This chapter presents the NMR investigations of synthetic polymer materials, including polymer blends and alloys, polymer nanocomposites, rubbers and elastomers, and other semicrystalline polymers: polystyrene/poly(vinyl methyl ether), poly(methacrylic acid)/poly(vinyl acetate), and poly-3-hexylthiophene/phenyl-C61-butyric acid methyl ester blends, polyketone/polyamide alloy, a healable polymer blend of poly(octamethylene thiourea)/poly(ether thiourea), poly(vinyl alcohol)/clay, polystyrene-poly(ethylene oxide) block copolymer/clay, nylon-6/clay, and poly(vinyl isobutyl ether)/poly(ε-L-lysine)/clay nanocomposites, and acrylonitrile butadiene rubbers are introduced. High-resolution solid-state 13C NMR spectra are sensitive to the existence of interpolymer interaction, particularly hydrogen bonds making chemical shift value down-field shift. Hetero-correlation NMR spectroscopy shows proof of the existence of the interaction between component polymer and clay. Analysis of the 1H spin–lattice relaxation (T1H) behavior influenced by the spin diffusion provides the degree of homogeneity, domain size of component polymers, and phase separation process for polymer blends/alloys. The paramagnetic effect on T1H gives information on the stability of an organic modifier on clay surfaces and of morphology of α and γ crystalline phases of nylon-6 in nylon-6/montmorillonite nanocomposites. Finally, the degradation of rubbers is discussed based on 1H spin–spin relaxation (T2H) analysis.