When we look at plastics in structural point of view, biodegradable plastics
have easily degradable linkages like anhydride, orthoester, esters. Due to the presence of these linkages the molecular weight of the plastic drops down to 1000s’ from lakhs as a result of degradation. Microorganisms are capable of assimilating these low molecular weight organic material whose molecular weight is less than 10,000 (approx.) and release CO2, H2O, and CH4 as metabolic products.
This process of converting organic carbon (Plastic) to inorganic carbon (metabolic product) is known as mineralization. Biodegradable plastics first degrade to low molecular weight products then they are mineralized further by biological agent. E.g., PLA, PLGA (They have hydrolysable ester group that initiate degradation)
Traditional plastics or normal plastics on other hand do not contain easily degradable linkages. Their main groups are hydrocarbon groups, amine groups, and ester members with hindering groups to prevent easy degradation. So they stay on land without degrading for years. However, when these plastics degrade to molecular weight range of 1000’s they will also be assimilated by biological agents as the case of biodegradable plastics. E.g.,PE,PP.