What is the fundamental difference in polymerization mechanism between LDPE and HDPE?

The fundamental difference in the polymerization mechanism between LDPE (Low-Density Polyethylene) and HDPE (High-Density Polyethylene) lies in the type of polymerization process and the resulting polymer structure, specifically the degree of branching. LDPE is produced through a high-pressure, free-radical polymerization process using initiators like peroxides. In this process, ethylene monomers add to a growing polymer chain in a random fashion. The high pressure and free-radical mechanism lead to significant chain branching, where short and long side chains are attached to the main polymer backbone. These branches prevent the polymer chains from packing closely together, resulting in a lower density and amorphous structure. HDPE, on the other hand, is produced using coordination polymerization with Ziegler-Natta or metallocene catalysts at lower pressures and temperatures. These catalysts allow for a more controlled polymerization process, resulting in linear polymer chains with very few branches. The linear structure allows the polymer chains to pack closely together, leading to a higher density and crystalline structure. Therefore, the key difference is that LDPE has significant branching due to the free-radical polymerization mechanism, while HDPE has minimal branching due to the coordination polymerization mechanism, resulting in different physical properties.