Synthetic polymers are made through a process called polymerization, where monomers (small, single molecules) are chemically bonded together to form long polymer chains. This can be achieved via two primary methods: addition polymerization and condensation polymerization. Addition polymerization involves monomers adding to each other without the loss of atoms from the original monomers, commonly initiated by heat, light, or catalysts, leading to polymers like polyethylene and polystyrene. Condensation polymerization, on the other hand, involves the co-joining of monomers with the concurrent elimination of small molecules such as water or alcohol, forming polymers like nylon and polyester. The choice of monomers and the specific conditions under which they're polymerized (temperature, pressure, catalysts) dictate the characteristics of the resulting polymer, including its strength, flexibility, and resistance to chemicals and temperature.

Polyethylene, a thermoplastic polymer consisting of long hydrocarbon chains, is known for its resistance to many chemical solvents, making it challenging to dissolve. However, certain chemicals have been identified that can break down or dissolve polyethylene. Among these, aromatic hydrocarbons like xylene and toluene are effective, albeit requiring high temperatures for the process to proceed efficiently. Additionally, some chlorinated solvents have also shown the ability to dissolve polyethylene, but their use is often limited due to environmental and health considerations. In research and industrial applications, a specialized solvent named "para-xylene" is noted for its ability to dissolve polyethylene at a faster rate and at somewhat lower temperatures. It's important to handle these chemicals with care due to their potential health and environmental hazards.

Putting PVC under a sidewalk involves the use of a technique called horizontal boring or pipe pushing. It avoids breaking up the sidewalk and is quite efficient. Start by digging a hole on each side of the sidewalk deep enough to be below the base of the sidewalk. The PVC pipe, attached to a boring tool or a piece of solid steel pipe (often referred to as a "bullet"), is then hammered through the soil beneath the sidewalk from one hole to the other. The process requires a sledgehammer or a mechanical driver, depending on the length of the sidewalk and soil conditions. Ensure the pipe is slightly angled upwards towards the exit point to help reduce friction and facilitate the movement under the sidewalk. Once the pipe is through, it can be used to run electrical wires, water lines, or for drainage. Always check for existing utilities before starting and consider consulting a professional for complicated projects.