Metallocene isotactic poly

Table 1 Comparison of degree of crystallinity for metallocene isotactic poly(propylenes) from wide-angle X-ray scattering analyzed by different methods...

Randall, J. C. Alamo, R. G Agarwal, P. K. Ruff, C. J. Crystallization rates of matched fractions of MgCl2-supported Ziegler-Natta and metallocene isotactic poly(propylene)s. 2. Chain microstructures from a supercritical fluid fractionation of a MgCl2-supported Ziegler-Natta isotactic poly(propylene). 

Simultaneous In-Situ SAXS and WAXS Study of Crystallization and Melting Behavior of Metallocene Isotactic Poly(propylene)... 

Dai, P.S., Cebe, P Capel, M Alamo, R.G., and Mandelkern, L. (2000) Simultaneous in-situ SAXS and WAXS study of crystallization and melting behavior of metallocene isotactic poly(propylene), in Scattering from Polymers. Characterization by X-Rays, Neutrons and Light (eds P. Cebe,... 

Figure 2 X-ray diffractograms recorded at room temperature, (a) Metallocene-synthesized isotactic poly(propylene), mmmm — 0.996 crystallized at 145°C. (b) Atactic poly(propylene). Reproduced with permission from Ref. [43], Copyright John Wiley Sons, Inc., 1999.

Table 8. Isotactic poly(propylene) polymerized by bridged- and unbridged-metallocene catalysts... 

Three stereoisomers are possible in the cholestanylindene-derived zir-conocene complexes illustrated in Scheme 67. Two are racem-like, and the other is meso-like depending on the geometry of the metallocene moiety. The stereochemistry of the reaction is controlled by both the structure of the metallocene skeleton and steroidal substituent. Polymerization of propylene with 0-C activated with MAO gave polypropylene of 240,000, about 40% mmmm approximately 70% is due to enantiomorphic site control and the rest is due to chain-end control. Use of the catalyst derived from a /3-A-B mixture produced a mixture of polymers. The a-A and a-B/MAO catalysts afforded isotactic poly-... 

On the other hand, the above phenomena do not apply to Cj-symmetric (nor to syndiospecific (7,-symmetric) metallocenes for these, decreasing monomer concentration either increases the isotacticity and melting point of iPP or has no relevant effect. This is due to the mechanism of site epimerization (also referred to as chain backskip, Scheme 28), in which the chain, at the lower monomer concentrations, has a higher chance to migrate to the less hindered site, which is usually also the more stereoselective. For the same reason, increasing the polymerization temperature either increases the melting point of an isotactic poly(a-olefin), or has no relevant effect.725... 

The resolution of the enantiomers of bridged bis(indenyl) metallocene dichlorides has been accomplished by the replacement of both halides by one enantiomer of binaphthol, followed by chromatography (179). Direct synthesis of enantiomer-ically pure precursors via chiral epoxides has been reported (180). Polymerization of a-olefins using such precursors does not lead to the production of appreciably chiral pol5mier due to the de facto mirror plane which exists in an isotactic poly(a -olefin) of reasonably high degree of polymerization (Fig. 10). 

Fischer, D. Miilhaupt, R. The influence of regio- and stereoirregularities on the crystallization behavior of isotactic poly(propylene)s prepared with homogeneous group IVa metallocene/methylaluminoxane Ziegler-Natta catalysts. MacromoZ. Chem. Phys. 1994,195, 1433-1441. 

C. The only homogeneous catalyst system able to produce highly 1,2-isotactic poly(4-MPD) hitherto reported is the MAO-activated post-metallocene catalyst dichloro[l,4-dithiabutanediyl-2,2 -bis(4,6-di-tert-butyl-phenoxy)]titanium (10, Figure 17.12)." This catalyst was first used to promote the isospecific polymerization of styrene " its successful use to produce highly isotactic l,2-poly(4-MPD) confirms the known similar polymerization behavior of these two monomers. 

The metallocene botactic poly(propylene) (m-iPP) used in thb study is an experimental product of Hocchst. Characterization of the material shows that the fractional content of isotactic pentads (mnunm) b low at 0.908 mol-%, the Mw b 335,500 g/mol, and the polydispersity b 2.3 [20]. Defect content was assessed by C-NMR [20] and the stereo and regio defects arc 1.68mol% and 0.67mol%, respectively. The material, received as pellets, was formed into films by compression molding at 200°C, then was quenched to room temperature in cold water. 

