The main processes that contribute to the degradation of these synthetic polymers can be grouped into:
Once the polymer is in the environment, photo and chemical degradation cause it to age, making it more brittle and susceptible to mechanical degradation \citep{Duwez_2001}.  Thanks to their movement through different habitats the particles erode into smaller particles ranging from size between 1 and 5000 micrometers. The degradation of these particles, classified as micro-plastics does nor end in this size range. In fact the increased surface area of the polymer particles, results in an increased degradation due to their higher reactivity leading to the formation of even smaller polymers identified as nanoplastics \citep{Lambert_2016}. The mechanisms that affect the deterioration of the different polymers highly depends on their chemical composition. UV radiation cause the compound to photo-oxidise which is eventually followed by chemical oxidation \citep{Andrady_2011}. The combination of these processes  weakens the polymer, breaking the molecules backbone \citep{2013} and decreasing the molecular weight of the polymers making it possible for microbial degradation to take place. The presence of these compounds in aquatic environments leads to a slower process of deterioration compared to their land based counterpart. The radiation from UV rays plays a major role to this process, being much stronger compared to its effect on particles submerged in water where the light intensity decreases as the depth increases \citep{Leonas_1993}. The rate of degradation is also dependant weather or not the compound has been treated with different additives (antioxidants) which will delay the process . 
For biodegradation on synthetic polymers to happen the environment conditions must be favourable. First and foremost, microorganisms that can depolymerize the polymers and then mineralize the monometric compounds with enzymes must be present, then temperature, pH, moisture and salinity must provide conditions needed for biodegradation. Finally, the morphology of the polymer particles has to allow the attachment of microorganisms and consequent formation of biofilm. The substrate structure of the same polymers, should not also prevent microbial actions  \citep{Klein_2017}
Since the size of synthetic polymers is generally too large to penetrate the cell membranes of microorganisms, the first step of biotic degradation is the cleavage of side chains or the polymer backbone and the formation of smaller polymer units (monomers, oligomers) by extracellular enzymes [64]. In most cases, this first step of depolymerization involves an enzymatically catalyzed hydrolysis of amides, esters, or urethane bonds. These smaller molecules can then be absorbed by microorganisms and metabolized. Of course, abiotic hydrolysis can also result in intermediates that are then further metabolized by microorganisms [65].
The complete biotic degradations of poly(ε-caprolactam) and water-soluble polyethylene glycol are well described in literature [66]. However, most of the plastics occurring in the environment are water insoluble, and many of the synthetic polymers present in the aquatic environment, such as PE, polypropylene (PP), PS, and PET, degrade very slowly or not at all. The degradation of these polymers is usually a combination of abiotic and biotic degradation pathways.
Polyolefins, such as PE and PP, represent a class of substances with high industrial production volumes and are determined frequently in environmental samples. These polymers are usually not biodegradable, as the alkyl backbone is not accessible for microorganism and must undergo an abiotic transformation. The alkyl backbone of polyolefins offers a high resistivity against hydrolysis but is usually susceptible to oxidative degradation. To prevent this, additives are added during the production process, and the oxidative or photooxidative degradation of the polymer is delayed until the antioxidants are consumed. After the initial oxidation of the surface of polyolefins, the degradation could occur in several weeks but results in the formation of microplastics as possible intermediates [67]. These smaller and oxidized plastic fragments are more susceptible to microbial attack, e.g., biodegradation of PE is described for pre-oxidized fragments of the original material by Pseudomonas sp. [68].
d chemicals from the marine environment and
pose danger to biota and humans when penetrating in and climbing
up the food chain (Ivar do Sul and Costa, 2007; Lusher, 2015; Li et al.,
2016; GESAMP, 2016; UNEP, 2016).