Ligands Dominate Highly Syndioselective Polymerization of Styrene by Using Constrained-geometry-configuration Rare-earth Metal Precursors

Theoretical investigations of 2-vinylpyridine stereoselective polymerization catalyzed by cationic yttrium complexes with different ancillary ligands

The polymerization mechanism of 2-vinylpyridine catalyzed by cationic yttrium complexes with diverse ancillary ligands, specifically [L1Y(CH2SiMe3)(THF)]+ [L1 = (2,6-Et2C6H3)NC(Me)CHC(Me)N(2,6-Et2C6H3)] (Y-1), [L2Y(CH2SiMe3)(THF)]+ [L2 = (2,6-Cl2C6H3)NC(Me)CHC(Me)N(2,6-Cl2C6H3)] (Y-2), and [L3Y(CH2SiMe3)(THF)]+ [L3 = (2,6-C6H5)NC(Me)CHC(Me)N(2,6-iPr2C6H3)] (Y-3), was studied using density functional theory (DFT) calculations. Having achieved an agreement between theory and experiment, it is found that isotactic selectivity induced by Y-1 or Y-2 results from a combination of smaller deformation of the catalyst and stronger electronic effects. Conversely, the Y-3 complex exhibits comparable energy barriers for proceeding via either isotactic or syndiotactic pathways, aligning with the production of atactic polymers as seen experimentally. To examine the steric effects on the kinetic and thermodynamic properties, a computational model of an analogue complex [L4Y(CH2SiMe3)(THF)]+ [L4 = (2,6-Cl2C6H3)NC(Me)CHC(Me)N(iPr2C6H3)] (Y-4), featuring increased steric hindrance, was analyzed. Distortion-interaction and topographic steric map analyses further affirmed that steric hindrance significantly influences stereoselectivity. A direct relationship was identified between the energy barriers of isotactic insertion transition states and the bulkiness of ancillary ligands; greater distortion energy of the catalyst correlates with higher barriers for isotactic polymerization. These findings enhance the mechanistic comprehension of 2-vinylpyridine polymerization and are expected to contribute valuable insights for the improvement of catalytic polymerization systems of 2-vinylpyridine.

Nopadiene: A Pinene-Derived Cyclic Diene as a Styrene Substitute for Fully Biobased Thermoplastic Elastomers

The bicyclic 1,2-substituted, 1,3-diene monomer nopadiene (1R,5S)-2-ethenyl-6,6-dimethylbicyclo[3.1.1]hept-2-ene was successfully polymerized by anionic and catalytic polymerization. Nopadiene is produced either through a facile one-step synthesis from myrtenal via Wittig-olefination or via a scalable two-step reaction from nopol (10-hydroxymethylene-2-pinene). Both terpenoids originate from the renewable β-pinene. The living anionic polymerization of nopadiene in apolar and polar solvents at 25 °C using organolithium initiators resulted in homopolymers with well-controlled molar masses in the range of 5.6-103.4 kg·mol-1 (SEC, PS calibration) and low dispersities (Đ) between 1.06 and 1.18. By means of catalytic polymerization with Me4CpSi(Me)2NtBuTiCl2 and (Flu)(Pyr)CH2Lu(CH2TMS)2(THF), the 1,4 and 3,4- microstructures of nopadiene are accessible in excellent selectivity. In pronounced contrast to other 1,3-dienes, the rigid polymers of the sterically demanding nopadiene showed an elevated glass temperature, Tg,∞ = 160 °C (in the limit of very high molar mass, Mn). ABA triblock copolymers with a central polymyrcene block and myrcene content of 60-75 mol %, with molar masses of 100-200 kg/mol were prepared by living anionic polymerization of the pinene-derivable monomers nopadiene and myrcene. This diene copolymerization resulted in thermoplastic elastomers displaying nanophase separation at different molar ratios (DSC, SAXS) and an upper service temperature about 30 K higher than that for traditional petroleum-derived styrenic thermoplastic elastomers due to the high glass temperature of polynopadiene. The materials showed good thermal stability at elevated temperatures under nitrogen (TGA), promising tensile strength and ultimate elongation of up to 1600%.

