Lanthanide-based Ziegler-Natta

Within the context of lanthanide chemistry it is interesting to note reports by Chinese (10,15) and Russian (12) scientists that activities of lanthanide-based Ziegler-Natta catalysts for polymerization of 1,3-dienes show much higher activity for early metals than for the end-row metals Er-Lu. The potential for higher activity analogs of the complexes described here is thus evident. ... 

Extensive efforts have been made to develop catalyst systems to control the stereochemistry, addition site, and other properties of the final polymers. Among the most prominant ones are transition metal-based catalysts including Ziegler or Ziegler-Natta type catalysts. The metals most frequentiy studied are Ti (203,204), Mo (205), Co (206-208), Cr (206-208), Ni (209,210), V (205), Nd (211-215), and other lanthanides (216). Of these, Ti, Co, and Ni complexes have been used commercially. It has long been recognized that by varying the catalyst compositions, the trans/cis ratio for 1,4-additions can be controlled quite selectively (204). Catalysts have also been developed to control the ratio of 1,4- to 1,2-additions within the polymers (203). 

The relevance of Ln/Al heterobimetallic complexes for the emulation of zirconocene-based polymerization catalysis [13-15] was later on also stressed by the Lanthanide Ziegler-Natta Model [16]. Accordingly, lanthanidocene alkyl complexes were not only successfully employed for clarifying major initiation, propagation, and termination steps (Scheme 3) [17,18]. 

Beyond this exclusive lanthanide Ziegler-Natta model, Ziegler-type multicomponent systems ( Mischkatalysatoren ) represent the only class of homogeneous rare-earth metal catalysts of considerable commercial relevance [40-43]. High-czs-1,4-polydienes are industrially produced from 1,3-dienes (butadiene and isoprene) in aliphatic or aromatic hydrocarbons by a number of Mischkatalysatoren based on the transition metals titanium, cobalt, and nickel, and the lanthanide element neodymium [40-47]. The... 

The discovery of Ziegler-Natta catalysts led to many industrial and academic investigations on other kinds of metallic complexes for polymerization of different monomers. Several organometallic and coordination compounds have been synthesized and probed as catalytic systems. They have been classified based on generations or groups, transition-metal type, the chemical structure, the type of activator, and their applications in polymerization processes [2]. Currently, there are different groups of initiator systems based on early and late transition metals or lanthanide complexes, which have been studied in polymerization catalysis [3]. 

The main parameters of diene polymerisation with lanthanide-based catalytic systems are similar to those of polymerisation with ion-coordinated catalysts on the basis of d-metals. This can be seen from the following facts polymerisation of dienes has an anionic coordinated character [18] at polymerisation temperatures from 20 to 25 C, the reaction is of first order with respect to the monomer and catalyst (this property is independent of the natures of catalyst and hydrocarbon solvent, the only exception to this rule being the system considered in work [18]) for most of catalysts studied [18, 21, 26, 28, 41] and, the apparent activation energy of the reaction of polymerisation of dienes is of the order of 33.5 kj/mol [20, 41]. For lanthanide catalysts, the concentration of active centres is somewhat higher than for conventional Ziegler-Natta catalysts, e.g., for neodimium-based catalysts their content varies from 6-10% [12, 41, 42, 50] to 15-20% [54-57]. 

Figure 3.9 Stereospecific polymerisation of isoprene (2-methyl-butadiene-l,3). The Russian industry has implemented technologies of c/s-l,4-polyisoprene synthesis with Ti-Al Ziegler-Natta catalysts (synthetic c/s-isoprene rubber SKI-3) and neodymium salts-based lanthanide catalysts SKI-5 grade synthetic rubber (TU 2294-051-16810126-96). SKI-3 rubbers contain up to 93 1% of as-1,4 links, while their content in SKI-5 grade rubber is 96 1%...

In conclusion, from our point of view, the growing interest in the synthesis of novel metal transition or lanthanide based catalyst systems could offer novel opportunities for further improvements in the nanocompounding process by in-situ polymerization via Ziegler-Natta coordination catalysis. 

The pre-eminent interest in lanthanides for the polymerization and copolymerization of conjugated diolefins is demonstrated by a large scientific and patent literature. A recent paper by one of us (24) reviewed the main worldwide contributions on butadiene and isoprene polymerization. Very recent contributions in this field come from Hsieh and Yeh (25). They observed that the butadiene polymerization in n-pentane evolves as a slurry process, the cis-polybutadiene being insoluble in the light hydrocarbon. They also measured a lower activation energy for Nd-based than for conventional Ziegler-Natta catalysts foi- both butadiene and isoprene polymerization. 

The appearance of an increasing number of processes with long-living active species, such as generation of Ziegler-Natta or lanthanide-based catalysts or new cationic and certain free radical polymerizations with controlled chain propagation, makes the results obtained especially important for polymerization engineering. 

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