Polymers shape human life but they also have been identified as pollutants in the oceans due to their long lifetime and low degradability. Recently, various researchers have studied the impact of (micro)plastics on marine life, biodiversity, and potential toxicity. Even if the consequences are still heavily discussed, prevention of unnecessary waste is desired. Especially, newly designed polymers that degrade in seawater are discussed as potential alternatives to commodity polymers in certain applications. Biodegradable polymers that degrade in vivo (used for biomedical applications) or during composting often exhibit too slow degradation rates in seawater. To date, no comprehensive summary for the degradation performance of polymers in seawater has been reported, nor are the studies for seawater‐degradation following uniform standards. This review summarizes concepts, mechanisms, and other factors affecting the degradation process in seawater of several biodegradable polymers or polymer blends. As most of such materials cannot degrade or degrade too slowly, strategies and innovative routes for the preparation of seawater‐degradable polymers with rapid degradation in natural environments are reviewed. It is believed that this selection will help to further understand and drive the development of seawater‐degradable polymers.

In order to promote the degradation of PLA in seawater, a series of seawater-degradable polyester blends PVA/PLA were prepared by blending biodegradable polylactic acid (PLA) with water-soluble modified polyvinyl alcohol (PVA) in this paper. ADR 4370S was introduced to bring a certain degree of improvement in compatibility of PVA/PLA blends. The results of degradation test in natural seawater for 180 days show that the weight loss of PVA/PLA blends in seawater is much higher compared with that of pure PLA. PVA can be used as an effective hydrolysis accelerator for PLA matrix, helping to significantly reduce the molecular weight of PLA. The channels caused by dissolution/swelling of PVA facilitate the entry of water and microorganisms into the materials to contact with PLA, thereby promoting the degradation process of PLA matrix itself. Thus, both dissolution/swelling of PVA and degradation of PLA occur in PVA/PLA blends, and the degree of rapid dissolution of PVA in the early stage determines the degree of degradation of PLA.

In view of the problem of marine plastic pollution and oil resource depletion, an innovative attempt was made to obtain biobased seawater-degradable polyesters by installing lactic acid (LA) into the poly(butylene succinate) (PBS) backbone and synthesizing a series of random copolyesters named poly(butylene succinate-l-lactide)s (PBSLs). Herein, biobased LA units are chosen as readily hydrolyzable points according to our theoretical calculation. The obtained PBSL copolyesters preserve the structure of PBS when LA units are inserted into this main chain mostly with a single unit. While maintaining excellent mechanical and thermal properties, PBSL shows notably improved degradation performance during a 440-day long-term test in natural seawater compared to PBS, demonstrating rapid loss of weight and mechanical properties, as well as a decrease in the molecular weight. A highly sensitive system for end-product CO2 detection during the seawater degradation process is designed and applied for the first time under conditions close to the natural seawater environment. A record mineralization rate of 16.2% in seawater after 379 days of degradation is achieved for PBSL30, which proved the occurrence of biodegradation in seawater. This result, combined with the analysis of intermediate products, clearly explained the whole degradation mechanism of PBSLs in seawater.