Synthesis of block copolymer with cis-1,4-polybutadiene and isotactic-rich polystyrene using α-diimine nickel catalysts

A series of styrene-butadiene di-block copolymers with high cis-1,4 unit content (greater than 92%) polybutadiene (PB) and isotactic-rich polystyrene (PS) (mmmm > 65%) was synthesized using α-diimine nickel catalysts (Ni-diimine). Four different Ni-diimine catalysts were synthesized via a complexing reaction between nickel (II) naphthenate and laboratory-made α-diimine ligands L1, L2, L3 and L4, which have different steric volume structures. The results indicate that the Ni-diimine catalyst prepared using the L4 ligand with a higher steric volume can effectively initiate the block polymerization of butadiene and styrene, and the resulting polymer has distinguished cis-1,4 structure unit PB and high isotactic-selective PS block. Differential scanning calorimetry and electrochemical performance tests show that these block copolymers with cis-1,4-regulated and isotactic-selective polymerization have advantages in terms of high-temperature and low-temperature resistance as well as corrosion resistance. Therefore, these copolymers are expected to be widely used in some harsh industrial environments.

High-Efficiency Mono-Cyclopentadienyl Titanium and Rare-Earth Metal Catalysts for the Production of Syndiotactic Polystyrene

Syndiotactic polystyrene (SPS) refers to a type of thermoplastic material with phenyl substituents that are alternately chirally attached on both sides of an aliphatic macromolecular main chain. Owing to its excellent physical and mechanical properties, as well as its chemical stability, high transparency, and electrical insulation characteristics, SPS is used in a wide variety of technical fields. SPS is commonly produced via the stereoselective transition metal-catalyzed coordination polymerization method mediated by stereospecific catalysts, which consists of anionic mono-cyclopentadienyl derivative η5-coordinated single active metal centers (referred to as "mono-Cp'-M"), with active center metals involving Group 4 transition metals (with an emphasis on titanium) and rare-earth (RE) metals of the periodic table. In this context, the use of mono-cyclopentadienyl titanocene (mono-Cp'Ti) catalysts and mono-cyclopentadienyl rare-earth metal (mono-Cp'RE) metallocene catalysts for the syndiospecific polymerization of styrene is discussed. The effects of the mono-cyclopentadienyl ligand structure, cationic active metal types, and cocatalysts on the activity and syndiospecificity of mono-Cp' metallocene catalysts are briefly surveyed.

Cationic Half-Sandwich Tetrahydrofluorenyl Rare-Earth Metal Complexes as Stable Single-Site Catalysts for (Co)Polymerization of Styrene and Butadiene

As indenyl-derivates, the tetrahydrofluorenyl ligands had an expanded "wingspan" with considerable steric hindrance. In this text, the rare-earth metal complexes bearing tetrahydrofluorenyl ligands have been synthesized and fully characterized by NMR (¹H and ¹³C) and X-ray diffraction analyses. Upon the activation by [Ph₃C][B(C₆F₅)₄], all the scandium complexes exhibited excellent catalytic activity for highly syndioselective polymerization of styrene with a narrow molecular weight distribution (M_w/M_n < 2.0), suggesting the beneficial influence of tetrahydrofluorenyl ligands in stabilizing the single-site active species during the polymerization. Moreover, the scandium-based catalytic systems also promoted the 1,4-regular polymerization of butadiene and its copolymerization with styrene, affording diblock copolymers featuring a highly syndiotactic polystyrene block and a 1,4-specific PBD block. The kinetics investigation revealed the huge gap in TMS-Sc-catalyzed polymerization reactivity ratios (r_BD/r_St > 300) between butadiene and styrene, and this further proved the block structure of styrene-butadiene copolymers. The morphology and mechanical property of the selected diblock copolymer were, respectively, investigated by atomic force microscopy and stress-strain experiments.

Syndiospecific Copolymerization of Styrene with para-Methoxystyrene Catalyzed by Functionalized Fluorenyl-Ligated Rare-Earth Metal Complexes

The development of efficient catalysts for the copolymerization of nonpolar monomers with polar monomers remains a great challenging task in polymer synthesis. A one-pot reaction of anhydrous LnCl₃ with pyridyl-methylene-functionalized octamethylfluorenyl lithium OctFlu-CH₂PyLi in a 1:1 molar ratio, followed by alkylation with 2 equiv of LiCH₂SiMe₃ in THF afforded the fluorenyl-ligated rare-earth metal bis(alkyl) complexes (OctFlu-CH₂Py)Ln(CH₂SiMe₃)₂(THF) [Ln = Sc (1), Y (2)]. Both complexes were isolated as neutral species and were characterized by NMR spectrum and elemental analysis. Complex 2 was subjected to single-crystal X-ray diffraction, which showed that the whole modified fluorenyl ligand was coordinated to Y³⁺ in the η⁵/κ¹ mode to form a constrained geometry configuration. In the presence of excess AlⁱBu₃, and on activation with 1 equiv of [Ph₃C][B(C₆F₅)₄] in toluene, complexes 1 and 2 became active for both styrene (St) and para-methoxystyrene (pMOS) polymerization, giving polymers with high syndiotacticity (rrrr > 99%) without solvent extraction. Moreover, the ternary catalyst system composed of complex 2/AlⁱBu₃/[Ph₃C][B(C₆F₅)₄] was highly effective for the syndiospecific copolymerization of styrene with pMOS, producing random copolymers with high molecular weights and narrow molecular weight distributions. The contents of pMOS in the copolymers could be easily tuned in a wide range (11-93 mol %) by simply changing the pMOS-to-St feed ratios.

