The propagation of microplastic in water requires research in the fragmentation of polymers under natural factors. UV exposure is the key impact causing a (relatively) fast degradation of polymers: solar radiation initiates autocatalytic thermal oxidation which is the main cause of polymer degradation in the environment. Wind and additional mechanical impacts, like friction against ground or other solid objects and the crushing of waves along the shoreline, molecular chains begin to break, which, in the absence of oxygen, results in increased material plasticity (so-called chemical-mechanical degradation). Mechanical fragmentation can be accelerated if the material has wear features due to other destructive factors. In this research, we conducted experiments on the accelerated aging of polymers through UV exposure at a rate of 1.16 W/m2 until the first signs of fragmentation occurred in polymer samples. UV exposure of polymers was carried out on the water surface in laboratory conditions. Based on the obtained experimental data, the authors calculated the duration of polymers (polyethylene/PE and polypropylene/PP) remaining in the waterways of Izhevsk under solar UV exposure until the first signs of fragmentation occur. The calculations showed that when PE and PP are in the water and experience natural UV exposure, the first signs of polymer fragmentation occur after 730 days in winter and 136 days in summer.