Phenoxy–Ether Ligated Ti Complexes for the Polymerization of Ethylene

Summary: Bis(phenoxy–ether) Ti complexes were investigated as ethylene polymerization catalysts. The complexes, combined with iBu₃Al/Ph₃CB(C₆F₅)₄ or methylaluminoxane (MAO) cocatalysts, can be highly active single‐site catalysts, which display activities ( turnover frequency, max. 2 065 min⁻¹) comparable with that of a highly active bis(phenoxy–imine) Ti complex/MAO system, and provide very high molecular weight polyethylenes ( 2 040 000–5 420 000) at 25 °C under atmospheric pressure.

Synthesis of polyethylene using bis(phenoxy–ether) Ti complexes, an example of which is shown.     

Bis(Phenoxy‐Imine) Zr Complexes/Et3Al/Heteropoly Compound Catalyst Systems for Ethylene Polymerization

Bis(phenoxy‐imine) Zr complexes upon activation with Et₃Al/(Ph₃C)_mH_n[PMo₁₂O₄₀] · 8 H₂O (average: m/n = 2:1) were demonstrated to be highly active catalysts for the polymerization of ethylene. One of the complexes formed narrow‐molecular‐weight distributed polyethylene ( 1.45) with a very high activity (5640 kg‐PE · mol‐cat⁻¹ · h⁻¹), representing the first example of a MAO‐ and borate‐free, highly active, single‐site catalyst system based on a Group 4 transition metal complex and a heteropoly compound.

Catalysis of ethylene with bis(phenoxy‐imine) Zr complexes activated with Et₃Al/(Ph₃C)_mH_n[PMo₁₂O₄₀] · 8 H₂O.     

Polymerization of Norbornene with CoCl2 and Pyridine Bisimine Cobalt(II) Complexes Activated with MAO

Addition polymerization of norbornene was performed with several pyridine bis(imine) cobalt dichloride complexes activated with methylaluminoxane (MAO), first described for ethylene polymerization. For the first time, norbornene was also polymerized with CoCl₂ associated to MAO. The influence of several reaction parameters has been investigated. Quite different behavior was observed compared with ethylene polymerization. Moreover, the copolymerization of ethylene and norbornene with these complexes was not possible but led to a mixture of both homopolymers.

The pyridine bis(imine) cobalt dichloride complexes used in this study.     

Effect of Cocatalysts on the Catalytic Activities of Tantalum‐ and Niobium‐Based Catalysts for Ring‐Opening Metathesis Polymerization of Norbornene

A series of pentavalent tantalum and niobium complexes with aryloxy ligands was prepared, and their catalytic behavior for the ROMP of norbornene was studied in the presence of an alkylaluminum cocatalyst. Tantalum complexes 1 – 4 showed very high activity for the ROMP of NBE in combination with ⁱBu₃Al to give high‐molecular‐weight polymers. In contrast, the niobium complexes 5 and 6 , as well as NbCl₅, exhibited very high activity upon activation with Me₃Al to give high‐molecular‐weight polymers.

     

Propylene Polymerization Performance of Isolated and Aggregated Ti Species Studied Using a Well‐Designed TiCl3/MgCl2 Ziegler‐Natta Model Catalyst

In propylene polymerization with MgCl₂‐supported Ziegler‐Natta catalysts, it is known that the reduction of TiCl₄ with alkylaluminum generates Ti³⁺ active species, and at the same time, leads to the growth of TiCl_x aggregates. In this study, the aggregation states of the Ti species were controlled by altering the Ti content in a TiCl₃/MgCl₂ model catalyst prepared from a TiCl₃ · 3C₅H₅N complex. It is discovered that all the Ti species become isolated mononuclear with a highly aspecific feature below 0.1 wt.‐% of the Ti content, and that the isolated aspecific Ti species are more efficiently converted into highly isospecific ones by the addition of donors than active sites in aggregated Ti species.

