Why doesn't PVC have chlorines on adjacent atoms?

Polyvinyl chloride, commonly known as PVC, is a widely used plastic with a distinctive structure responsible for its physical properties and applications. The structure of PVC can be represented as a long chain of repeating units, known as monomers, that are linked together to form a polymer. The monomer unit for PVC is vinyl chloride, which consists of a two-carbon chain with one chlorine atom attached to one of these carbons.

The high degree of chlorine content in PVC gives it unique characteristics such as rigidity, resistance to moisture and chemicals, and flame retardance. It's essential to recognize that in the polymer chain, each chlorine atom is bound to a carbon atom, with the chlorine atoms alternating from one side of the carbon chain to the other. This alternating pattern arises from the specifics of the PVC polymerization process, which we will delve into later. The lack of chlorines on adjacent carbon atoms is due to the orientation of the monomers during the polymerization.

The vinyl chloride monomer (VCM) is the building block of PVC and holds a simple, yet vital structure that determines the final properties of the polymer. Understanding VCM is crucial to grasping the overall structure of PVC. VCM's structure consists of an ethylene molecule (a two-carbon chain bonded by a double bond) with one chlorine atom substituting a hydrogen atom on one of the carbons.

To transform this monomer into the polymer polyvinyl chloride, it undergoes a chemical reaction called polymerization. At this point, it's key to understand that the double bond in vinyl chloride is highly reactive, setting the stage for this transformation. When numerous VCM molecules polymerize, they form the long and repeating chain that characterizes PVC, which we will discuss in the context of the polymerization process.

The head-to-tail polymerization process is central to explaining why chlorine atoms do not end up adjacent to each other in the PVC polymer. The 'head' refers to one end of the vinyl chloride monomer, where the double bond and chlorine atom are located, while the 'tail' refers to the other end of the monomer.

During polymerization, these monomers link together in a specific manner: the reactive 'head' of one monomer bonds to the less reactive 'tail' of another. This connection continues, one monomer after the other, creating a linear, repeating structure. This sequential linking ensures chlorines are attached to alternating carbon atoms in the chain, resulting from the monomer's innate preference to join in this orderly fashion.

Further clarity arises from realizing that in vinyl chloride, the chlorine atom is always attached to the same carbon in the monomer (call it the 'chlorinated carbon'). When the double bond breaks to form a bond with an adjacent monomer during polymerization, it's always the non-chlorinated carbon that initiates the connection, thus perpetuating the alternating pattern of chlorines in the final PVC polymer chain.