Copolymers late transition metal catalysts

After five decades of catalyst research there is slowly emerging a family of discrete late transition metal catalysts that are capable of generating high molecular weight, linear, random copolymers of ethylene and polar comonomers such as acrylates. Further advances in the efficiency of these catalysts will likely give rise to new families of commercial polyolefins with a wealth of new performance properties imparted by the polar groups attached to the polymer backbone. 

W. Kaminsky and M. Amdt-Rosenau, "Tactic norbomene homo- and copolymers made with early and late transition metal catalysts," in L.S. Baugh and J.A.M. Canich, eds., Stereoselective Polymerization with Single-Site Catalysts, chapter 16, pp. 413-444. CRC Press, Boca Raton, FL, 2008. 

The addition copolymerization of norbornene-type monomers with a-olefins [21] forms the basis of EPDM (ethylene propylene diene monomer) technology. Incorporation of smaU amounts of DCPD or ethylidene norbornene (ENB) in olefinic vinyl addition polymers provides latent crosslink sites in EPDM elastomers. It is weU known in the hterature that incorporation of higher amounts of rigid, bulky multicychc olefins results in materials with higher TgS [22]. In fact, more recent work has concentrated on increasing the Tg of norbornene-type monomer/a-olefin copolymers [23]. The use of late transition metal catalysts to prepare such copolymers is reviewed in Section 4.3. 

In conclusion, the expanded scope of ADMET is largely due to the development of well-defined late-transition metal catalysts that are functional group-tolerant and easy to handle and synthesize. The potential for ADMET is very great, in that it is a mild method of forming useful linear condensation polymers and copolymers. The apphcation of ADMET in modeling polyolefins and other polymers is just being discovered, and this aspect of ADMET is expected to further the understanding of these enormously important polymers. 

Another important issue in olefin polymerization is copolymerization of different types of monomers. If one can freely produce copolymers of non-polar and polar monomers, which are difficult to copolymerize with conventional initiators, it would provide useful polymer materials. The Ziegler type catalysts using trialkylaluminum is not suitable for polymerizing polar monomers, whereas late transition metal catalysts are more tolerant of polar monomers. Recently catalysts using late transition metal catalysts have been intensively studied [89]. Because of the obvious importance of these polymeric materials in industrial use, further studies are expected on the applicability of late transition metal complexes for polymerization. 

Metallocenes or late transition metal catalysts were iiKxeas-ingly utilized over the years for the (co)polymetization of olefins including the synthesis of graft copolymers. In this last case coordination polymerization had to be combined with other polymerization processes to give access to macromonomets. However, only a few examples of coordination homopolymer-ization or copolymerization of macromonomets aimed to design comb-shaped polymers were described in the literature. In the following these examples will be described briefly. 

I C Cyclopentene Homo- and Copolymers Made with Early and Late Transition Metal Catalysts... 

Metallocene/methylaluminoxane (MAO) and other single site catalysts for olefin polymerization have opened a new field of synlhesis in polymer chemistry. Strained cyclic olefins such as cyclobutene, cyclopentene, norbornene (NB), and their substituted compounds can be used as monomers and comonomers in a wide variety of polymers." Much interest is focused on norbornene homo- and copolymers because of the easy availability of norbornene and the special properties of their polymers. Norbornene can be polymerized by ring opening metathesis polymerization (ROMP), giving elastomeric materials, or by double bond opening (addition polymerization). Homopolymerization of norbornene by double bond opening can be achieved by early and late transition metal catalysts, namely Ti, Zr, Hf, Ni, - ° and Pd (Scheme 16.1). 

Norbomene can be copolymerized with olefins such as ethylene, propylene, 1-butene, and longer-chain a-olefins using early and late transition metal catalysts. The resultant copolymer properties depend on different parameters, such as comonomer content and distribution throughout the polymer chain, as well as the conformational orientation of the comonomer units. The microstmcture of the copolymer can be controlled by the appropriate choice of reaction conditions and catalyst stmcture. The most powerful method to determine copolymer microstmcture is NMR spectroscopy. In the past years, much progress has been achieved in making peak assignments for olefin-norbornene copolymers. - ... 