     

Synthesis of Multinuclear Pyridylimine Based Palladium(II) Complexes with a Functionalized Star Polystyrene Core and Evaluation of Their Catalytic Activity Towards Ethylene Oligomerization

Summary: The synthesis of a novel series of multi‐nuclear macroligated pyridylimine based palladium(II) complexes conjugated to a synthetic star polymer core has been well‐described. Star shaped polystyrene with a finite number of arms were prepared by atom transfer radical polymerization (ATRP) and the chain ends were chemically modified in order to introduce the desired end functionality. Utilizing this polymer as organic supports for pyridylimine‐based ligands, palladium(II) complexes were synthesized. On activating with methylaluminoxane (MAO), this complex showed substantially high activity towards ethylene oligomerization, producing C₄ and C₆ compositions as major products. The plausible mechanistic insight concerning the process of ethylene oligomerization is also broached.

An example of trinuclear pyridylimine based palladium(II) complex having a functionalized star polystyrene core.     

Sodium Hydride/Trialkylaluminum Complexes for the Controlled Anionic Polymerization of Styrene at High Temperature

Summary: A new generation of anionic initiators (butyllithium free), based on trialkylaluminum and a readily available alkali metal hydride, has been developed for the control of styrene polymerization at high temperature. Triisobutylaluminum and sodium hydride form heterocomplexes that are efficient for the initiation of styrene polymerization at 100 °C in toluene or in bulk. To be active under such conditions, these systems require the presence of an excess of metal hydride with respect to AlR₃ ([Al]/[Na] < 1). PS chains are specifically initiated by the hydrides coming from NaH, and molar masses are controlled in the range 0.8 < [Al]/[Na] < 1. Fast exchange between dormant 1:1 and active 1:2 complexes (Al:Na), and ligand rearrangements within the 1:2 complex, can explain the observed results.

Initiation of styrene with i‐Bu₃Al/NaH systems.     

Titanium complexes bearing carbamato ligands as catalytic precursors for propylene polymerization reactions

This report describes propylene polymerization reactions with titanium complexes bearing carbamato ligands, Ti(O₂CNMe₂)Cl₂ ( I ) and Ti(O₂CR₂)₄ [R₂ = NMe₂ ( II ), NEt₂ ( III ) and ( IV )]. Combinations of these complexes and MAO form catalysts for the synthesis of atactic polypropylene, as confirmed by FT‐IR, DSC and ¹³C NMR analysis. Effects of main reaction parameters on the catalyst activity were studied including the type of complex, solvent, temperature, and the [Al]/[Ti] molar ratio. The highest activity was observed when chlorobenzene was used as a solvent and AlMe₃‐depleted MAO was employed as a cocatalyst. © 2013 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2013 , 51, 4095–4102     

Modeling of Slurry Polymerization of Ethylene Using a Soluble Cp2ZrCl2/MAO Catalytic System

The slurry homopolymerization of ethylene catalyzed by a Cp₂ZrCl₂/MAO catalytic system was studied. A simple kinetic model including initiation, propagation, transfer to monomer and cocatalyst, spontaneous transfer and spontaneous deactivation was developed to predict dynamic yield of polymerization and molecular weight of final products. Kinetic constants were estimated by numerical solution of polymerization kinetic model, combined with Nelder‐Mead simplex method. The model predicts that the propagation reaction has the lower activation energy in relation to chain transfer reactions which leads to decrease of molecular weight at elevated temperatures. The initiation reaction has however, the highest activation energy that results in raising the peak of reaction rate at higher temperatures.

     

Triflic‐Acid‐Mediated Polycondensation of Carbonyl Compounds with Aromatic Hydrocarbons – A Theoretical Study

Mono‐ and diprotonated reaction intermediates involved in the acid‐catalyzed polyhydroxyalkylation of aldehydes and ketones of the general formula R₁COR_2, (R₁ = H, CH₃, CF₃ and R₂ = Ph, CH₃, CF₃) with benzene and biphenyl, were studied theoretically at PBE0/aug‐cc‐PVTZ//PBE0/6‐31 + G** level of theory. The calculations performed for sulfuric acid and TFSA‐catalyzed reactions showed that for all studied reactions the enhancement of the reactivity of diprotonated species is not sufficient to compensate for the large positive Gibbs energy of second protonation. An alternative mechanism has been proposed for the reaction between benzene and benzaldehyde in TFSA involving only monoprotonated species. The low reactivity of carbonyl compounds with electron donating substituents is due to excessive stabilization of monoprotonated species rendering the reaction thermodynamically impossible.

