Metallocenes block

Metallocene catalysts produce high-comonomer content products, such as polypropylene block... 

T. C. Chung, New utilities of metallocene catalysts and borane reagents in the functionalization and block/graft reactions of polyolefins, MetCon 95 Proceedings, USA, May 1995. 

A new generation coordination catalysts are metallocenes. The chiral form of metallocene produces isotactic polypropylene, whereas the achiral form produces atactic polypropylene. As the ligands rotate, the catalyst produces alternating blocks of isotactic and atactic polymer much like a miniature sewing machine which switches back and forth between two different kinds of stitches. 

In the polymer filed, new-generation metallocenes, which are currently used in many polyethylene and polypropylene processes, can polymerize proplylene in two different modes alternating blocks of rigid isotactic and flexible atactic. These new developments and other changes and approaches related to polymerization are noted in Chapters 11 and 12. 

Many other shapes are possible for complexes. The simplest are linear, with coordination number 2. An example is dimethylmercury(O), Hg(CI l,)2 (4), which is a toxic compound formed by bacterial action on aqueous solutions of I Ig ions. Coordination numbers as high as 12 are found for members of the / block, but they are rare in the d block. One interesting type of d-mctal compound in which there are 10 links between the ligands and the central metal ion is ferrocene, dicyciopentadi-enyliron(O), [Fe(C5H5)2] (5). Ferrocene is an aptly named sandwich compound, with the two planar cyclopentadienyl ligands the bread and the metal atom the filling. The formal name for a sandwich compound is a metallocene. 

Markel E.J., Weng W., Peacock A.J., and Dekmezian A.H. Metallocene-based branched-block thermoplastic elastomers. Macromolecules, 33, 8541, 2000. 

Ferrocene has been extensively used as a building block to prepare organo-metallic polymers and heteroatom-bridged poly(metallocenes)(G )-(G ")(Fig 4). 

Polymers with blocks containing different tactcities can be produced, e.g., atactic PP (amorphous)/isotactic PP (semi-crystalline) can be made using metallocene catalysts. They behave in a manner similar to SBS thermoplastic elastomers. 

A half-metallocene iron iodide carbonyl complex Fe(Cp)I(CO)2 was found to induce the living radical polymerization of methyl acrylate and f-bulyl acrylate with an iodide initiator (CH3)2C(C02Et)I and Al(Oi- Pr)3 to provide controlled molecular weights and rather low molecular weight distributions (Mw/Mn < 1.2) [79]. The living character of the polymerization was further tested with the synthesis of the PMA-fc-PS and PtBuA-fi-PS block copolymers. The procedure efficiently provided the desired block copolymers, albeit with low molecular weights. 

Metallocene catalysis has been combined with ATRP for the synthesis of PE-fr-PMMA block copolymers [123]. PE end-functionalized with a primary hydroxyl group was prepared through the polymerization of ethylene in the presence of allyl alcohol and triethylaluminum using a zirconocene/MAO catalytic system. It has been proven that with this procedure the hydroxyl group can be selectively introduced into the PE chain end, due to the chain transfer by AlEt3, which occurs predominantly at the dormant end-... 

Based on Chien s research results, Collins et al. modified the basic structure of the catalysts and also achieved elastic material [8,18,19]. In both cases the elastic properties of the polymers are justified in a block structure with isotactic and atactic sequences. In 1999 Rieger et al. presented a couple of asymmetric, highly active metallocene catalysts, e.g., the dual-side catalyst rac-[l-(9-r 5-fluorenyl)-2-(5,6-cyclo-penta-2-methyl-l-q5-indenyl)ethane]zirconium dichloride (Fig. 3). These catalysts allowed building of isolated stereoerrors in the polymer chain to control the tacticity and therefore the material properties of the polymers [9],... 

Llinas GH, Dong SH, Mallin DT, Rausch MD, Lin YG, Winter HH, Chien JCW (1992) Crystalline-amorphous block polypropylene and nonsymmetric ansa-metallocene catalyzed polymerization. Macromolecules 25 1242-1253... 

As stated above, we postulated that fast, reversible chain transfer between two different catalysts would be an excellent way to make block copolymers catalytically. While CCTP is well established, the use of main-group metals to exchange polymer chains between two different catalysts has much less precedent. Chien and coworkers reported propylene polymerizations with a dual catalyst system comprising either of two isospecific metallocenes 5 and 6 with an aspecific metallocene 7 [20], They reported that the combinations gave polypropylene (PP) alloys composed of isotactic polypropylene (iPP), atactic polypropylene (aPP), and a small fraction (7-10%) claimed by 13C NMR to have a stereoblock structure. Chien later reported a product made from mixtures of isospecific and syndiospecific polypropylene precatalysts 5 and 8 [21] (detailed analysis using WAXS, NMR, SEC/FT-IR, and AFM were said to be done and details to be published in Makromolecular Chemistry... 

