Chain-Shuttling and Chain-Transfer Polymerization

28.7 ppm for the fluorinated complex, because it is located close to the electron deficient metal center (Figure 1-8).⁶⁰

Figure 1-8. ¹H-¹³C HSQC NMR spectra (toluene-d8/o-difluorobenzene, -45 °C) of the chain-carrying titanium species generated from reaction of the ¹³C-MAO activated complexes 1 (left) and VII (right) by reaction with ¹³C-ethylene.⁶⁰

The ortho-fluorine substituents in the cationic titanium alkyl complex generated from 1 shield the metal center from close contact to the β-methylene group by a noncovalent interaction with the titanium atom. In this way β-hydrogen elimination is inhibited. Without ortho-fluorine substituents the polymer chain bends to a conformation with close proximity of the β-methylene group to the electron-deficient titanium center. As a result weak β-agostic interactions might be established, which ultimately results in β-hydrogen elimination.

1.1.3 Chain-Shuttling and Chain-Transfer Polymerization

The production of well-defined polymers by living polymerization is limited by the fact, that only one polymer chain per initiator or active metal center is formed.

Therefore, this process is commercially unviable for expensive transition metal

Post-Metallocene Catalyzed Olefin Polymerization

12

catalysts. The chain-shuttling and chain-transfer polymerization processes overcome this limitation for the large scale production of block copolymers and well-defined polyolefins.

Figure 1-9. Chain shuttling polymerization process affording multiblock copolymers.

Block copolymers are an interesting class of materials, which find application for example as thermoplastic elastomers. In these materials the elasticity of amorphous, low Tg polymers can be combined with the strength of crystalline polymers.

Commonly, block copolymers are prepared by living polymerization with sequential addition of the monomers. However, the chain-shuttling polymerization process developed by Arriola and coworkers at Dow represents an alternative route to olefin block copolymers (OBCs). In this solution polymerization process the growing polymer chain is transferred reversibly with a chain transfer agent such as ZnEt2

between a catalyst that does not incorporate comonomer significantly (e.g. a phenoxyimine Zr complex), and a catalyst with a high propensity for comonomer incorporation (e.g. a pyridyl-amido Hf complex). This yields multiblock copolymers with highly crystalline polyethylene and amorphous poly(ethylene-co-1-olefin) blocks (Figure 1-9). As this polymerization system is not stoichiometric but catalytic in terms of polymer chains produced per metal center, commercially profitable polymer quantities can be produced with relatively low catalyst loadings.^62,63 Dow and Mitsui Chemicals entered a joint research agreement for the development of catalyst systems for the chain shuttling polymerization process in 2006.⁶⁴ The multiblock copolymers, which are marketed as INFUSE^TM OBC by Dow, can be used for flexible parts fabricated by injection molding such as soft touch grips. Another application is for example the fabrication of flexible films for diapers. Very recently, Dow announced a new block copolymer product (termed INTUNE^TM) on the basis of

13 ethylene and propylene. These polymers are supposed to allow an efficient compounding of polypropylene with various nonpolar and polar polymers such as polyethylene, polyolefin elastomers, ethylene/vinyl alcohol copolymers and polyamides.⁶⁵

By using multifunctional chain transfer agents, the chain shuttling concept was further expanded to the synthesis of telechelic polymers and blockcopolymers, which can be employed as ingredients for coatings, sealants, adhesives, and rocket fuel binder.^66,45 In this context also a double chain shuttling process is mentioned, for which the cheaper and less pyrophoric aluminum alkyls can be employed as surrogates. As chain transfer to aluminum is not as efficient as to zinc, small amounts of zinc alkyl compounds are used to catalyze the chain transfer from the transition metal to the aluminum compound.^66a

Figure 1-10. Structures of the indenyl-amido complex VIII, oxygen chelated complex IX, imino-amido complex X and acetamidinate complex XI.

Besides phenoxyimine zirconium and pyridyl-amido hafnium complexes, additional post-metallocenes are referred to in the context of the chain shuttling polymerization process. These complexes include a broad variety of constrained geometry catalysts such as complex VIII, diamido-amine coordinated hafnium complexes similar to complex XIII (vide infra), the oxygen chelated complex IX and imino-amido complexes such as X (Figure 1-10).^63,66 Imino-amido catalysts can be employed in the chain-shuttling process instead of the phenoxyimine catalyst, to produce the hard block of the multiblock copolymer. The octene incorporation of these catalysts is slightly higher than of the phenoxyimine complex and can be adjusted by variation of

Post-Metallocene Catalyzed Olefin Polymerization

14

substituents and the metal center, thus allowing for a fine tuning of the properties of the hard block.⁶⁷

Another approach toward large scale production of precisely defined polyolefins with very narrow molecular weight distributions is the catalyzed chain growth (CCG) or catalytic chain transfer polymerization (CCTP).⁶⁸ In this process the polymer chain is transferred reversibly from the catalytically active metal center to a surrogate, typically a zinc or main group metal alkyl compound, with the chain transfer rate being faster than the chain growth rate. This leads to a simultaneous growth of all polymer chains and thus narrow molecular weight distributions. The number of polymer chains is not stoichiometric in the costly catalyst, but in the much cheaper chain transfer agent. The metal containing end-groups of these polymers can be further converted to a great variety of functional groups. Various metals, including late and early transition metals and lanthanides, were employed as catalysts. Group 4 cyclopentadienyl acetamidinate complexes (XI) in combination with chain transfer agents were utilized by Sita and coworkers for the synthesis of various well defined, end-functionalized polyolefins.⁶⁹ With this catalyst system ethylene/1-hexene multiblock copolymers can be prepared by dynamic counterion exchange of a tightly ([MeB(C6F5)3]^-) and a weakly ([B(C6F5)4]^-) binding borate anion, which leads to a modulation of the 1-olefin enchainment rate.⁷⁰ In order to reduce the amount of zinc alkyl compounds a ternary chain transfer system with aluminum alkyls as major component can be employed.⁷¹

1.1.4 Late Transition Metal Polymerization Catalysts and Hybrid