Broadening this comparison to include copolymers prepared by both early and late transition metal catalysts, the results discussed immediately above show that Ci-symmetric zirconocenes such as 9/MAO produce only copolymers with isolated norbornene units or alternating structures (at 30 C), mainly with isotactic (meso) configurations. C2-symmetric zirconocenes such as 2/MAO readily produce norbornene dyads that are exclusively meso-linkcd (isotactic). In accordance with their catalyst structures, Q-symmetric zirconocenes such as 8/MAO produce norbornene dyads with a rac-linkage (syndiotactic), although with a generally lower stereoselectivity. Palladium a-diimine catalysts, despite the homotopic nature of their coordination sides (that would be expected to give a mixture of meso and racemic blocks), produce norbornene dyads that are solely rac-connected. This behavior can be attributed to a chain-end control type polymerization mechanism. 

The late-transition-metal catalysts have two key advantages over metallocenes first, they allow oxygen and other functional groups, including polar monomers such as acrylates, to be tailored onto an ethylene backbone as a consequence, novel ethylene copolymers can be synthesized. Second, these catalysts can... 

E-N copolymers made by single-site catalysts are characterized by narrow molecular weight distributions, which make technical processing easier. The first commercial E-N copolymer products by early transition metal catalysts are already available. In contrast, late transition metal catalysts, which are more tolerant to polar functional groups, need further developments to be efficiently used in olefin-cycloolefin copolymerizations. 

Coordination polymerization of dienes has progressed significantly within the last decade. Selective polymerization of 1,3-dienes is reinforced by conventional transition metal catalysts and by new organolanthanide catalysts. Nonconjugated dienes also polymerize selectively to produce polymers with cyclic units or vinyl pendant groups. Living polymerization of dienes has become common, which enabled preparation of block copolymers of dienes with alkenes and other monomers. Another new topic in this field is the polymerization of allenes and methylenecycloalkanes catalyzed by late transition metal complexes. These reactive dienes and derivatives provide polymers with novel structure as well as functionalized polymers. The precision polymerization of 1,2-, 1,3-, and l,n-dienes, achieved in recent years, will be developed to construct new polymer materials with olefin functionality. 

In contrast to Group IV-based polymerization catalysts, late transition metal complexes can carry out a number of useful transformations above and beyond the polyinsertion reaction. These include isomerization reactions and the incorporation of polar monomers, which have allowed the synthesis of branched polymer chains from ethylene alone, and of functional polyolefins via direct copolymerization. The rational design of metallocene catalysts allowed, for the first time, a precise correlation between the structure of the single site catalyst and the mi-crostructure of the olefin homo- or copolymer chain. A similar relationship does not yet exist for late transition metal complexes. This goal, however, and the enormous opportunities that may result from new monomer combinations, provide the direction and the vision for future developments. 

Brookhart and coworkers combined late transition metals with sterically bulky ligands and noncoordinating anions to synthesize olefin-acrylate copolymers (Scheme 10). The bulky ligands block associative displacement and thus minimizing chain transfer. Anions like B(Ar)4 promote a more active cationic catalytic species and improve the solubility of these systems. However, the current late transition metal ca ysts still don t achieve the activities (productivities) of the conventional Ziegler-Natta or metaUocene catalysts for nonfunctional monomers, such as ethylene. 

The sheer size and value of the polyethylene industry ensure that there is continued research, progress, and development in catalysis, for their potential commercial impact. Although this whole subject is not within the scope of this chapter, we mention a couple of aspects of the progress, which offer the potential to impact this industry. In 1995, DuPont introduced work, carried out with them at the University of North Carolina—via the largest patent applicafion ever in the USA. They disclosed what are described as post-metallocene catalysts. These are transition and late transition metal complexes with di-imine ligands, which form part of the DuPont Versipol technology. Such catalysts create highly branched to exceptionally linear ethylene homopolymers and linear alpha-olefins. Late transition metals offer not only the potential for the incorporation of polar comonomers, which until now has only been possible in LDPE reactors, but also their controlled sequence distribution, compared to the random composition of free radical LDPE copolymers. Such copolymers account for over 1 million tons per annum [20]. Versipol has so far only been cross-licensed and used commercially by DuPont Dow Elastomers (a former joint venture, now dissolved) in an EPDM plant. 

The homo- and copolymerization of methacrylates and acrylates and the synthesis of block copolymers is also reported [108], Recently, it is reported that new organometallic compounds (late metals) also produce ethylene/MMA copolymers [54, 55]. The development of polymerization catalysts incorporating late transition metals is a promising area of research, since late metals are typically less oxophilic, and thus more functional group tolerant, than early metals. Examples of the functional group tolerance of late metals in insertion-type reactions include reports on Ru, Rh, Ni, and Pd catalysts. 

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