Some exchange reactions of complexes 112 and 115 have been studied. From the X-ray crystal structure analyses, it appears that trimethylgallium is rather loosely bound to the (r 2-aryne)- and (r 2-cydohexyne) metallocene building blocks in the dimetalla-bicyclic complexes 112 and 115, respectively (Scheme 7.34). Therefore, it was tempting to investigate whether it was possible to reverse the reactions depicted in Schemes 7.32 and 7.33 using these specific examples to carry out thermally induced exchange reactions. 

Metalloboranes, 4 172 exopolyhedral, 4 208-210 main group element, 4 207-208 transition element, 4 205-207 Metallo-carbohedrene clusters, 4 648 Metallocarboranes, 4 170 as catalysts, 4 217-218 economic aspects, 4 229 exopolyhedral, 4 215-216 f-block element, 4 225-226 host-guest chemistry-carborane anticrowns, 4 216-217 structural systematics, 4 176-179 transition metal, 4 210-215 Metallocene catalysis, MAO in, 16 92-93. 

Recent advances in the development of well-defined homogeneous metallocene-type catalysts have facilitated mechanistic studies of the processes involved in initiation, propagation, and chain transfer reactions occurring in olefins coordi-native polyaddition. As a result, end-functional polyolefin chains have been made available [103].For instance, Waymouth et al.have reported about the formation of hydroxy-terminated poly(methylene-l,3-cyclopentane) (PMCP-OH) via selective chain transfer to the aluminum atoms of methylaluminoxane (MAO) in the cyclopolymerization of 1,5-hexadiene catalyzed by di(pentameth-ylcyclopentadienyl) zirconium dichloride (Scheme 37). Subsequent equimolar reaction of the hydroxyl extremity with AlEt3 afforded an aluminum alkoxide macroinitiator for the coordinative ROP of sCL and consecutively a novel po-ly(MCP-b-CL) block copolymer [104]. The diblock structure of the copolymer... 

The effect of blending LDPE with EVA or a styrene-isoprene block copolymer was investigated (178). The properties (thermal expansion coefficient. Young s modulus, thermal conductivity) of the foamed blends usually lie between the limits of the foamed constituents, although the relationship between property and blend content is not always linear. The reasons must he in the microstructure most polymer pairs are immiscible, but some such as PS/polyphenylene oxide (PPO) are miscible. Eor the immiscible blends, the majority phase tends to be continuous, but the form of the minor phase can vary. Blends of EVA and metallocene catalysed ethylene-octene copolymer have different morphologies depending on the EVA content (5). With 25% EVA, the EVA phase appears as fine spherical inclusions in the LDPE matrix. The results of these experiments on polymer films will apply to foams made from the same polymers. 

Unbridged metallocenes rarely achieve highly stereoselective polymerizations because free rotation of the r 5-ligands results in achiral environments at the active sites. An exception occurs when there is an appreciable barrier to free rotation of the r 5-ligands. Fluxional (con-formationally dynamic) metallocenes are initiators that can exist in different conformations during propagation. Stereoblock copolymers are possible when the conformations differ in stereoselectivity and each conformation has a sufficient lifetime for monomer insertion to occur prior to conversion to the other conformation(s). Isotactic-atactic stereoblock polymers would result if one conformation were isoselective and the other, aselective. An isotactic-atactic stereoblock polymer has potential utility as a thermoplastic elastomer in which the isotactic crystalline blocks act as physical crosslinks. 

Block copolymers have been successfully synthesized because many metallocene polymerizations of MMA proceed as living polymerizations, and it is possible to have a single one-way crossover from carbanion (alkene) polymerization to MMA (enolate) polymerization with metallocene and related initiators, especially when group 3 transition metal initiators are used [Boffa and Novak, 2000 Desurmont et al., 2000a,b Jin and Chen, 2002 Yasuda et al., 1992],... 

Group 4 metallocene catalysts are also applicable to the above sequential block co-polymerization method to furnish polyolefin and polar polymer block co-polymers. Frauenrath et al. and Chen and Jin " reported the synthesis of PE-/ -PMMA and PP-3-PMMA, respectively, using metallocene catalysts (e.g., raz--(C2H4)(Ind)2ZrMe2/ B(C5F5)3, iPP-/ -iPMMA PP segment 4/ = 8900, PDI= 1.90 block co-polymer 4/n = 10900, PDI = 1.66, MMA content = 17.1 mol%). 

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