Polyolefins isotactic

Polypropylene is the second most important commercial polyolefin. Isotactic PP has the lowest density (0.90-0.91 g/mL) of the major plastics. It has a high crystalline melting point of 165°C. The first commercial production of polypropylene was in the 1950s following the discovery of Z-N catalysts. 

Polymorphism. Many crystalline polyolefins, particularly polymers of a-olefins with linear alkyl groups, can exist in several polymorphic modifications. The type of polymorph depends on crystallisa tion conditions. Isotactic PB can exist in five crystal forms form I (twinned hexagonal), form II (tetragonal), form III (orthorhombic), form P (untwinned hexagonal), and form IP (37—39). The crystal stmctures and thermal parameters of the first three forms are given in Table 3. Form II is formed when a PB resin crystallises from the melt. Over time, it is spontaneously transformed into the thermodynamically stable form I at room temperature, the transition takes about one week to complete. Forms P, IP, and III of PB are rare they can be formed when the polymer crystallises from solution at low temperature or under pressure (38). Syndiotactic PB exists in two crystalline forms, I and II (35). Form I comes into shape during crystallisation from the melt (very slow process) and form II is produced by stretching form-1 crystalline specimens (35). 

Analytical and test methods for the characterization of polyethylene and PP are also used for PB, PMP, and polymers of other higher a-olefins. The C-nmr method as well as k and Raman spectroscopic methods are all used to study the chemical stmcture and stereoregularity of polyolefin resins. In industry, polyolefin stereoregularity is usually estimated by the solvent—extraction method similar to that used for isotactic PP. Intrinsic viscosity measurements of dilute solutions in decahn and tetraHn at elevated temperatures can provide the basis for the molecular weight estimation of PB and PMP with the Mark-Houwiok equation, [rj] = KM. The constants K and d for several polyolefins are given in Table 8. 

Polyolefins. The most common polyolefin used to prepare composites is polypropylene [9003-07-0] a commodity polymer that has been in commercial production for almost 40 years following its controlled polymerisation by Natta in 1954 (5). Natta used a Ziegler catalyst (6) consisting of titanium tetrachloride and an aluminum alkyl to produce isotactic polypropylene directly from propylene ... 

Blends of isobutylene polymers with thermoplastic resins are used for toughening these compounds. High density polyethylene and isotactic polypropylene are often modified with 5 to 30 wt % polyisobutylene. At higher elastomer concentration the blends of butyl-type polymers with polyolefins become more mbbery in nature, and these compositions are used as thermoplastic elastomers (98). In some cases, a halobutyl phase is cross-linked as it is dispersed in the polyolefin to produce a highly elastic compound that is processible in thermoplastic mol ding equipment (99) (see Elastomers, synthetic-thermoplastic). ... 

Aliphatic Polyolefins other than Polyethylene, and Diene Rubbers 11.1.4 Additives for isotactic polypropylene... 

The brittleness of isotactic polystyrenes has hindered their commercial development. Quoted Izod impact strengths are only 20% that of conventional amorphous polymer. Impact strength double that of the amorphous material has, however, been claimed when isotactic polymer is blended with a synthetic rubber or a polyolefin. 

Polypropylene block and graft copolymers are efficient blend compatibilizers. These materials allow the formation of alloys, for example, isotactic polypropylene with styrene-acrylonitrile polymer or polyamides, by enhancing the dispersion of incompatible polymers and improving their interfacial adhesion. Polyolefinic materials of such types afford property synergisms such as improved stiffness combined with greater toughness. 

Brosse et al. [41] modified isotactic polypropylene and other polyolefins by a cold plasma. In isotactic polypropylene, plasma treatment results in a polypropylene crystallization of paracrystalline or smectic form into a a-crystalline form. Further, the active films are susceptible to react with monomers in a postgrafting reaction. 

As with MAH, the extent of grafting varies dramatically vi ith the polyolefin substrate. Some differences have been attributed to variations in the type and amount of stabilizers present in the polyolefins substrate.326 In the case of isotactic PP, the maximum graft levels attained with 27 were found to correspond to only one unit of DEM per PP molecule 5 0 This would support a mechanism whereby grafts appear only at the chain ends. Higher graft levels were obtained with atactic PP. The higher reactivity of the atactic PP (and atactic sequences in... 

The metal catalyzed production of polyolefins such as high density polyethylene (HDPE), linear low density polyethylene (LLDPE) and polypropylene (PP) has grown into an enormous industry. Heterogeneous transition metal catalysts are used for the vast majority of PE and all of the PP production. These catalysts fall generally within two broad classes. Most commercial PP is isotactic and is produced with a catalyst based on a combination of titanium chloride and alkylaluminum chlorides. HDPE and LLDPE are produced with either a titanium catalyst or one based on chromium supported on silica. Most commercial titanium-based PE catalysts are supported on MgCl2. 

