APP/iPP

Appending asymmetric aryl groups to Cp in CpMCl3 (M = Ti, Zr) has been shown to induce the formation of PP-containing aPP/iPP stereoblocks, the length of which strongly depends on the polymerization temperature.710 711... 

PEOD) (OD =11 mol%), but immiscible with poly(ethylene-co-dodecene)(PEDD)(DD = 19.5 mol%), based on solid state NMR results [347]. The effect of stereo-structure of PP blends was reported, showing aPP/iPP blends were miscible, whereas aPP/sPP blends were phase separated [348]. Theoretical predictions using solubility parameter concepts were in agreement with experimental results. Similar results were noted by Phillips [349], where aPP/iPP blends were noted to be miscible with an ucst below 155 °C. The iPP/sPP blends were noted to be immiscible in the melt. 

In reality, finding a suitable solvent is not as easy as simply matching the polymer s solubility parameter (8 value). It is also important to take into account the effects of polymer crystallinity (as in the case of aPP and iPP, LDPE and HDPE). Because of their various chemical structures, it may be necessary to experiment with solvent, temperature, and time conditions to optimise the extraction strategy. 

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... 

AFM Atomic force microscopy aPP Atactic polypropylene DSC Differential scanning calorimetry HDPE High-density polyethylene iPP Isotactic polypropylene LLDPE Linear low-density polyethylene MD Microdomain ODT Order-disorder transition PB Poly(butadiene)... 

FIGURE 2.8 Representation of (a) a crystalline portion from isotactic polypropylene (iPP), and (b) an amorphous portion from atactic polypropylene (aPP). 

Which would you expect to form better helical structures (a) iPP or (b) aPP ... 

Atactic or amorphous forms of PP are also used. Initially, aPP was obtained as a byproduct of the production of iPP. As an inexpensive by-product it is used as a modifier for asphalt for roofing and in adhesives. As the effectiveness of catalyst systems becomes better, less aPP is available so that today some aPP is intentionally made for these applications. 

What are some physical properties that iPP would have in comparison with aPP ... 

The reactions catalyzed by prenyltransferases are unique and interesting from a mechanistic viewpoint. The reaction starts with elimination of the diphosphate ion from an allylic diphosphate (APP) to form an allylic cation, which is attacked by the IPP molecule, with stereospecific removal of a proton to form a new C-C bond and a new double bond in the product. By repeating this type of condensation between IPP and the allylic prenyl diphosphate product, prenyltransferase can synthesize a prenyl diphosphate with a certain length and stereochemistry fixed by ifs specificify (seeFig. 10) [252]. 

Rubber is synthesized and sequestered on cytsolic vesicles known as rubber particles. Rubber transferase is localized to the surface of the rubber particles, and biosynthesis is initiated through the binding of an allylic pyrophosphate (APP, a pyrophosphate, produced by soluble trans- rtnyl transferases) primer. Progressive additions of IPP molecules ultimately result in the formation of high molecular weight cjT-l,4-polyisoprene. The rubber transferase also requires a divalent cation, such as Mg + or Mn +, as cofactor. 

Enzymatically active, partially purified (washed) rubber particles can be isolated such that, when provided with an appropriate APP primer, magnesium ion cofactor, and IPP monomer, rubber is produced in vitro [253-255]. Fresh latex can be separated by centrifugation into three phases. The bottom fraction (20% of the latex) contains membrane-bound organelles. The middle fraction is called the C-serum. The top fraction phase contains the rubber particles. Biochemical smdies have established that latex in this fractionated form is unstable. These smdies also suggest that the bottom fraction is required for initiation of polymer synthesis. 

Rubber molecules are synthesized from one APP molecule, which initiates the reaction, and the rubber polymer (cw-l,4-polyisoprene) is then polymerized by sequential condensations of the non-allylic IPP (magnesium cations are a required cofactor) with release of a diphosphate at each condensation. After initiation and elongation, a termination event occurs in which the rubber molecule is released from the enzyme. Despite the similar process, remarkable differences exist between plant species with respect to enzymatic reaction mechanisms and product molecular weight. 

Although many different APPs are effective initiators of rubber biosynthesis, only IPP can be used as the source of isopentenyl monomer for the c/x-1,4-polymerization of the rubber polymer. 

Some properties of isotactic, syndiotactic, and atactic PP are listed in Table 12.2.17 The insolubility of iPP in hydrocarbon solvents at room temperature often is used to separate iPP from atactic polypropylene (aPP). 

The feedstock materials used in this work included medium density polyethylene (PE), atactic-polypropylene (aPP), isotactic-polypropylenc (iPP), beech wood, pine wood, cellulose and hydrolytic lignin. The size of the wood biomass and the plastic particles was less 0,1 mm. 

As shown in Figure 23, PP is no longer able to crystallize when the stereoregularity of the chains is reduced below a threshold value (below about 70% m diad, or 40% mmmm pentad content for iPP, or below about 60% rrrr pentad content for sPP), and it becomes amorphous (amPP). When statistical randomness in the sequence of chirotopic methynes in the polymer chain is reached, the polymer is called atactic (aPP). In this case the pentad distribution is perfectly random Bcrnoullian mmmm mmmr rmmr mmrr (rmrr+ mrmm) mrmr rrrr rrrm mrrm 1 2 1 2 4 2 1 2 1. 

Materials. The amorphous polypropylene (APP) used is that of unstabilized Eastobond from Tennessee Eastman. The semicrystalline polypropylene (IPP) is the Profax 6501 from Hercules Incorporated. It has a crystallinity of 61% as determined from its density (4). 

Pyrolysis of Polypropylene. For the measurement of rates of pyrolysis, the temperature range is limited to that of conveniently measurable rates. Figure 2 shows the thermograms of APP and IPP which provides the choice of temperature for pyrolysis. 

Even though the pyrolysis of polypropylene is mechanistically complicated (vide infra), the kinetics is first order because the rate determining step is the homolysis of the C—C bond describable by a well-defined rate constant. We found the activation energy for pyrolysis to be 51 and 56 kcal mol"1, respectively, for IPP and APP. Other literature values are 60 kcal mol"1 measured at 350°-400°C by Wall and Straus (10), 58 kcal mol 1 in the range 336°-366°C reported by Madorsky and Straus (12) and 55 kcal mol"1 found by Moissev et al. (13) in the temperature range 320°-420°C. 

The choice of date range is arbitrary. The number of journal articles for each year was obtained from a search of electronic version of English-based polymer and polymer-related journals using the keywords polyolefin and blends. Within polyolefin keyword, the subkeywords used in the search were polyethylene (PE, LLDPE, LDPE, HDPE, UHMWPE, PE, etc.), polypropylene (PP, iPP, sPP, aPP, etc.), polybutene-1, poly-4-methylpentene-l, ethylene-diene monomer, ethylene-propylene-diene terpolymer, ethylene propylene rubber, thermoplastic olefins, natural rubber (NR), polybutadiene, polyisobutylene (PIB), polyisoprene, and polyolefin elastomer. For the polyolefin blends patent search, polymer indexing codes and manual codes were used to search for the patents in Derwent World Patent Index based on the above keywords listed in the search strategy. 

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