Polyolefins atactic

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 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. 

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.)... 

Alkane dehydrogenation has been demonstrated as a suitable method for the functionalization of polyolefins such as atactic poly(l-hexene) under homogeneous conditions (Equation 12.6) [23]. 

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. 

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. 

Resconi, L. Synthesis of Atactic Polypropylene Using Metallocene Catalysts. In Metallocene-based Polyolefins Preparation, Properties and Technology Scheirs, J., Kaminsky, W., Eds. Wiley Chichester, 2000 Vol. 1, p 467. 

However, the polyolefinic dispersed phase cannot contribute to a large increase in impact performance, due to its crystalline structure, as shown previously by HEIKENS et al. (22). In order to verify this point, a low concentration of atactic polypropylene was introduced in certain blends in the polyolefin phase. 

The most important cases of symmetry-related polymerizations (that is based on the mechanism of site control) of propene are discussed in sections III and IV. When the two sources of chiral induction are either absent or too weak to be effective, atactic or nearly atactic polyolefins are produced. 

Pol5 ropylene is one of the most common and versatile thermoplastics. It is closely related to polyethylene, both of which are members of a group known as polyolefins. The polyolefins are composed of only hydrogen and carbon. Within the chemical structure of PP, a distinction is made between isotactic PP and atactic PP. The isotactic form accounts for 97% of the polypropylene produced. This form is highly ordered having the structure shown below ... 

A significant breakthrough came with the separation of polyolefins by adsorption on a carbon-based colunm (Hypercarb) Macko and Pasch [113] obtained a separation of isotactic, syndiotactic, and atactic polypropylene together with linear polyethylene using a gradient of decanol-TCB in a very short analysis time, as shown in Fig. 30. 

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