The organometallic mediated radical polymerization (OMRP) of methyl methacrylate (MMA), styrene and vinyl acetate, mediated by a novel tert-butyl substituted amine−bis(phenolate) iron(II) complex in the absence of a halide source, accesses an organometallic-only route to controlled radical polymerization. Using a low temperature radical initiator, V-70, detailed kinetic and end group studies were used to further understand the mechanism of control, and the relative rates of propagation and termination reactions. For the polymerization of MMA, propagation is favored at low conversions, with good control and reasonable dispersities achieved. Mechanistic studies suggest propagation proceeds through a RT-OMRP mechanism, while termination reactions become dominant at higher conversions. The polymerization temperature greatly affects the nature of termination, tuning whether bimolecular termination or catalytic chain transfer (CCT) dominates. With careful control of reaction conditions, the polymerization of styrene also shows good control, with dispersities as low as 1.27, and while not comparable to ATRP conditions, represents the most effective iron-mediated OMRP of styrene to date.

Radical initiation of methyl methacrylate polymerizations mediated by titanium amino-phenolate complexes gave exceptional control without the need for any co-catalyst or activator. The polymerization proceeds not by a classical controlled radical or coordination insertion mechanism but via a unique bimetallic group transfer process. Detailed experimental and computational studies support a polymerization mechanism with a titanium(IV)-enolate complex and a second titanium(III) complex that delivers an activated monomer, and provides new insight into both titanium-mediated radical reactions and polymerizations. Titanium is a biocompatible, abundant, inexpensive metal, and thus is attractive as a catalyst in organic synthesis, 1 building from its long history in polymer chemistry, particularly in the coordination insertion polymerization of alkenes. 2-9 In particular, with two well-defined and accessible oxidation states (III and IV), the use of discrete complexes of titanium in CC bond formation via single electron transfer processes is increasingly popular. 10-12 The use of titanium complexes in controlled radical polymerization (CRP), where functional monomers are polymerized via a radical mechanism to predictable molecular weights and narrow dispersities (Đ), has received little attention, especially in comparison with other first-row transition metals. 13-16 While Asandei and others used titanium metallocenes (Scheme 1a) 17-26 and nonmetallocenes 27-29 as mediators of radical polymerisations of styrenes and (meth)acrylates, 18,30-32 these methods had uncertain mechanisms and required expensive, pyrophoric activators (Scheme 1b). Building from our long-standing interest in sustainable, non-toxic mediators of controlled radical polymerizations, 33-37 we sought to better design and indeed better understand these systems. We now show that a conventional azo-initiator, in the presence of novel titanium amine-phenolate complexes, can be used not to initiate a radical polymerization but rather to establish an unprecedented Ti(III)/Ti(IV) polymerization mechanism (Scheme 1c). The reaction is facile to conduct and offers

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