Polymers are a unique class of modern materials. Despite their light mass density, their mechanical properties are strong enough for a variety of applications either through crystallization or vitrification to achieve sufficient rigidity. However, high Young's modulus is not the only necessary characteristic. Desirably, polymeric materials should be ductile, for example, tolerating considerable bending. In this regard polymers (at room temperature) can be classified as brittle or ductile polymers. A ductile or brittle polymer structurally, can be either amorphous or semicrystalline. Moreover, a polymeric material (either glassy or semicrystalline) could remain ductile at temperatures much lower than its glass transition temperature (Tg). Ductile polymers include semicrystalline polymers such as polyethylene (PE) and polypropylene (PP) with low Tg, and polyamide with high Tg, as well as some amorphous glassy polymers such as bisphenol A polycarbonate (bpA-PC) and polyethylene terephthalate (PET). Examples of the brittle polymers under ambient conditions, are semicrystalline polymers with high Tg such as semicrystalline PET or Poly(lactic acid) (PLA), or glassy amorphous polymers such as polystyrene (PS) and poly(methyl methacrylate) (PMMA). For decades mechanical behavior of polymers is an important research topic in polymer science and despite the tremendous researches in this filed, there is no comprehensive molecular picture that can universally describe the mechanical behavior of polymeric materials at the molecular level. Consequently, previous studies are not able to propose a method to efficiently improve the mechanical properties of polymers without additives, e.g. turning a brittle polymer to a ductile one.This dissertation consists of two parts: PART A is about the molecular mechanics of glassy amorphous polymers, and PART B focuses on the molecular mechanics of semicrystalline polymers. In each part, we carried out three independent projects to address several important questions in the mechanics of polymer glasses and semicrystalline polymers. Influenced by our recent phenomenological molecular model, this work comprehensively emphasizes on the important role of the chain networking, arising from intra-chain connectivity and chain uncrossability, on affording ductility into the polymeric materials.Chapter I is a general introduction into the PART A. In Chapter II we study several different polymer glasses in tensile, creep and stress relaxation mechanical experiments in order to understand the crazing behavior of polymer glasses. Based on our inclusive experiments and counterintuitive results, an alternative molecular mechanism and model is proposed to explain craze initiation in polymer glasses. This chapter also includes a refrainment to our recent phenomenological molecular model. Chapter III studies the two-phase rubber-toughened polymer glasses. It is indicated that the commercial rubber-toughened polymer glasses do not undergo molecular level yielding characterized by the increased segmental mobility, thus we title the strain softening point of these materials as apparent yield point. Methods to switch the mechanism of yielding from apparent to the real one is also proposed. A novel rubber-toughening mechanism that is operative in the recently synthesized nano-rubber-toughened PMMA by Dow chemicals, is suggested. In Chapter IV, real-time birefringence measurements during different mechanical experiments of PMMA and PS is performed in order to achieve molecular level insights into the deformation of polymer glasses. For the first time, birefringence changes at the post-yield deformation of PS and PMMA are measured. Attempts are made to explain the molecular reasons of the birefringence changes and its correlation/decorrelation with stress, during different mechanical experiments including uniaxial tensile, creep and stress relaxation experiments.By an analogy with PART A, PART B of dissertation, starts with a general introduction into the molecular mechanics of semicrystalline polymers in Chapter V. In Chapter VI, our general framework and universal molecular picture of the deformation of semicrystalline polymers are introduced. Chapter VII investigates the molecular mechanics of semicrystalline PLA which is an ideal model semicrystalline polymer, through a comparative study with the mechanics of its amorphous state. Reasons behind mechanical weakness of this polymer and in general any other semicrystalline polymer below Tg, and lower drawability above Tg, are explained in light of the results from time-resolved polarized optical microscope (POM) observations during large deformation of semicrystalline PLA. After identifying the origins of the brittle response of semicrystalline polymers, a universal strategy is proposed in Chapter VIII to turn brittle semicrystalline polymers to ductile. Finally, Chapter IX summarizes the overall body of the dissertation and offers main conclusions.