Polymers are large molecules formed by chemical bonding of many smaller molecules called monomers. There are two main types of polymers: thermoplastics and thermosets. Thermoplastics can be remelted and reshaped, while thermosets harden permanently after heating. Common polymerization reactions include bulk, suspension, solution, and emulsion polymerization. Biodegradable polymers break down into natural byproducts, while bioplastics are made from renewable sources. Polymer properties depend on factors like tacticity, polydispersity index, and glass transition temperature. Examples of important polymers include polyethylene, nylon, polyesters, PVC, Teflon, and conducting polymers.