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 ].
The ability to control polyethylene branching density is of great interest as a means by which a polymer’s thermomechanical properties may be tailored. One particularly interesting way in which this can be achieved is by altering the electronic characteristics of Pd- and Ni-based α-diimine catalysts through the inclusion of electron-withdrawing or electron-donating substituents onto the ligand scaffold; however, a few critical fundamental studies are absent from the literature. These include a systematic examination of electronic perturbations of Ni-based α-diimine catalysts, as well as how placement of donating or withdrawing substituents on the backbone versus N-aryl moieties of the α-diimine ligand framework impact polymer topology. In addition, no method currently exists by which the polymer topology may be predicted based on an intrinsic characteristic of the (pre)catalyst or ligand without requiring extensive polymerization studies. Herein, we use both experimental and computational methods to understand how the placement of electron-donating or electron-withdrawing substituents on Ni α-diimine catalysts affects PE branching density, and compare those results to the analogous unsubstituted catalyst. We will show that inclusion of electron-withdrawing substituents decreases resultant PE branching density, whereas electron-donating substituents exhibit little to no change in PE branching density. Finally, we will show that as the placement and identity of donating or withdrawing substituents are varied, so too is the redox half-wave potential (E1/2) of the precatalysts, which can be used to generate a predictive curve by which PE branching density may be estimated for other substituted Ni-based α-diimine catalysts without the need for extensive polymerization studies.
A polymer's properties and functionality are directly related to the constituent monomers from which it was synthesized, the order in which these monomers are assembled, and the degree to which monomers are enchained. Furthermore, a standing challenge in the field of polymer synthesis is to provide temporal polymerization control that can be leveraged to access a variety of advanced polymer architectures. Though many polymer classes are attractive for various applications, polyesters have drawn considerable recent interest due to the potential of these materials to provide biodegradable alternatives to other, often petroleum derived, polymeric materials that create concerning, long-term environmental impacts. Many of these biodegradable polyesters can be produced via the transition-metal catalyzed ring-opening polymerization of cyclic ester and cyclic ether monomers. Through researchers' quest to access precise and well-defined polyesters via ring-opening polymerization, an intriguing class of stimuli-responsive catalysts have emerged. More specifically, catalyst systems have been developed in which their electronic nature may be modulated via either ligand-based or active metal site-based redox-switchability. These redox-switchable catalysts have been shown to exhibit altered chemoselectivity and kinetic modulation as a function of catalyst redox-state. Herein, we will discuss the beginnings, select recent advancements, and an outlook on the field of redox-switchable ring-opening polymerizations.
Without solvents present, the often far-fromequilibrium environment in a mechanochemically driven synthesis can generate high-energy, non-stoichiometric products not observed from the same ratio of reagents used in solution. Ball milling 2 equiv.with CaI 2 yields a non-stoichiometric calciate, K[CaA' 3 ], which initially forms a structure (1) likely containing a mixture of piand sigma-bound allyl ligands. Dissolved in arenes, the compound rearranges over the course of several days to a structure (2) with only η 3 -bound allyl ligands, and that can be crystallized as a coordination polymer. If dissolved in alkanes, however, the rearrangement of 1 to 2 occurs within minutes. The structures of 1 and 2 have been modeled with DFT calculations, and 2 initiates the anionic polymerization of methyl methacrylate and isoprene; for the latter, under the mildest conditions yet reported for a heavy Group 2 species (one-atm pressure and room temperature).
Milling two equivalents of K[1,3‐(SiMe3)2C3H3] (=K[A′]) with MgX2 (X=Cl, Br) produces the allyl complex [K2MgA′4] (1). Crystals grown from toluene are of the solvated species [((η6‐tol)K)2MgA′4] ([1⋅2(tol)]), a trimetallic monomer with both bridging and terminal (η1) allyl ligands. When recrystallized from hexanes, the unsolvated 1 forms a 2D coordination polymer, in which the Mg is surrounded by three allyl ligands. The C−C bond lengths differ by only 0.028 Å, 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 η3‐allyl configuration on Mg is energetically preferred over the η1‐ (σ‐bonded) arrangement, but the Mg must be in a low coordination environment for it to be experimentally realized. Methyl methacrylate is effectively polymerized by 1, with activities that are comparable to K[A′] and greater than the homometallic magnesium complex [{MgA′2}2].
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