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Polyethene as a Polymer

 
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Low Density Polyethene

Low Density Polyethene (LDPE) was the first polymer of ethylene to be made. It is made under high pressures and temperatures using initiator such as benzoyl peroxide. Keep going along this trail to find out more about initiation reactions on the "Free Radical Polymerisation" page. (Initiation into the order of polymer scientists isn't as bad as you might think, promise!)


The structure of the benzoyl peroxide (BPO) initiator. The O-O bond breaks homolytically (i.e. one electron from the bond goes onto each O) making two radicals. These radicals then start the polymerisation reaction.

Under these conditions, the molecules of polyethene are not simple long chains, but have numerous branches. Just like branches reduce the melting point of small hydrocarbons, these branches keep polyethene from packing closely together, making it relatively soft. Low Density Polythene is used wherever a soft plastic is required - for example, those plastic bags that are often mistaken by turtles for jellyfish are almost invariably LDPE. For more about the chemistry of LDPE formation, head down to "Chain Polymerisation".


The structure of Low Density Polyethylene (LDPE).

High Density Polyethene

High Density Polyethene (HDPE) is made at relatively low temperatures and pressures using special catalysts. These are called Ziegler-Natta catalysts after the two men who were awarded the Nobel Prize for their development and application to polymerisation of ethene and propene.


A typical reactive centre in Ziegler-Natta polymerisation

Use of these catalysts gives an unbranched product that is much harder and more crystalline than LDPE.


The structure of High Density Polyethylene (HDPE).

Linear Low Density Polyethene

Linear Low Density Polyethene (LLDPE) is a polymer made by copolymerisation with other alkenes using Ziegler-Natta catalysts that only has very short branches, so though it is low density it has better mechanical properties than LDPE. Over the last few decades, blends of LLDPE and LDPE have become used for most polyethene film applications - the LDPE gives strength in the molten state so that the polymer can be extruded, while the LLDPE gives the good mechanical properties to the final product.

In recent years, there has been great interest in metallocene catalysts. These perform the same functions as Ziegler-Natta catalysts, but produce polyethenes with superior properties.


A metallocene catalyst for polyethylene polymerisation

The melt strength branching index is a method used in industrial laboratories to gauge how branched a polyethene is - the more long chain branches are present, the harder it is for the molecules to flow past each other, and the more stress a molten strand of the polymer can take.

Short chains are formed by what is called a "backbiting" reactions which tends to give branches only four carbon units long - this happens when the growing polymer chain reacts with itself, pulling a hydrogen off the backbone chain and generating a radical site there that can then add more monomer units. This is illustrated below for polyethene.


The mechanism for short-chain branching in polyethene.