I never knew polymers properties were attributed to their chemical composition. As example, substituents create less dense polymers.
Interesting note, the density depends on the amount of pressure and heat.
High density polymers are made by slight high pressure and a catalyst. In a macular view, these molecules are able to line up together.
Low density polymers are caused by high pressure and O₂ . The conditions are depending on low or high density that react with the polymers.
Below is an image of the families of esters created using MarvinJS.