Synthetic
polymer chemistry endeavors to imitate the spatial and
temporal control exhibited within biological systems to obtain well-defined
polymeric materials with unique structures, properties, and applications.
This is often approached through the development of dynamic catalyst
(or initiator) systems that use external stimuli to elicit discrete,
site-specific transformations that impact the polymerization. Herein
we highlight developments in polymerizations that are modulated by
external stimuli, with particular focus on those systems that enable
notable changes in kinetics, monomer selectivity, polymer architecture,
or tacticity. Examples of external stimuli include chemical oxidants
or reductants, light, applied voltage, and mechanical force.
A study designed to ascertain the impact that ligand symmetry, number of redox-active moieties, and identity of the active metal center have on the catalytic ring-opening polymerization performance of redox-switchable catalysts.
Milling two equivalents of K[1,3-(SiMe 3 ) 2 C 3 H 3 ] (= K[A']) with MgX 2 (X = Cl, Br) produces the allyl complex [K 2 MgA' 4 ](1). Crystals grown from toluene are of the solvated species [((h 6 -tol)K) 2 MgA' 4 ]([1·2(tol)]), atrimetallic monomer with both bridging and terminal (h 1 )a llyl ligands.W hen recrystallized from hexanes,t he unsolvated 1 forms a2 D coordination polymer,inwhich the Mg is surrounded by three allyl ligands.T he CÀCb ond lengths differ by only 0.028 , indicating virtually complete electron delocalization. This is an unprecedented coordination mode for an allyl ligand bound to Mg. DFT calculations indicate that in isolation, an h 3 -allyl configuration on Mg is energetically preferred over the h 1 -(sbonded) arrangement, but the Mg must be in al ow coordination environment for it to be experimentally realized. Methyl methacrylate is effectively polymerizedby1,with activities that are comparable to K[A']a nd greater than the homometallic magnesium complex [{MgA' 2 } 2 ].
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