In general, the term "plastic" refers to a wide array of materials that are formed on the molecular level by repeating carbon atoms units. Such a long chain of atoms bonded together is referred to as a polymer. So, the simplest and most common plastics are carbon polymers. The repeating carbon units may be thought of as the “backbone” of the plastic. Different types of plastics are produced by changing the groups that are attached to this backbone. This causes properties such as heat tolerance, hardness, resiliency and more to vary between different plastics. This also allows plastics to be fashioned into fibers, sheets, and solids.
Polyethylene is the simplest plastic possible in regards to its elemental composition. It consists of a carbon backbone with two hydrogen atoms attached to each carbon.
Polyethylene is probably the most common plastic in the world. It is found in grocery bags, numerous plastic bottles, toys, and even Kevlar, the protective material within bulletproof vests. You might wonder how the same material can be used to produce both flimsy grocery bags and bulletproof vests, and that is a good question. The difference lies in both the length and arrangement of the backbone. Grocery bags are made of polyethylene that is relatively short and branched. On the other hand, Kevlar consists of tremendously long unbranched polyethylene that can be folded onto itself on the molecular level to become extremely compact and dense.
Polypropylene is also a very common plastic. Its elemental composition is very similar to that of polyethylene. It consists of a carbon backbone. The only difference is that every other carbon of the backbone has a methyl group (CH3) attached to it.