     

Reductive Formation of Isospecific Ti Dinuclear Species on a MgCl2 (110) Surface in Heterogeneous Ziegler‐Natta Catalysts

Dinucleation of TiCl₄ on the MgCl₂ (100) surface has been conventionally believed as the origin of the stereospecificity of heterogeneous Ziegler‐Natta catalysts for propylene polymerization, while the MgCl₂ (110) surface has been regarded as non‐stereoselective in the absence of organic donors. Based on periodic density functional calculations, we propose a new isospecific Ti dinuclear species on the MgCl₂ (110) surface, which is formed as a result of reduction of Ti from 4+ to 3+. The new species closely resembles the dinuclear species on the (100) surface, but two Ti ions are obliquely stacked along the (110) surface through Cl bridges. The results address the importance of the reduction and re‐distribution of surface Ti species after contacting with cocatalysts in considering the origin of the stereospecificity.

     

Ethylene Polymerization Catalyzed by Titanium(IV) Complexes of a Triaryloxoamine Ligand [TiX{(OArCH2)3N}]

Summary: Titanium complexes containing a triaryloxoamine ligand, [TiX{(O‐2,4‐R₂C₆H₂‐6‐CH₂)₃N}] (R = Me, tBu; X = OiPr, O‐2,6‐iPr₂C₆H₃), exhibited notable catalytic activity for ethylene polymerization in the presence of MAO, especially at temperatures between 100 and 120 °C. Their activity increased upon the addition of a small amount of AlMe₃.

     

Living Ring-Opening Polymerization of Cyclic Esters with Epoxide-Derived Titanium Alkoxides

Summary: The Ti^IIICp₂Cl-catalyzed radical ring opening of epoxides produces Ti alkoxides which initiate the ring-opening polymerization of ε-caprolactone. A linear dependence of on conversion, linear kinetics, low values, and the synthesis of block copolymers demonstrate a living process, while NMR spectroscopy confirms the presence of the initiator chain end. Epoxides are thus introduced as a new class of initiators for the Ti-catalyzed living ring-opening polymerization of cyclic esters.

The TiCp₂Cl-catalyzed radical ring opening of epoxides followed by the initiation of the living ring-opening polymerization of ε-caprolactone.     

Propylene Polymerization with Bis(phenoxy‐imine) Group‐4 Catalysts Using iBu3Al/Ph3CB(C6F5)4 as a Cocatalyst

The catalytic behavior of three bis(phenoxy‐imine) group‐4 transition‐metal complexes (M = Ti, Zr, Hf), with ⁱBu₃Al/Ph₃CB(C₆F₅)₄ cocatalyst systems towards propylene polymerization was investigated under atmospheric pressure at 25 °C. The Ti complex produced ultrahigh‐molecular‐weight atactic poly(propylene), whereas Zr and Hf complexes formed high‐molecular‐weight isotactic poly(propylene)s via a site‐control mechanism. The isotactic poly(propylene) obtained with the Hf complex displayed a high melting temperature of 123.8 °C.

     

Living Polymerization of N,N‐Diphenylacrylamide with Triisobutylaluminum

Summary: N,N‐Diphenylacrylamide was polymerized in a living fashion with triisobutylaluminum in THF at 0 °C. The polymerization results showed an increase of molecular weight proportional to the amount of monomer consumed and a first‐order kinetics at −78 °C. The intermediates obtained with excess initiator at −78 °C revealed that the polymerization was initiated through 1,4‐addition of hydride from a triisobutyl group in the triisobutylaluminum and then proceeded through aluminum‐oxygen bond interchange.

N,N‐Diphenylacrylamide was polymerized in a living fashion with triisobutylaluminum in THF at 0 °C.     

Synthesis of Chlorotitanium(IV) Schiff‐Base Complexes and their Application to Styrene Polymerization

Summary: A [TiCl₂(salen)] complex and its derivatives with the formula [TiCl₂(L)] [L = salen(tBu), salen(di‐Me), salen(di‐tBu), salen(Me)] were synthesized in high yield by reacting the Schiff‐base ligands with TiCl₄. [TiCl₂{salen(tBu)}] and [TiCl₂{salen(di‐tBu)}] have been characterized by single‐crystal X‐ray diffraction. Styrene polymerizations carried out with [TiCl₂(salen)] and its derivatives co‐catalyzed by MAO yielded syndiotactic polystyrenes. The catalytic activity and syndiospecificity were dependent on the bulkiness of the ortho substituents in the aryl ring of ligand.

