Why isn’t there LDPP, HDPP?

Question

Unlike polyethylene that there are LDPE, HDPE, or LLDPE, when it comes to polypropylene the issue becomes about copolymer (homo/ copo/ block copolymer) or location of methyl group (isotactic/ atactic/ syndiotactic)? Why does polypropylene not have designations based on density?

Answer 1

First off, lets understand how densities of poly(ethylene) are differentiated. The prototypical polyethylene is often represented as a simple repeating ethylene monomer units as shown below:

This is what high-density polyethlyene (HDPE) is constructed of; simple linear monomer units that form closely packed crystal structures. This polymer is synthesized by free-radical polymerization with catalyses that encourages radical at the end of the chain, keeping the polymer linear. low density polyethylene (LDPE) is free-radically polymerized without this control, which causes branching at random points in the polymer chain. This branching prevents the crystallization that made polyethylene more dense, thus it is more amorphous and lower density. To make a linear low density polyethylene (LLDPE), ethylene is co-polymerized with longer olefins such as 1-butene, 1-hexene or other olefins but with catalysts controlling polymerization similar to polymerization of HDPE. A diagram demonstrating molecular structure differences is shown below:

_{Source: Plastics Technologies}

Bringing this back to about polypropylene, the co-polymers are a different monkey that are used for precisely tuning properties and that is not useful for this discussion. But as you mention the tactility is important. There are two reasons for this. 1.) there is no reason to produce low-density polypropylene as LDPE and LLDPE are cheaper materials that are more mature in their development with plenty of material data available and developed recycling streams, thus there is little incentive to investigate LDPP. 2.) polypropylene is already controlled by its tacticity. Isotactic and syndiotactic polymers crystallize fairly well while atactic polymers do not. Since polypropylenes rigidity and density can be controlled by using certain polymerization catalysts there is again little incentive to produce branched polypropylene