Insertion/Isomerization Polymerization of 1,5-Hexadiene: Synthesis of Functional Propylene Copolymers and Block Copolymers

Polymerization of 1,5-hexadiene with a bis(phenoxyimine) titanium catalyst system is reported. The microstructure of the polymer contains the expected methylene-1,3-cyclopentane units as well as the unexpected 3-vinyl tetramethylene units. A mechanism for formation of this polymer is proposed. This unusual reaction is also employed in the synthesis of vinyl-functional polypropylene copolymers and block copolymers with low polydispersity indices

A New Catalyst for Highly Syndiospecific Living Olefin Polymerization: Homopolymers and Block Copolymers from Ethylene and Propylene

A New Catalyst for Highly Syndiospecific Living Olefin Polymerization:  Homopolymers and Block Copolymers from Ethylene and Propylen

Mechanism of Propylene Insertion Using Bis(phenoxyimine)-Based Titanium Catalysts: An Unusual Secondary Insertion of Propylene in a Group IV Catalyst System

A highly regioselective secondary enchainment of propylene in a group IV catalyst system is reported. End-group analysis of polypropylene formed using the phenoxyimine-based titanium catalysts revealed a reversal in the regioselectivity of insertion for this class of catalysts. To the best of our knowledge, bis(phenoxyimine)-based titanium complexes are the only known group IV catalysts that insert propylene with exclusive 2,1-regiochemistry. Insertion of propylene into the initiating titanium hydride occurs with high 1,2-regiochemistry. Subsequent insertions into primary titanium alkyls are regiorandom, while insertions into secondary titanium alkyls proceed with high 2,1-regioselectivity. Cyclopolymerization and ethylene/propylene copolymerization strategies are employed to support this proposal

Mechanism of Activation of a Hafnium Pyridyl−Amide Olefin Polymerization Catalyst: Ligand Modification by Monomer

We have investigated the olefin polymerization mechanism of hafnium catalysts supported by a pyridyl−amide ligand with an ortho-metalated naphthyl group. Ethylene−α-olefin copolymers from these catalysts have broad molecular weight distributions that can be fit to a bimodal distribution. We propose a unique mechanism to explain this behavior involving monomer modification of the catalyst, which generates multiple catalyst species when multiple monomers are present. More specifically, we present evidence that the hafnium alkyl cation initially undergoes monomer insertion into the Hf−naphthyl bond, which permanently modifies the ligand to generate new highly active olefin polymerization catalysts. Under ethylene/octene copolymerization conditions, a plurality of new catalysts is formed in relative proportion to the respective monomer concentrations. Due to the asymmetry of the metal complex, two “ethylene-inserted” and eight “octene-inserted” isomers are possible, but it is a useful approximation to consider only one of each in the polymerization behavior. Consequently, gel permeation chromatography data for the polymers can be fit to a bimodal distribution having a continuous shift from a predominantly low molecular weight fraction to predominantly higher molecular weight fraction as [octene]/[ethylene] is increased. Theoretical calculations show that such insertions into the Hf−aryl bond have lower barriers than corresponding insertions into the Hf−alkyl bond. The driving forces for this insertion into the Hf−aryl bond include elimination of an eclipsing H−H interaction and formation of a stabilizing Hf−arene interaction. These new “monomer-inserted catalysts” have no β-agostic interaction, very weak olefin binding, and olefin-insertion transition states which differ on the two sides by more than 4 kcal/mol. Thus, the barrier to site epimerization is very low and high polymerization rates are possible even when the chain wags prior to every insertion. Experimental evidence for aryl-insertion products is obtained from reactions of ethylene (13C2H4 NMR studies) or 4-methyl-1-pentene (4M1P) using relatively low monomer/catalyst ratios. Quantitative generation of monomer-inserted products is complicated by slow initiation kinetics followed by fast polymerization kinetics. However, NMR evidence for reaction with 13C2H4 was observed in situ at low temperature, and the attachment of monomer to ligand was confirmed by GC/MS and 13C NMR after quenching. Furthermore, a 4M1P-appended ligand was isolated from a polymerization reaction (50:1 monomer:catalyst) by column chromatography followed by multiple recrystallizations. One isomer was characterized by X-ray crystallography, which unequivocally shows a 4-methylpentyl substituent at the 2-position of the naphthyl, consistent with 1,2-insertion into the Hf−aryl bond. NMR suggests a second diastereomer (not isolated) is formed from a 1,2-insertion of opposite stereoselectivity

Continuous Production of Ethylene-Based Diblock Copolymers Using Coordinative Chain Transfer Polymerization

Continuous Production of Ethylene-Based Diblock Copolymers Using Coordinative Chain Transfer Polymerizatio

An Exploration of the Effects of Reversibility in Chain Transfer to Metal in Olefin Polymerization