     

CF3SO3H Initiated Cationic Polymerization of Diisopropenylbenzenes in Macrobatch and Microflow Systems

CF₃SO₃H was found to serve as a more superior initiator for the polymerization of diisopropenylbenzenes than the conventionally used acids such as H₂SO₄. Much faster polymerization at lower temperatures seems to be ascribed to the higher acidity of CF₃SO₃H. The use of microflow systems was also found to be effective in increasing the indane unit content, especially for 1,4‐DPB. Fast mixing and uniformity of the temperature seem to be responsible. The thus‐obtained polymers of high indane unit content serve as useful materials having high thermal resistance and low dielectric constants.

     

Highly Efficient Synthesis of Low Polydispersity Core Cross‐Linked Star Polymers by Ru‐Catalyzed Living Radical Polymerization

The efficient formation of low polydispersity core cross‐linked star (CCS) polymers via controlled/living radical polymerization (LRP) and the arm‐first approach was found to be dependant on the mediating catalyst system. The Ru catalyst, Ru(Ind)Cl(PPh₃)₂ Cat. 1 , and tertiary amine co‐catalyst were used to synthesize highly living poly(methyl methacrylate) (PMMA) macroinitiators, which were then linked together with ethylene glycol dimethacrylate (EGDMA) to form PMMA_armPEGDMA_core CCS polymers. The quantitative and near‐quantitative synthesis of CCS polymers were observed for low to moderate molecular weight macroinitiators ( = 8 and 20 kDa), respectively. Lower conversions were observed for high‐molecular weight macroinitiators ( ≥ 60 kDa). Overall, an improvement of between 10 and 20% was observed when comparing the Cat. 1 system to a conventional Cu‐catalyzed system. This significant improvement in macroinitiator‐to‐star conversion is explained in the context of catalyst system selection and CCS polymer formation.

     

Compartmentalization in Atom Transfer Radical Polymerization (ATRP) in Dispersed Systems

Summary: Compartmentalization in atom transfer radical polymerization (ATRP) in dispersed systems at low conversion (<10%) has been investigated by means of a modified Smith–Ewart equation focusing on the system n‐butyl acrylate/CuBr/4,4′‐dinonyl‐2,2′‐dipyridyl at 110 °C. Compartmentalization of both propagating radicals and deactivator was accounted for in the simulations. As the particle diameter (d) decreases below 70 nm, the polymerization rate (R_p) at 10% conversion increases relative to the corresponding bulk system, goes through a maximum at 60 nm, and thereafter decreases dramatically as d decreases further. This behavior is caused by the separate effects of compartmentalization (segregation and confined space effects) on bimolecular termination and deactivation. The very low R_p for small particles (d < 30 nm) is due to the pseudo first‐order deactivation rate coefficient being proportional to d⁻³.

Simulated propagating radical concentration ([P•]) as a function of particle diameter (d) at 10% conversion for ATRP of n‐butyl acrylate ([nBA]₀ = 7.1 M , [PBr]₀ = [CuBr/dNbpy]₀ = 35.5 mM ) in a dispersed system at 110 °C. The dotted line indicates the simulated [P•] in bulk at 10% conversion.     

Direct Synthesis of Pyridyl Disulfide‐Terminated Polymers by RAFT Polymerization

A trithiocarbonate RAFT agent was modified with a pyridyl disulfide group and used in the direct synthesis of endgroup pyridyl disulfide‐functionalized homo‐ and amphiphilic block copolymers of oligo(ethyleneglycol) acrylate (PEG‐A) and butyl acrylate (BA). Both the homo‐ and copolymerizations were found to be well controlled via the RAFT mechanism. The NMR analysis indicated that both the homopolymers of PEG‐A and the amphiphilic diblock copolymers of PEG‐A and BA possessed pyridyl disulfide terminal groups. A UV‐Vis absorption test revealed that the pyridyl disulfide endgroup of the polymer could be efficiently used to couple thiol‐bearing molecules to the polymer without the need for any post‐polymerization modification. This communication presents the first efficient direct synthesis of thiol‐reactive endgroup‐functionalized well‐defined polymers via the RAFT technique.