More than 50 years have passed since Ziegler and Natta shared the Nobel Prize in Chemistry for their discovery of olefin polymerization catalysts. The field of metal-catalysed polymerization has since matured, in no small part owing to the development of several high-performance catalysts. Although polymerization research has in many ways been driven by catalyst development, this has often occurred as a result of trial and error discovery of a promising motif, followed by extensive tuning of the steric and electronic properties of the ligand(s) present in the lead complex. Recently, some alternative design strategies have emerged that afforded new classes of olefin polymerization catalysts. This Perspective highlights recently designed catalyst motifs and the novel reactivity patterns they enable. Special attention is given to methods specifically designed for the copolymerization of ethylene with polar-functionalized co-monomers — challenging reactions that showcase these creatively designed catalyst motifs. The development of high-performance olefin polymerization catalysts is a major driving force in polyolefin studies. This Perspective discusses some alternative strategies for catalyst design — strategies in which existing systems are tuned beyond merely modifying the electronic and steric properties.

Designing catalysts for olefin polymerization and copolymerization: beyond electronic and steric tuning

Polymers have become one of the largest and most important materials we use in daily life. Their popularity has stemmed from their wide range of material properties combined with their low cost of ...

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Recent Trends in Catalytic Polymerizations

Ultrahigh molecular weight polyethylene: Catalysis, structure, properties, processing and applications

Polyolefin thermoplastic elastomers from polymerization catalysis: Advantages, pitfalls and future challenges

Titanium is the second most abundant transition metal and is already a key player in important industrial processes (e.g. polyethylene). Titanium is an attractive metal to use for catalytic transformations as it is a versatile and inexpensive metal of low-toxicity and of established biocompatibility. However, its potential use as a catalyst for the synthesis of fine chemicals, pharmaceuticals and agrochemicals is often overlooked due to its oxophilic, Lewis acidic character, which renders complexes of titanium less functional group tolerant than their late transition metal counterparts. Nevertheless, three different fields of research in titanium catalysis have drawn attention in recent years: formal redox catalysis, hydroamination and hydroaminoalkylation. For these reactions, titanium offers new approaches and alternative pathways/mechanisms that are complementary to late transition metal-based catalysis. This review focuses on advances in fine chemical synthesis by titanium-catalyzed reactions featuring redox transformations and two important hydrofunctionalization reactions, hydroamination and hydroaminoalkylation. Starting from the late 90s, we provide an overview of historic inspirational contributions, both catalytic and stoichiometric, and the latest insights in catalyst design efforts, mechanistic details and utility of the three different classes of transformations. Insights to enhance catalyst activity as well as catalyst controlled regio- and stereoselectivities are presented. Illustrative examples that highlight substrate scope and the application of titanium catalysis to the synthesis of complex organic small molecules, natural products and materials are shown. Finally, opportunities and strategies for on-going research and development activities in titanium catalysis are highlighted.

Titanium catalysis for the synthesis of fine chemicals – development and trends

The discovery of transition metal-catalysed polymerisation of olefins in the 1950s by Ziegler and Natta was of huge importance. Over the last 30 years, the interest in homogenous polymerisation has not only grown but has changed focus from primarily studying the metallocene complexes of Group 4 to widespread exploration of post-metallocene systems. This is a consequence of extensive patenting of the metallocene catalyst systems, as well as for general interest in the improvement of polyolefin catalysis. The plastics industry produces more than 107 tonnes of polyethylene, polypropylene and polystyrene each year worldwide, and the study of well-defined homogenous catalysts is invaluable in uncovering the mechanism of polymerisation and the fundamental properties of the active species. This short Tutorial Review gives an overview of homogenous transition metal Ziegler–Natta catalysis in general as well as an overview of a selection of Group 4 post-metallocene catalysts. An overview of the synthesis and reactions of alkyl cations relevant to olefin polymerisation is also given. Finally, this review summarises recent advances in bimetallic catalysts for olefin polymerisation.

https://link.springer.com/content/pdf/10.1007%2Fs13203-015-0105-2.pdf

Group 4 metal complexes for homogeneous olefin polymerisation: a short tutorial review

Reaction of K[HC(C(Me)NAr′)2] (Ar′ = 2,6-C6H3iPr2) with Mg(BH4)2 afforded the pseudo four-coordinate tetrahydroborate complex Mg{HC(C(Me)NAr′)2}(BH4)(THF) (1). The corresponding reaction of Ca(BH4)2(THF)2 or Sr(BH4)2(THF)2 gave the pseudo five-coordinate analogues M{HC(C(Me)NAr′)2}(BH4)(THF)2 (M = Ca (2) or Sr (3)). All three compounds 1–3 have been structurally characterised. According to X-ray crystallography and IR spectroscopy all possess κ3-bound BH4 ligands in the solid state. This coordination mode is also maintained in THF solution for 2 and 3, whereas complex 1 appears to form a bis(THF) complex containing a κ2-bound BH4. Reaction of 1 with K[HC(P(Ph2)NAr′)2](THF)2 formed Mg{HC(P(Ph2)NAr′)2}(BH4)(THF)2 (4) possessing a κ3-bound BH4 ligand in both the solid state and solution. Compounds 1, 2 and 4 are highly active for the ring-opening polymerisation of e-caprolactone forming dihydroxytelechelic PCL. Compound 1 is also extremely active for the ROP of rac-lactide forming highly heterotactic PLA with good agreement between predicted and measured Mn, in accord with previous studies of alkoxide and amide initiators based on this metal and ligand class. Compounds 2 and 4 were less productive and gave PLA with poorer control of Mn and negligible heterotactic enrichment. MALDI-ToF MS analysis of the PLA formed with all three catalysts showed a mixture of both –CH(Me)CHO and –CH(Me)CH2OH termini arising from the M–BH4 initiating groups.

Synthesis and rac-lactide ring-opening polymerisation studies of new alkaline earth tetrahydroborate complexes

Monometallic and Bimetallic Titanium κ1-Amidinate Complexes as Olefin Polymerization Catalysts

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Papers

Group 4 metal complexes for homogeneous olefin polymerisation: a short tutorial review

Synthesis and rac-lactide ring-opening polymerisation studies of new alkaline earth tetrahydroborate complexes

Monometallic and Bimetallic Titanium κ1-Amidinate Complexes as Olefin Polymerization Catalysts