.

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مواضيع ذات صلة

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

Controlled/“Living” Free-Radical Polymerization

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