Materials Similar to Polyethylene

Materials that are quite similar to polyethylene can be obtained from other starting materials. The most prominent is formation of polymethylene and similar high molecular weight paraffin hydrocarbons from diazoalkanes. The reaction was originally carried out by Pechmann [27] when small quantities of a white flocculent powder formed in an ether solution of diazomethane. Bamberger and Tschirner [28] showed that this white powder is polymethylene -(-CH₂-), that melts at 128°C. The synthesis was improved since by introduction of various catalysts. The reaction can yield highly crystalline polymers that melt at 136.5°C [29] with the molecular weight in millions [30]. Among the catalysts, boron compounds are very efficient [30]. Bawn et al. [31] postulated the mechanism of catalytic action. It consists of initial coordination of a monomer with the initiator, BF3. This is followed by a loss of nitrogen and a shift of a fluorine atom from boron to carbon. The successive additions of molecules of diazoalkane follow a similar path with a shift of the chain fragment to the electron-deficient carbon:

The resulting macromolecules are still reactive toward additional diazoalkanes. The above step-growth polymerization reactions can also yield block copolymers:

Formation of polymethylene by this reaction is not practical for commercial utilization. Colloidal gold and fine copper powder also catalyze diazoalkane polymerizations. The reaction appears to precede by formation of alkylidine or carbene species that are bound to the surfaces of metals [31–33]. The initiations are completed by additions of diazoalkanes to the bound carbenes followed by liberation of nitrogen. Termination may take place by chain transfer, perhaps to a monomer, or to the solvent [31–33]. Many different diazoalkanes lend themselves to these polymerization reactions. Polypentylidine, polyhexylidine, polyheptilidine, and polyoctylidine form with a gold complex catalyst, AuCl3 pyridine [34]. An entirely different route to preparation of macro paraffins is through a high-pressure reaction between hydrogen and carbon monoxide. Transition metals, like finely divided ruthenium, catalyze this reaction. At pressures of about 200 atm and temperatures below 140C, polymethylene of molecular weight as high as 100,000 forms [35]:

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مواضيع ذات صلة

Manufacturing Techniques

Polypropylene

Commercial High-Density Polyethylene, Properties, and Manufacture

Preparation of Polyethylene by Coordination Mechanism

Preparation of Polyethylene by a Free-Radical Mechanism

Polyethylene and Related Polymers

Thermodynamics of Ring-Opening Polymerization

Ring-Opening Polymerizations by a Free Radical Mechanism

Polymerization of Trioxane

Ring-Opening Polymerizations of Cyclic Acetals

Polymerization of Oxepanes

The Propagation Reaction