From Stereochemically Tunable Homopolymers to Stereomultiblock Copolymers: Lewis Base Regulates Stereochemistry in the Coordination Polymerization of 2-Vinylpyridine

Enhanced Control of Isoprene Polymerization with Trialkyl Rare Earth Metal Complexes through Neutral Donor Support

The development of catalysts for stereospecific polymerization of 1,3-dienes is an area of interest due to the robust nature of poly(1,3-diene)s' physical and mechanical properties, as well as the material's versatility in many applications. Dialkyl rare earth metal complexes supported by a diverse cast of ligand frameworks are selective for the polymerization of 1,3-dienes and are an exciting option for examination. However, development in this area has been hampered by the focus on complex catalyst systems that are costly to make. In this study, we synthesize a series of simple homoleptic trialkyl rare earth metal precatalysts and highlight their efficacy for isoprene polymerization using 1 or 2 equiv of [Ph3C][B(C6F5)4] activator. We investigated the addition of commercially available in situ donors, leading to the identification of triphenylphosphine as an ideal support to enhance the dispersity control and prevent loss of catalyst activity. We demonstrated how the activation and reaction conditions, including the order/time of reagent addition and donor electronics, had a major impact on the rate, control, and selectivity for the polymerization of 1,3-dienes. Further interrogation of the catalyst system signals the crucial role of triphenylphosphine in providing enhanced stability and control in this living catalyst system.

Unveiling the Detailed Mechanism and Origins of Chemo-, Regio-, and Stereoselectivity of Rare-Earth Catalyzed Alternating Copolymerization of Polar and Nonpolar Olefins

The direct copolymerization of polar and nonpolar olefins is of great interest and significance, as it is the most atom-economical and straightforward strategy for the synthesis of functional polyolefin materials. Despite considerable efforts, the precise control of monomer-sequence and their regio- and stereochemistry is full of challenges, and the related mechanistic origins are still in their infancy to date. Herein, the mechanistic studies on the model reaction of Sc-catalyzed co-syndiospecific alternating copolymerization of anisylpropylene (AP) and styrene were performed by DFT calculations. The results suggest that the subtle balance between electronic and steric factors plays an important role during monomer insertions, and a new amino-dissociated mechanism was proposed for AP insertion at chain initiation. AP insertion follows the 2,1-si-insertion pattern, which is mainly controlled by steric factors caused by the restricted MeO···Sc interaction. As for styrene insertion, it prefers the 2,1-re-insertion manner and its regio- and stereoselectivities are influenced by steric repulsions between the inserting styrene and the polymer chain or the ligand. More interestingly, it is found that the alternating monomer-sequence is mainly determined by the "steric matching" principle, which is quantitatively expressed by the buried volume of the metal center of the preinserted species. The concept of steric pocket has been successfully applied to explain the different performances of several catalysts and other alternating copolymerization reactions. The insightful mechanistic findings and the quantitative steric pocket model present here are expected to promote rational design of new rare-earth catalysts for developing regio-, stereo-, and sequence-controlled copolymerization of specific polar and nonpolar olefins.

Rare Earth Starting Materials and Methodologies for Synthetic Chemistry

The number of rare earth (RE) starting materials used in synthesis is staggering, ranging from simple binary metal-halide salts to borohydrides and "designer reagents" such as alkyl and organoaluminate complexes. This review collates the most important starting materials used in RE synthetic chemistry, including essential information on their preparations and uses in modern synthetic methodologies. The review is divided by starting material category and supporting ligands (i.e., metals as synthetic precursors, halides, borohydrides, nitrogen donors, oxygen donors, triflates, and organometallic reagents), and in each section relevant synthetic methodologies and applications are discussed.

Crystalline sulfur-functionalized poly(α-olefin) synthesized by scandium-catalyzed coordination polymerization

Combined with organoborate, the scandium complex catalyzed monomers to produce a series of syndiotactic sulfur-functionalized poly(α-olefin)s (rrrr of up to 0.95) which formed crystalline materials with a Tm of up to 91 °C.

