Formation of Polyethylene (1 Viewer)

[cutemouse]

Hi,

Can anyone please give me the reactions for the production of LDPE and HDPE?

Thanks,

Jason

 

[minijumbuk]

LDPE: Under high pressure (1000 to 3000 times atmospheric) and high temperature ie. 300^oC

1. Initiation:
An organic peroxide is added to a pool of chloroethene monomers. These organic peroxides have the general formula CH₃(CH₂)_n- O - O - (CH₂)_nCH₃
These O - O bond breaks, and form what I call half peroxides (I'm not sure if it's a real term), and become 'initiators' for the reaction.

E.g. CH₃ - O - O - CH₃ --> 2CH₃O.

A chloroethene molecule (monomer) attaches itself to the half peroxide.

CH₃O. + CH₂=CH₂ --> CH₃-O-CH₂CH₂.

2. Propagation:
More monomers join to the previous, forming a polymer chain consisting of repeating units of -(CH₂-CH₂)-
CH₃-O-CH₂CH₂. + n (CH₂=CH₂) --> CH₃-O-CH₂CH₂CH₂CH₂CH₂CH₂CH₂CH₂.

Note: After showing around 3 repeating units, you can stop. I'll show you the formula to write for polymerisation after the last step.

3. Termination:
Either another polymer half chain attaches to another half to form 1 full polymer chain, or an alkyl group (if I remember correctly) is added to terminate the propagation process.

So basically:

n(CH₂=CH₂) --> —(CH₂CH₂)_n—
n is typically 2000-3000.

Note that you don't have to include the half peroxide at each end of the polymer in this equation, as it contributes so little to the molecular properties, that it is insignificant.

This process produces highly branching polymer chains, which means that it's less likely to pack together, decreasing density, hence the name "Low density PE".

HDPE:
This process is called the Ziegler-Natta process. It uses few times atmospheric pressrue, ad about 60^oC, and uses a catalyst, which is a mixture of titanium (III) chloride and a trialkylaluminium compound, e.g. triethylaluminium.

No initiators are used in this production method, and this process forms unbranched PE molecules, which allows them to pack closely, increasing the density and hence the name "High density PE"

 

[danz90]

LDPE: Under high pressure (1000 to 3000 times atmospheric) and high temperature ie. 300^oC

1. Initiation:
An organic peroxide is added to a pool of chloroethene monomers. These organic peroxides have the general formula CH₃(CH₂)_n- O - O - (CH₂)_nCH₃
These O - O bond breaks, and form what I call half peroxides (I'm not sure if it's a real term), and become 'initiators' for the reaction.

E.g. CH₃ - O - O - CH₃ --> 2CH₃O.

A chloroethene molecule (monomer) attaches itself to the half peroxide.

CH₃O. + CH₂=CH₂ --> CH₃-O-CH₂CH₂.

2. Propagation:
More monomers join to the previous, forming a polymer chain consisting of repeating units of -(CH₂-CH₂)-
CH₃-O-CH₂CH₂. + n (CH₂=CH₂) --> CH₃-O-CH₂CH₂CH₂CH₂CH₂CH₂CH₂CH₂.

Note: After showing around 3 repeating units, you can stop. I'll show you the formula to write for polymerisation after the last step.

3. Termination:
Either another polymer half chain attaches to another half to form 1 full polymer chain, or an alkyl group (if I remember correctly) is added to terminate the propagation process.

So basically:

n(CH₂=CH₂) --> —(CH₂CH₂)_n—
n is typically 2000-3000.

Note that you don't have to include the half peroxide at each end of the polymer in this equation, as it contributes so little to the molecular properties, that it is insignificant.

This process produces highly branching polymer chains, which means that it's less likely to pack together, decreasing density, hence the name "Low density PE".

HDPE:
This process is called the Ziegler-Natta process. It uses few times atmospheric pressrue, ad about 60^oC, and uses a catalyst, which is a mixture of titanium (III) chloride and a trialkylaluminium compound, e.g. triethylaluminium.

No initiators are used in this production method, and this process forms unbranched PE molecules, which allows them to pack closely, increasing the density and hence the name "High density PE"

spot on