Triboelectric (nano)generator (TENG) devices recently has gained a top importance for mechanical energy harvesting to power autonomous microdevices and portable electronics [1]. The output from TENG is dependent from the magnitude of surface charge forming on polymer after contact-separation. Mechanisms of polymer triboelectrification are under the debate. Three main mechanisms are considered for polymer electrification: (i) electron transfer, (ii) heterolytic covalent bond break and transfer of charged fragments from macromolecules, and (iii) ion transfer between surface water adsorbate layers. Here we report a series of experiments indicating that the polymer triboelectrification is due to transfer of charged molecular fragments. The surface charge was measured by contacting-separating sample films under controlled contacting force and frequency, as well as separation gap and speed and environmental conditions. Sample films with constant thickness – 100 μm were produced from different polymers and polymer composites. The surface roughness was controlled to be similar between different samples (otherwise specified) to avoid any additional influencing factors. Sample films were adhered or deposited on ITO coated glass substrate. Over 300 combinations of polymer vs. polymer or polymer vs. ITO or metal were tested. The results indicates that the surface charging is mainly dependent from deformative properties of polymer films in contact. The surface charges increases with decreasing the elastic modulus of the polymer or increasing the difference in hardness between contacted polymers. The strong surface charge can be measured when contacting chemically identical polymers with different thermal history, surface roughness or filler content. The surface charge value is rising with increasing in the discrepancy in the parameter. Moreover, an order of magnitude increase of triboelectric charge density of glassy polymers such as polystyrene (PS), poly- carbonate (PC) and poly(methyl methacrylate) (PMMA) can be observed when heated above the glass transition temperature. Further, measurements reveal that higher surface charge is formed on more adhesive polymer surfaces, thus confirming covalent bond cleavage and material transfer as a mechanism for contact electrification of polymers. The material transfer was confirmed also by AFM and XPS studies. The established understanding of contact electrification provides a new paradigm for developing high performance materials for applications in triboelectric mechanical energy harvesting.