The major appeal of biodegradation is that, in theory, the polymer will be completely consumed in the environment without needing complex waste management and that the products of this will be non-toxic. 
Most common plastics biodegrade very slowly, sometimes to the extent that they are considered non-biodegradable. As polymers are ordinarily too large to be absorbed by microbes, biodegradation initially relies on secreted extracellular enzymes to reduce the polymers to manageable chain-lengths. This requires the polymers bare functional groups the enzymes can recognise, such as ester or amide groups. Long-chain polymers with all-carbon backbones like polyolefins, polystyrene and PVC will not degrade by biological action alone and must first be oxidised to create chemical groups which the enzymes can attack.
Oxidation can be caused by melt-processing or weathering in the environment. Oxidation may be intentionally accelerated by the addition of biodegradable additives. These are added to the polymer during compounding to improve the biodegradation of otherwise very resistant plastics. Similarly, biodegradable plastics have been designed which are intrinsically biodegradable, provided they are treated like compost and not just left in a landfill site where degradation is very difficult because of the lack of oxygen and moisture.