Synthetic polymers simply mean “made by chemical synthesis” or “man-made”, and can be derived from petroleum/fossil fuels or bio-based materials (like corn), and once processed, can have the same properties, including lack of ability to degrade, regardless of raw material.
Synthetic plastics frequently contain toxic additives to make them have preferred qualities like being malleable, water-resistant, heat resistant, and more. There are no assurances that synthetic polymers can completely biodegrade within a specific timeline, meet compostable standards, or be “marine degradable”.