## 1 Introduction  
Total plastic production in the 1950s has been shown to be around several million metric tons (million MT), but at present, cumulative production has been estimated as \(6.98\times 10^{9}\)-MT during the period 1950 to 2015 [1; 2]. Waste plastic from land sources is continually flowing into world oceans via rivers due to accidents or carelessness. In 1972, [PERSON] and [PERSON] pointed out marine contamination from plastics drifting on the surfaces of the Sargasso Sea [3]. In 2001, [PERSON] et al. [4] reported debris plastics to form garbage patches in the Pacific Ocean and to increase by 17-fold (by weight) or 95 times (by pieces) as much the amounts of contaminants in the past 30 years. [PERSON] et al. [5], [PERSON] [6], [PERSON] [7] and [PERSON] et al. [8] have shown this drifting plastic to break up into a great many small pieces by the action of waves and effects of light to form micro/nanoplastics which in turn lead to massive plastic contamination in bulk. All drifting plastic quite likely break up into small pieces in this manner [9]. By 2050, debris plastics will have attained a weight exceeding that of all fish throughout the oceans of the world [10].  
The authors have been engaged in the collection of coastal waters and beach sands from around the world for the past 20 years so as to determine what chemicals are generated from plastics in oceans worldwide [11; 12; 13; 14; 15]. However, the manner in which various chemicals are derived from plastics hasyet to be fully clarified. Plastics decomposition has long been a topic of intensive research [16; 17; 18; 19; 20; 21; 22] at temperatures of 250 \({}^{\circ}\)C or higher.  
In order to estimate the amounts of SOs from drifting PS in the ocean, the rate of PS decomposition needed to be determined at a living temperature range in the nature. However, to date, no kinetic research on PS decomposition has been conducted at the lower temperature range in the natural environment. Plastic debris undergoes degradation called as weathering in the ocean and on a beach, being exposed to salt water or to the sun light in the presence of the air. Temperature is an essential factor among various potential factors to govern plastic degradation. PS decomposition at low temperature was thus examined in the present study using polyethylene glycol (PEG1540) as a heating medium.  
In this study, kinetic parameters of PS decomposition at a lower temperature range, 30-150 \({}^{\circ}\)C, was determined to obtain the rate of SOs formation from PS. The amounts of SOs and PS in the ocean with time were simulated based on the kinetic parameters obtained although there are various factors other than reaction temperature.  
Using purified PS without contamination of SOs, low-temperature decomposition was carried at 30 to 150 \({}^{\circ}\)C. The products obtained were styrene oligomer (SOs), of styrene (styrene monomer, SM), 1,3-diphenylpropane (styrene dimer, SD\({}_{1}\)), 2,4-diphenyl-1-buttene (styrene dimer, SD\({}_{2}\)) and 2,4,6-triphenyl-1-hexene (styrene trimer, ST). The composition ratio of SM:SD:ST was 1:1:5 and the main product was ST. The rate of ST formation by PS decomposition was measured at 30 to 150 \({}^{\circ}\)C. Kinetic parameters of the PS decomposition were determined and the activation energy of the conversion of PS into ST was given as 45.0 kJ mol\({}^{-1}\). One MT of PS was found to decompose at a rate of 0.3 g per year at 30 \({}^{\circ}\)C.  
Simulation results indicated that the total amount of degraded PS in the ocean has been as much as 430-MT during the period, 1950 to 2050. The results indicated PS to have little stability toward heat, kinetically. Drifting macro PS not only breaks up into micro/nanosized pieces, but subsequently degrades into basic structure units of SOs in the ocean.