Scandium, titanium and vanadium complexes supported by PCP-type pincer ligands: synthesis, structure, and styrene polymerization activity

A series of first-row early transition metal dialkyl complexes bearing pincer ligands [(POCOP)M(CH₂SiMe₃)₂] (POCOP: (2,6-(^tBu₂PO)₂-C₆H₃); 1-Sc: M = Sc; 1-Ti: M = Ti; 1-V: M = V) and [(PCP)M(CH₂SiMe₃)₂] (PCP: (2,6-(^tBu₂PCH₂)₂-C₆H₃); 2-Sc: M = Sc; 2-Ti: M = Ti) have been synthesized. These dialkyl complexes were characterized by single-crystal X-ray diffraction, NMR spectroscopy, and solution magnetic susceptibility (Evans method) analyses appropriately. All the complexes exhibited square pyramidal geometries with different extents of distortion. The activities of these complexes were further explored in styrene polymerization, in which combinations of scandium complexes (1-Sc or 2-Sc) with [Ph₃C][B(C₆F₅)₄] were found to be active catalytic systems for highly syndiospecific (>99% rrrr) polymerization of styrene. Meanwhile, the Ti(III) complexes 1-Ti and 2-Ti showed rather low activity in styrene polymerization, which stands in sharp contrast to those in previous reports involving Ti(III) catalysts bearing cyclopentadienyl derivative ligands.

Synthesis, structure, and coordination chemistry of a neutral pyrrolyl-functionalized amidinate ligand

A new o-phenylene-bridged ligand incorporating neutral pyrrolyl and amidino moieties was designed and synthesized. The reactivity of the ligand was investigated and showed versatile coordination modes towards alkali- and rare-earth metals.

Access to Derivatizable Octahydrofluorenyl Ligand: Half-Sandwich Rare-Earth Metal Complexes for Highly Syndiospecific (Co)Polymerization of Styrene

A simple and facile synthetic pathway for accessing new derivatizable bulky-demanding octahydrofluorenyl (OHF) ligands has been developed, and a series of half-sandwich rare-earth metal (Sc, Y, Lu) complexes bearing the OHF ancillary ligands have been synthesized. In conjunction with a borate, the OHF-ligated Sc complexes exhibited high catalytic activity for styrene (co)polymerization to afford polymers with highly syndiotactic polystyrene sequence (>99% rrrr).

Chain Transfer to Toluene in Styrene Coordination Polymerization

The chain-transfer reaction is rather important in coordination polymerization regarding catalytic efficiency, adjustment of molecular weight, and control of chain structure. To date, chain transfer to H₂ and Al, Mg, and Zn alkyl compounds and β-H elimination are the commonly encountered modes. Now a novel chain transfer to toluene is reported. By introducing fluorine atoms into the β-diketimine ligands, an inert catalytic system for styrene (St) coordination polymerization was transferred into the highly active one. The activity increased with an increase in the number of fluorines in the ligands. Surprisingly, the molecular weights of resultant polystyrenes are very low (M_n =2000-6600 Da) despite of St loadings, corresponding up to 121 chains per active species. The mechanisms were investigated by DFT simulation, MALDI-TOF MS, isotope tracing experiment and 2D NMR spectrum analyses, which revealed that the fluorine activated the polymerization and directed chain transfer to toluene.

Syndio-and cis-1,4 dually selective copolymerization of polar fluorostyrene and butadiene using rare-earth metal catalysts

Synthesizing functional butadiene–styrene rubber through coordination polymerization is a theoretical challenge for polymer science, since functional monomers usually deactivate to the applied catalysts.