Although PFE lacks a proven total concept for in-polymer analysis, as in the case of closed-vessel MAE (though limited to polyolefins), a framework for method development and optimisation is now available which is expected to be an excellent guide for a wide variety of applications, including non-polyolefinic matrices. Already, reported results refer to HDPE, LDPE, LLDPE, PP, PA6, PA6.6, PET, PBT, PMMA, PS, PVC, ABS, styrene-butadiene rubbers, while others may be added, such as the determination of oil in EPDM, the quantification of the water-insoluble fraction in nylon, as well as the determination of the isotacticity of polypropylene and of heptane insolubles. Thus PFE seems to cover a much broader polymer matrix range than MAE and appears to be quite suitable for R D samples. 

Inspired by the design of metallocence catalysts, there has been research into the use of C2-symmetric Ni a-diimine catalysts for the preparation of polyolefins with stereoregularity. Such catalysts were shown to afford higher degrees of isotacticity in polypropylenes as compared to the standard C2v-symmetric catalysts, which afford mostly syndiotactic polymer [106], Coates and coworkers have studied... 

The rhodium-catalyzed borylation of alkanes is applied to regiospecific functionalization of polyolefines.165,165a The reaction of polypropylenes (atactic, isotactic, and syndiotactic) with B2pin2 in the presence of Cp Rh( 74-C6Me6) catalyst at 200 °G affords the borylated polymers, which are treated with basic hydrogen peroxide in a mixture of THF and H20 to oxidize the boronate esters to the corresponding alcohols (Scheme 20). The hydroxylated polymers contain 0.2-1.5% hydroxymethyl side-chains. 

Data concerning the chain conformations of isotactic polymers are reported in Table 2.1. In all the observed cases the torsion angles do not deviate more than 20° from the staggered (60° and 180°) values and the number of monomeric units per turn MIN ranges between 3 and 4. Chains of 3-substituted polyolefins, like poly(3-methyl-l-butene), assume a 4/1 helical conformation (T G )4,45,46 while 4-substituted polyolefins, like poly(4-methyl-1-pentene), have less distorted helices with 7/2 symmetry (T G )3.5-39 When the substituent on the side group is far from the chain atoms, as in poly(5-methyl-1-hexene), the polymer crystallizes again with a threefold helical conformation (Table 2.1). Models of the chain conformations found for the polymorphic forms of various isotactic polymers are reported in Figure 2.11. 

Similarly large anisotropies were later reported for highly emissive blends of alkoxy-substituted bis(phenylethynyl)benzene derivatives and polyolefins such as linear low-density polyethylene (LLDPE) and isotactic polypropylene (z-PP) [8,9]. The latter systems reach high levels of anisotropy at very low draw ratios, which is advantageous from a processing point of view. 

Like polypropylene, PVC has the problem of stereospecificity. The carbon atom to which the chlorine atom is attached is asymmetrical. (See Figure 23-8.) As a result, PVC molecules can be iso tactic, syndiotactic, and atactic. Commercial PVC is only 5—10% crystalline—low percent isotactic. It is more dense, 1.3 to 1..8 g/cc, than the polyolefins, (fe Figure 23—9.)... 

Single crystals with a Tm of 423 K have been produced from low-density polyethylene (ldpe). Isotactic PP crystals have a Tm of 444 K and syndiotactic PP has a Tm of 411 K, whereas atactic PP is amorphous and has a Ts of 255 K. Isotactic polyolefins with pendant groups, such as polyhexene, have high Tm values. Random copolymers of ethylene and propylene are amorphous, but block copolymers of these monomers are crystalline. 

An analysis of the ionic factors for the polymerization of dienes to cis and trans structures is possible in the same way as for isotactic mono-enes. The mechanism which controls the steric structure of poly 1,4 dienes is parallel to that we have already seen for the mono-olefins. Roha (2) listed the catalysts which polymerize dienes according to the polymer structures produced. It was shown that the highly anionic as well as the highly cationic catalyst systems with increasing ionic separation produced trans-poly-1,4-dienes. This is analogous to the production of syndiotactic polyolefins. 

Highly crystalline isotactic polyolefins are not soluble in organic solvents at room temperature. However, most amorphous polyolefins and oligomers of cr-olefins are easily soluble in saturated and aromatic hydrocarbons at ambient temperature. This difference in solubility can be used to separate amorphous atactic components of polyolefins from crystalline isotactic material in crude polyolefins mixtures. 

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