Lewis acid-mediated radical polymerization

Mechanistic Perspectives on Stereocontrol in Lewis Acid-Mediated Radical Polymerization Lessons from Small-Molecule Synthesis... 

Scheme 5 The necessary requirements for effective isotactic control in Lewis acid-mediated radical polymerization.

Benjamin Noble and Michelle Coote s chapter on stereocontrol in Lewis acid-mediated radical polymerization brings much-needed order into the important area of polymer synthesis the lack of tacticity control is now the greatest obstacle to the synthesis of precisely controlled macromolecules by versatile and efficient radical-based processes. They bring mechanistic insights from small-molecule synthesis to bear on the problem, as they outline and assess some of the key factors likely to undeilie the success or failure of Lewis acid-mediated isotactic control in radical polymerization. 

Lewis acids 436 metal complex-mediated radical polymerization 484-6 molecular weight distributions 251,453-4, 458-60,490-1.499-501 molecular weight conversion dependence 452-3,455... 

Although copper reagents, hahdes and triflates, are widely used in atom-transfer polymerization reactions (ATRP) [63], these processes do not fall under the category of Lewis acid-mediated reactions. Sherrington and co-workers have shown that a vinyl monomer coordinated to a chiral copper Lewis acid (122) undergoes stereoselective polymerization (Sch. 29) [64]. A chiral block-copolymer 124 was prepared under radical conditions. 

Further discussion on the effects of the reaction media and Lewis acids on lacticily appears in Section 7.2. Attempts to control laciicily by template polymerization and by enzyme mediated polymerization are described in Section 7.3. Devising effective means for achieving stereochemical control over propagation in radical polymerization remains an important challenge in the field. 

Simultaneous control of stereosequence and molecular weight distribution has long been one of the holy grails in the field of radical polymerization. Nitroxide mediated polymerization (NMP), atom transfer radical polymerization (ATRP) and RAFT all offer control over molecular weight distribution. However, polymers produced by these methods show similar tacticity to those obtained by the conventional process. Recently there have been reports of tacticity control of homopolymers " (which enables the synthesis of stereoblock copolymers ) and control of the alternating tendency for copolymerizations in ATRP or RAFT polymerization through the use of Lewis acids as additives. 

In general, the compounds of the Group 4 metals, such as halides and alkoxides, are well known as Lewis acids to catalyze two-electron electrophilic reactions, and their metallocenes coupled with alkylation and/or reduction agents were effective catalysts for the coordination polymerization of olefins. For the transition metal-catalyzed radical polymerization, their alkoxides, such as Ti(Oi-Pr)4, have also been employed as an additive for a better control of the products. Contrary to the common belief that the Group 4 metals rarely undergo a one-electron redox reaction under mild conditions, there have been some reports on the controlled radical polymerization catalyzed or mediated by titanium complexes, although the conflict in the mechanism between the (reverse) ATRP and OMRP is also the case with the Group 4 metal complexes. 

Not only the highly Lewis acidic early transition metal-based polymerization catalysts suffer from poisoning by coordination of functional groups. Even in late transition metal-based complexes, the possible o-coordination in certain functional groups has a negative impact on polymerization reactions. The prominent example here is the still ongoing search for active acrylonitrile (AN) copolymerization catalysts. This reaction can serve as an ideal example to illustrate the challenges in late transition metal-catalyzed insertion polymerizations with polar functionalized comonomers. The metal-mediated copolymerization of AN has numerous appearances in literature however, in most cases, the reaction mechanism seems to be of ionic or radical nature. 

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