Yttrium tris(trimethylsilylmethyl) complexes grafted onto MCM-48 mesoporous silica nanoparticles

A series of tris(trimethylsilylmethyl) yttrium donor adduct complexes was synthesized and fully characterized by X-ray diffraction, ¹H/¹³C/²⁹Si/³¹P/⁸⁹Y heteronuclear NMR and FTIR spectroscopies as well as elemental analyses. Treatment of Y(CH₂SiMe₃)₃(thf)_x with various donors Do led to complete (Do = TMEDA, DMAP) and partial displacement of THF (Do = NHC^iPr, DMPE). Exceptionally large ⁸⁹Y NMR shifts to low field were observed for the new complexes. Complexes Y(CH₂SiMe₃)₃(tmeda) and Y(CH₂SiMe₃)₃(dmpe)(thf) were chosen to perform surface organometallic chemistry, due to a comparatively higher thermal stability and the availability of the ³¹P nucleus as a spectroscopic probe, respectively. Mesoporous nanoparticles of the MCM-48-type were synthesized and used as a 3^rd generation silica support. The parent and hybrid materials were characterized using X-ray powder diffraction, solid-state-NMR spectroscopy, DRIFTS, elemental analyses, N₂-physisorption, and scanning electron microscopy (SEM). The presence of surface-bound yttrium alkyl moieties was further proven by the reaction with carbon dioxide. Quantification of the surface silanol population by means of HN(SiHMe₂)₂-promoted surface silylation is shown to be superior to titration with lithium alkyl LiCH₂SiMe₃.

From isoselectivity to syndioselectivity: Lewis base regulates stereochemistry in 2-vinylpyridine polymerization

We have developed an effective synthesis of syndiotactic poly(2-vinylpyridine) by a novel catalytic system. Switching from isoselective to syndioselective polymerization was achieved by adding Lewis base.

Regioselective, stereoselective, and living polymerization of divinyl pyridine monomers using rare earth catalysts

The first regioselective, stereoselective, and living polymerization of divinyl pyridine monomers, mediated by simple rare earth catalysts, is reported.

Induced Circular Dichroism of Isotactic Poly(2-vinylpyridine) with Diverse and Tunable "Sergeants-and-Soldiers" Type Chiral Amplification

Facile and efficient construction of the helical structure with diverse and tunable chiral amplification from an achiral polymer is attractive but remains a challenge to develop multiple functional materials. We report herein a macromolecular acid-base complex of highly isotactic poly(2-vinylpyridine) (mmmm > 99%), (+)-camphorsulfonic acid, and dodecylbenzensulfonic acid. The asymmetric induction of the chiral additive through the ionic interactions between pyridinium pendants and acid ions drives the polymer backbone to twist in a preferred direction in CHCl₃ and its mixture with CH₃CN. The sign and intensity of induced circular dichroism rely on the base to acid ratio, the chiral acid content, and the solvent nature. By systematically tuning the solvent composition, four distinct types of "sergeants-and-soldiers" mode chiral amplification are achieved for the first time within a single system owing to the solvent dependent bias of chiral-chiral and chiral-achiral ion pairs.

“C–H⋯π Interaction” regulates the stereoselectivity in olefin polymerization

DFT calculations reveal that a coordinating THF plays an essential role in regulating the stereoselectivity via C–H⋯π noncovalent interaction.

Origin of stereoselectivity and multidimensional quantitative analysis of ligand effects on yttrium-catalysed polymerization of 2-vinylpyridine

Through a combination of density functional theory (DFT) calculations and multivariate regression analysis, the origin of the stereoselectivity of yttrium-catalysed polymerization of 2-vinylpyridine (2VP) has been investigated.

Controlled iso-specific polymerization of 2-vinylpyridine catalyzed by arylamide-ligated rare-earth metal aminobenzyl complexes

Arylamide rare-earth metal aminobenzyl complexes were employed as highly active catalysts for iso-specific polymerization of 2-vinylpyridine.

Perfectly isoselective polymerization of 2-vinylpyridine promoted by β-diketiminato rare-earth metal cationic complexes

Highly isotactic poly(2-vinylpyridine)s were produced with rare-earth metal complexes supported by a symmetric β-diketiminate ligand in the presence of borate.