In the second method, the alkoxyamine-ftmctionalized backbone is prepared by a chemical modification of a preformed polymer. Abbasian and Entezami prepared alkoxyamine-functionalized poly(vinyl chloride) (PVC) in a three-step procedure. PVC was first arylated with toluene by Friedel-Crafts acylation followed by a bromination step using N-bromosuccinimide. The bromine atom was finally reacted via nucleophilic substitution by the TEMPO hydro-xylamine anion. PVC-g-PS was finally obtained after TEMPO-mediated polymerization of styrene. A TEMPO-functionalized isotactic poly(l-butene) macroinitiator was synthesized by Jo et al. who used a rhodium-catalyzed activation of the alkane C-H bonds and subsequent transformations of the boronate ester group into an hydroxyl pendant group. This reactive moiety was then used to attach a TEMPO-based alkoxyamine bearing another hydroxy function by an ether linkage. A method to prepare PE-g-PS from a poly(ethylene-co-m,p--methylstyrene) obtained by metallocene-catalyzed polymerization was also reported. The macroalkoxya-mine was synthesized after bromination with N-bromosuccinimide followed by a nucleophilic reaction with the TEMPO hydroxylamine anion. 

Thereafter, many reports on lanthanocenes for stereoselective MMA polymerization have appeared (Fig. 15). Table 1 summarizes some examples of lanthanide-catalyzed stereoselective polymerizations of MMA. Marks et al. developed Ci-symmetric ansa-metallocenes of lanthanides that give isotactic poly(MMA) [ 125]. They introduced a (+)-neomenthyl group on a Cp ring and achieved high isotactidty (31, [mm]=0.94, Mn= 1.04x10, M /Mn>1.8). Similar... 

The non-metallocene ytterbium compound 26 gives highly isotactic poly(MMA) of high molecular weight in toluene, whereas syndiotactic polymer was obtained in THF [130]. Diaza-pentadienyl 27 and azaaUyl lanthanides 28, 29 produce highly isotactic poly(MMA), although the polymer was bimodal and another peak consisted of atactic polymer [131]. 

Soga and Shiono used some C -symmetric ansa-metallocenes for their ZnEtj-assisted system. Typical catalyst precursors for isospecific propylene polymerization, such as 44,45 and 46, produced highly isotactic poly(MMA) with catalyst control [141, 142]. Marks showed that the binuclear cationic species [(rac-MejSillndljZrljlp-Me)] with PBB anion gave highly isotactic poly(MMA) ([mnj]=0.93) [135]. 

Fig. 9.56a Overall birefringence change as a function of crystallization temperature and time for a metallocene catalyzed isotactic poly(propylene), M = 5.75 X 10 with 0.3% chain structural defects. (From Alamo and Chi (209))...

Stevens et al. (2) observed that the haze content of poly(ethylene-co-[isotactic]-propylene) was directly related to the monomer content and whether the material was prepared using a metallocene catalyst. Low haze values were obtained for polymers prepared using nonmetallocene catalytic agents with polymers having moderately high ethylene levels. 

Stereospecific Polymerization. In the early 1950s, Ziegler observed that certain heterogeneous catalysts based on transition metals polymerized ethylene to a linear, high density material at modest pressures and temperatures. N atta showed that these catalysts also could produce highly stereospecific poly-a-olefins, notably isotactic polypropylene, and polydienes. They shared the 1963 Nobel Prize in chemistry for their work. More recently, metallocene catalysts that provide even greater control of molecular structure have been introduced. 

PP-b-PMMA (Mn = 22220, Mw/Mn = 1.14) was produced by CRP via another route. Terminally vinyl PP (Mn = 3100, Mw/Mn = 1.45, isotactic-ity = 32%) prepared using a zirconocene catalyst was converted to terminally brominated PP via PP-SiH prepared by hydrosilylation [70]. The resulting PP-b-PMMA was purified by extraction of unreacted PP with diethyl ether. Poly(ethylene-co-butene)-bZocfc-poly(methyl methacrylate) (EBR-b-PMMA) was synthesized through the bromination of terminally hydroxy-lated EBR (Mw = 3600 g/mol, Mw/Mn = 1.05), which was commercially available [71]. An atactic PP/PMMA had been synthesized by a combination of metallocene catalyses, Cp2ZrCl2 and Me2Si(CpMe4)(.W-f-Bu)TiCl2, and ATRP [72]. 

During the last decade, a variety of new catalysts have been presented for the stereospecific polymerisation of a-olefins, based on non-bridged metallocene or stereorigid ansa-metallocene as the procatalyst and a methylaluminoxane activator [29,30,37,105-107,112-114,116-135], Apart from isotactic [118,119,124, 131,132] and syndiotactic [23,118,124,133] polypropylenes and other poly(a-olefin)s [121], hemiisotactic [112,121,124], isoblock [131,132,134], syndioiso-block (stereocopolymer) [127], stereoblock isotactic [135] and stereoblock isotactic atactic [116,128,129] polypropylenes have been obtained using these new catalysts. 

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