Question

* Why bulk polymarization of Polyethylene requires much higher pressure and temperature compared to gas phase polymarization of polyethylene?

* Between LDPE (0.92 g/cm3) and LLDPE (0.89 g/cm3), which would have better permeability for gases and why?

Answer 1

1...  the polymerization is an exothermic process, i.e. negative enthalpy change, since addition of a monomer to the growing polymer chain involves the conversion of π bonds into σ bonds, or a ring–opening reaction that releases the ring tension in a cyclic monomer. so we know that the sigma bond are stronger bond then pii bond and sigma bond require more energy to break.

In the industrial high pressure polymerization process, manufacturing of ethylene-based copolymers like poly(ethylene-co-acrylic acid) has to be performed at very high pressures (up to 300 MPa) to avoid de-mixing due to unfavourable interactions between product (polymer) molecules and molecules of the educt mixture (ethylene and co-monomer).

2..LDPE is softer, more flexible and melts at a lower temperature than HDPE. LDPE is used in sandwich bags and other types of packaging. HDPE is harder, has a higher chemical resistance and can withstand higher temperatures.