Syndiotactic melting temperature

With MAO activation, Zr- and Hf-FI catalysts 1 and 3 exhibit fairly high reactivity toward propylene and produce propylene oligomers [64, 65], Conversely, the corresponding Ti-FI catalyst/MAO 2 forms semicrystalline PP (1 °C polymerization), which displays a peak melting temperature of 97 °C, indicative of the formation of a stereoregular polymer. To our surprise, microstructural analysis by 13C NMR indicates that the resultant polymer is syndiotactic (rr 19%), and that a chain-end control mechanism is responsible for the observed stereocontrol, regardless of the C2 symmetric catalyst ([28] for the first report on syndiospecific propylene... 

Fig. 34 a, b. Effect of the mixing ratio of isotactic po y(methyi methacrylate) (iso-PMMA) and syndiotactic PMMA cm the melting temperature (Tm) and specific area of the melting endotherms299, (a) Relationship between Tm and crystallization temperature (Tc) for different mixtures of stereoregular PMMA [iso-PMMA]/[synd-PMMA] 1/2, O 1/1, C 2/1, 4/1, (b) Specific areas of melting endotherms of stereocomplexes with different [iso-PMMA]/[synd-PMMA] ratio in bulk mixtures. Ciystallization time O 4 h, O 16 h, 4 days, 2 weeks... 

On the other hand, the temperature of melting and crystallinity measurement can describe only those stereosequences that crystallize. In propylene oxide polymers which we have studied, for example, isotactic sequences are the only ones that crystallize. Therefore, in these polymers we can determine only the stereosequence length of isotactic units and the average length of the uncrystallizable units. The syndiotactic and atactic sequences in the uncrystallizable blocks are not distinguishable from the crystallinity and melting temperature measurements. 

Syndiotactic polystyrene (sPS) represents an important achievement in olefin polymerization catalysis. Syndiotactic PS is an industrially relevant thermoplastic material produced by Dow Chemical and Idemitsu Kosan Co. under the tradenames Questra and Xarec , respectively. Industrial interest on sPS originates from the remarkable properties exhibited by this highly crystalline polymer. The high melting temperature, 270 °C, the relatively fast crystallization rate (at least much faster than that of iPS), the high heat resistance, the low dielectric constant, the high elastic modulus, and an excellent resistance to chemicals explain the industrial interest for this material. Syndiotactic PS was considered as an innovative new resin option for the automotive, electrical, and electronic markets, appliances such as... 

In 1985, Ishihara [10] at Indemitsu, Japan, reported that monocyclopentadienyl, indenyl, or fluorenyl titanium complexes, with different substituents on the Cp derivatives, were highly active for styrene polymerization, producing syndiotactic polystyrene with high melting temperature. Ishihara smdied early transition metals from group 3 or 4 systems based on late transition metals such as Ni were inactive for styrene homo- or copolymerization. Monocyclopentadienyl complexes, also... 

The thermotropic behavior of both the isotactic and syndiotactic poly n-[4 -(4"-methoxyphenyl)phenoxy]alkyl methacrylate s is summarized in Table 11. All of the tactic polymers crystallize. With the exception of poly 2-[4 -(4"-methoxyphe-nyl)phenoxy]ethyl methacrylate] (n=2), the melting temperature of the isotactic polymers is almost independent of the spacer length. In contrast, the syndiotactic polymers melt with a large odd-even alternation. However, only the syndiotactic polymers with at least four carbons in the spacer exhibit an enantiotropic smectic mesophase, which occurs over only a very narrow temperature range. The greater order of the isotactic polymers is evidently due to the greater segmental mobility of isotactic versus syndiotactic polymethacrylate backbones [246, 247]. 

Figure 10-19. Dependence of the melting temperature Tm and glass transition temperature Tg of a 90% syndiotactic l,2-poly(butadiene) on the X-ray crystallinity, olx.

Figure 2. Variation of melting temperature with crystallization temperature for atactic isotactic (O),and syndiotactic (l ) PPBA...

Most polyacrylates, especially the atatic and syndiotactic polyacrylates, are amorphous due to the irregular array of pendant ester groups, which prevent the parallel alignment of the polymer backbones. The T s of the different polyacrylates vary from subambient temperature to values above 100 °C. The T and the melting temperature for some polyacrylates are listed in Table 2. 

SMMA Styrene-methyl methacrylate block copolymer sPS Syndiotactic polystyrene S-S Simha and Somcynsky cell-hole theory SSE Single-screw extmder SSSE Solid-state shear extrusion TD Transverse direction TEM Transmission electron microscopy Tg Glass transition temperature THE Tetrahydrofuran TIBA fr -Isobutyl aluminum (°C) Melting temperature TMA fr -Methyl aluminum TMS Trimethylsilyl... 

The development of metallocene catalysts has made it possible to synthesize syndiotactic polypropylene (sPP). sPP has nearly the same melting temperature as aiPP, but its industrial development seems hindered by its lower overall crystallization rate. As a consequence, the search for appropriate nucleating agents may be essential. sPP has been shown to interact with low molecular weight organics and polymers, and notably aiPP. 

Polybutadiene. Unlike cis- and trans-1,4-polybutadiene, high vinyl 1,2-polybutadiene has a chiral center which can exist in one of three different stere-ochemically related forms. The material can either be atactic, leading to an amorphous elastomer, or it can be isotactic or syndiotactic, both of which are crystalline. The elastomeric amorphous form has found utility in tire tread applications (271) and although certain molybdenum (272) coordination catalysts can produce this material, commercialization has focused on anionic alkali metal initiators modified with Lewis bases. Of the two crystalline forms, isotactic 1,2-polybutadiene with a melting temperature of 126° C is the most elusive isomer. A few chromium systems based on soluble salts and aluminum alkyls have been reported to give 45% of the isotactic polymer in a mixture of the atactic isomer (273,274). 

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