Half-sandwich rare-earth metal complexes bearing a C5Me4-C6H4-o-CH2NMe2 ligand: synthesis, characterization and catalytic properties for isoprene, 1-hexene and MMA polymerization

A new ortho-dimethylaminomethylphenyl-tetramethylcyclopentadienyl ligand C₅Me₄H-C₆H₄-o-CH₂NMe₂ (HL) and a series of rare-earth metal complexes bearing this ligand were synthesized. Of these complexes, two binuclear alkyl complexes [(C₅Me₄-C₆H₄-o-CH₂N(Me)CH₂-μ)Ln(CH₂SiMe₃)]₂ (Ln = Sc (1a) and Y (1b)) were obtained from the alkane elimination reaction of the free ligand with Ln(CH₂SiMe₃)₃(THF)₂, followed by an intramolecular C-H activation process of a NMe group in the ligand with a CH₂SiMe₃ group, two binuclear dichloro complexes (C₅Me₄-C₆H₄-o-CH₂NMe₂)₂Y₂Cl₄[LiCl(THF)₂] (2a) and [(C₅Me₄-C₆H₄-o-CH₂NMe₂)LuCl(μ-Cl)]₂ (2b) were synthesized by the reaction of anhydrous yttrium or lutetium trichloride with the lithium salt of the ligand LiL, and the binuclear bis(borohydrido) complexes [(C₅Me₄-C₆H₄-o-CH₂NMe₂)Ln(μ-BH₄)BH₄]₂ (Ln = Sm (3a) and Nd (3b)) were synthesized by the reaction of Ln(BH₄)₃(THF)₃ (Ln = Sm and Nd) with the lithium salt of the ligand. The molecular structures of all complexes 1a, 1b, 2a, 2b, 3a and 3b were determined by single-crystal X-ray crystallography. Upon activation with AlR₃/Ph₃CB(C₆F₅)₄, MAO or MMAO, the binuclear alkyl complexes 1a and 1b show good catalytic activity for isoprene cis-1,4 enriched regioselective polymerization and moderate catalytic activity for 1-hexene polymerization. Complexes 3a and 3b were studied as catalysts for methyl methacrylate polymerization reaction under different conditions and were found to show moderate to high catalytic activity.

Sequence-controlled ethylene/styrene copolymerization catalyzed by scandium complexes

Sequence controlled E/St copolymerization was achieved by tuning the electrophilicity of scandium center and the steric around it, isolating multi-block, random, “pseudo random” and alternating E/St copolymers.

Alkaline earth metal complexes stabilized by amidine and guanidine ligands: synthesis, structure and their catalytic activity towards polymerization of rac-lactide

A series of amidine ligands RAmDIPP (R is backbone substituent, R = 2-methylpyridine (HL1), N,N,2-trimethylaniline (HL2), N,N-dimethylpropan-1-amine (HL3); DIPP (2,6-diisopropylphenyl) is N substituent) and a (Z)-1,1-diethyl-2,3-bis(2-methoxyphenyl)guanidine ligand (HL4) have been synthesized. Reaction of HL1-HL4 with Ca[N(SiHMe2)2](THF) or Ca[N(SiMe3)2](THF)2, respectively, afforded complexes 1-8 (1: [L1AmDIPPCa(SiHMe2)2]2, 2: L1AmDIPPCaN(SiMe3)2·(THF)2, 3: L2AmDIPPCaN(SiHMe2)2·(THF)2, 4: L2AmDIPPCaN(SiMe3)2·(THF), 5: L3AmDIPPCaN(SiHMe2)2·(THF)2, 6: L3AmDIPPCaN(SiMe3)2·(THF)2, 7: [L4CaN(SiHMe2)2]2, 8: [L4CaN(SiMe3)2]2). All the complexes were well-defined by NMR spectrum analyses and the molecular structures of 3 and 5-8 were further determined by single crystal X-ray diffraction analyses. In combination with an excess of PhOH as the chain transfer agent, complexes 1-8 catalyzed immortal ring-opening polymerization of rac-lactide in a controlled manner and exhibited moderate catalytic activity at room temperature. The end-group fidelity of the resultant polymer was certified by NMR and MALDI-TOF mass spectra.

Highly syndioselective coordination (co)polymerization of isopropenylstyrene

Coordination (co)polymerization of para-isopropenylstyrene (pIPSt) and meta-isopropenylstyrene (mIPSt), initiated by scandium (Sc) based catalysts, afforded new type of products, bearing pendant isopropenyl groups with perfect syndiotacity (rrrr > 99%).

Copolymerization of ethylene with styrene catalyzed by a scandium catalyst

The pseudo-random copolymers of ethylene and styrene can also be generated by a rare-earth metal based catalyst system.