Chemistry
How Polymers Degrade Through Photolysis and Hydrolysis in Environmental Conditions
Quick fact
Photolysis can break polymer bonds with UV light, but its efficiency is low for many common plastics because they don't absorb UV well; instead, impurities often trigger the reaction. Hydrolysis, on the other hand, is a key driver for polyester degradation, but it is extremely slow for polyolefins like polyethylene, making them persist for centuries in the environment.
Why this is interesting
You've seen plastic bottles and bags that seem to last forever, yet some plastics crumble after a few months outdoors. What makes the difference?
Read the full explanation
Understanding How Polymers Degrade Through Photolysis and Hydrolysis in Environmental Conditions
Think of a polymer as a long chain of monomers linked by chemical bonds. When a polymer is exposed to the environment, two main forces can break these chains: light and water. Photolysis is the direct breaking of chemical bonds by photons, especially in the ultraviolet (UV) range. When a polymer absorbs a high-energy photon, the bond can become excited and break, producing two shorter chains. However, most common polymers (like polyethylene or polypropylene) have strong carbon-carbon bonds that do not absorb UV light well. Instead, impurities—such as residues from manufacturing, catalysts, or absorbed pollutants—can absorb the light and transfer energy, starting the degradation process. Hydrolysis involves water molecules attacking bonds that are susceptible to nucleophilic attack. In a condensation polymer like an ester or amide, the water molecule can insert itself into the bond, breaking the polymer chain into two parts. This is why polyesters, like PET, are more easily hydrolyzed than polyolefins, which lack such vulnerable bonds. Both processes eventually cause chain scission, which reduces the polymer's molecular weight and changes its physical properties, leading to brittleness, cracking, loss of strength, and eventually disintegration into smaller fragments.
A deeper explanation
Photolysis mechanism: Polymers absorb UV photons (energy ~300-400 kJ/mol) that match the bond dissociation energy (C-C ~350 kJ/mol). Absorption leads to electronic excitation and possibly direct homolytic bond cleavage, creating free radicals. These radicals can then react with oxygen (photooxidation) or initiate chain reactions that cause further scission and crosslinking. The efficiency depends on the polymer's absorption spectrum—aromatic groups in polystyrene absorb UV, but aliphatic C-C bonds in polyethylene do not. Therefore, impurities or chromophoric defects are often the initiating sites. Hydrolysis mechanism: In a condensation polymer, the functional group (e.g., ester, amide) is susceptible to nucleophilic attack by water. For an ester, the carbonyl carbon is electrophilic; water attacks, forming a tetrahedral intermediate that collapses, breaking the O-C bond and yielding a carboxylic acid and an alcohol. The rate depends on the bond's reactivity, temperature, pH, and water availability. Polyesters hydrolyze at moderate rates; polyamides (nylons) are slower; polyolefins have no hydrolysable bonds, so they are essentially inert to water, which is why they accumulate in the environment. These degradation pathways are crucial not only for plastic pollution but also for designing materials with controlled lifetimes. Biodegradable plastics like PLA rely on hydrolysis, while outdoor polymers are often stabilized against photolysis to extend their service life. Understanding the balance between these processes allows chemists to engineer materials that degrade when desired, or resist degradation when needed.