High-density polyethylene may be chlorinated by inducing the random substitution of chlorine, atoms for hydrogen. (a) Determine the concentration of \(\mathrm{Cl}\) (in wt \%) that must be added if this substitution occurs for \(8 \%\) of all the original hydrogen atoms. (b) In what ways does this chlorinated polyethylene differ from poly(vinyl chloride)?

Polyethylene, a versatile polymer used in numerous applications, can be chemically modified to enhance its properties. One such modification is chlorination.

Chlorination involves substituting chlorine atoms for hydrogen atoms in the hydrocarbon polymer. When chlorine atoms are added to polyethylene, it becomes 'chlorinated polyethylene' (CPE). This process not only affects its chemical structure but also its physical properties. Chlorinated polyethylene, having a higher chlorine content compared to its non-chlorinated counterpart, shows increased resistance to oils, fuels, and chlorine itself, making it advantageous for various industrial uses.

For instance, if chlorination occurs for 8% of all the hydrogen atoms in high-density polyethylene (HDPE), the material can see an improvement in fire retardancy and thermal stability, but these enhancements must be balanced against potential changes in processability and flexibility.

Understanding the molecular weight of polymers like polyethylene is crucial for predicting their physical properties. To calculate the molecular weight of a chlorinated polyethylene, initially, we consider the molecular weight of pure polyethylene, which consists of repeated units of ethylene (C2H4).

Each unit's molecular weight is the sum of its atomic weights: for two carbon atoms (12 amu each) and four hydrogen atoms (1 amu each), yielding a molecular weight of 28 amu. During chlorination, some of these hydrogen atoms are replaced by chlorine atoms—significantly heavier at 35.5 amu each.

When a certain percentage of hydrogen, for example, 8%, is replaced by chlorine, the molecular weight increases proportionally to the number of substituted atoms and their mass difference from hydrogen. This is a fundamental step in modifying the polymer's characteristics, as molecular weight greatly influences qualities such as strength, temperature resistance, and solubility.

Chlorinated polyethylene (CPE) and poly(vinyl chloride) (PVC) may seem similar due to their chlorine content, but their structure and properties differ greatly. CPE is essentially modified polyethylene with chlorine atoms randomly substituted along the polymer chain. This randomness can lead to inconsistency in the material's properties across the polymer.

On the other hand, PVC has a very orderly structure with chlorine always attached to the same position in the repeating unit, making its properties more predictable and uniform. This standardized structure of PVC contributes to its rigidity and makes it a popular choice for construction materials.

Despite their chlorinated nature, CPE retains more of the original polyethylene's flexibility and softness, whereas PVC tends to be more rigid. PVC's rigidity can be tempered with the addition of plasticizers, which is not often necessary for CPE due to its inherent flexibility. This comparison is essential when deciding between materials for specific applications.