A kinetic model was derived to investigate the effects of reversibility in chain transfer to metal in olefin polymerization. The model predicts both number- and weight-average molecular weights, Mn and Mw, as a function of several reactor input variables including the constants for chain transfer and reversible transfer. A number of interesting and nonobvious insights into molecular weight distributions are gained from these simulations. The most revealing result is the variation of the molecular weight distribution (Mw/Mn) with conversion. In the absence of reversible transfer, the Mw/Mn is always greater than or equal to 2. Regardless of the magnitude of the reversible transfer constant, Mw/Mn approaches 2 as Mn approaches its maximum value. More dramatic deviations from Mw/Mn = 2 are observed for higher chain transfer constants. Polymerization data from two bis(phenoxyimine)zirconium catalyst systems are presented to demonstrate these effects. These results indicate that singular polymerizations should not be used to explore for reversible transfer characteristics, but rather a series of polymerizations should be conducted over a range of polymer conversions

Orientation Control in Thin Films of a High‑χ Block Copolymer with a Surface Active Embedded Neutral Layer

Directed self-assembly (DSA) of block copolymers (BCPs) is an attractive advanced patterning technology being considered for future integrated circuit manufacturing. By controlling interfacial interactions, self-assembled microdomains in thin films of polystyrene-<i>block</i>-poly­(methyl methacrylate), PS-<i>b</i>-PMMA, can be oriented perpendicular to surfaces to form line/space or hole patterns. However, its relatively weak Flory interaction parameter, χ, limits its capability to pattern sub-10 nm features. Many BCPs with higher interaction parameters are capable of forming smaller features, but these “high-χ” BCPs typically have an imbalance in surface energy between the respective blocks that make it difficult to achieve the required perpendicular orientation. To address this challenge, we devised a polymeric surface active additive mixed into the BCP solution, referred to as an embedded neutral layer (ENL), which segregates to the top of the BCP film during casting and annealing and balances the surface tensions at the top of the thin film. The additive comprises a second BCP with a “neutral block” designed to provide matched surface tensions with the respective polymers of the main BCP and a “surface anchoring block” with very low surface energy that drives the material to the air interface during spin-casting and annealing. The surface anchoring block allows the film to be annealed above the glass transition temperature of the two materials without intermixing of the two components. DSA was also demonstrated with this embedded neutral top layer formulation on a chemical patterned template using a single step coat and simple thermal annealing. This ENL technology holds promise to enable the use of high-χ BCPs in advanced patterning applications

Photonic Polyethylene from Self-Assembled Mesophases of Polydisperse Olefin Block Copolymers

We have discovered a synthetic process capable of producing olefin block copolymers (OBCs) with controlled block length polydispersity. Certain compositions of these OBCs self-assemble in the melt to form ordered mesophases. The morphologies and dielectric contrast of the semicrystalline and amorphous blocks produce transparent films exhibiting a partial photonic band gap for frequencies in the visible spectrum. The domain spacings are not only much larger than expected for monodisperse block copolymers of similar molecular weight, but they also exceed the predictions of recent theories for polydisperse block copolymers. An extension to Strong Segregation Theory demonstrates that many molecules have a weak preference for segregation to the interface versus the center of a domain. Minor perturbations can then produce highly swollen but relatively stable domains

On the First Insertion of α-Olefins in Hafnium Pyridyl-Amido Polymerization Catalysts

Reactions of [{N−,N,Cnaphthyl−}HfMe][MeB(C6F5)3] (2) precatalyst with a series of α-olefins have been investigated in order to intercept the active polymerization species generated by an in situ modification of the precursor by insertion of a single monomer unit into the Hf−CAryl bond. In all cases the first migratory insertion of monomer occurs into the Hf−CAryl bond rather than the Hf−CAlkyl bond. A low-temperature polymerization with 170 equiv of 1-hexene activated with tris(pentafluorophenyl)borane (FAB) allows for the complete NMR characterization of a Hf−CAlkylaryl methyl cation. This structure agrees with DFT studies of the kinetically favored diastereomer, although a number of other structures are more stable thermodynamically. Attempts to trap an inserted catalyst through the stoichiometric addition of 2-vinylpyridine or 3-ethoxy-1-propylene led to complicated reactions, but in both cases, experimental and computational data suggest that both processes initiate through insertion of the substrate into the Hf−CAryl bond. In addition, an activated Hf-dibutyl complex is studied in an attempt to minimize differences in the rates of initiation and propagation, as a butyl group is a reasonable mimic for a propagating polymer chain. Tritylborate activation of this complex cleanly generates 1 equiv of 1-butene in close proximity to the Hf-butyl cation via β-hydride abstraction. This reaction results in formation of isotactic poly(1-butene) with a fairly high molecular weight rather than butene oligomers. The observed molecular weight is consistent with a small fraction of active species, and quenching studies show a similar fraction of butene-modified ligand. These results are consistent with slow insertion into the Hf−CAryl bond followed by fast polymerization kinetics, with the latter rate constant being 3 orders of magnitude faster than the former

“Uni et Trini”: In Situ Diversification of (Pyridylamide)hafnium(IV) Catalysts

“Uni et Trini”: In Situ Diversification of (Pyridylamide)hafnium(IV) Catalyst