Natural polymers are macromolecular substances produced by living organisms and composed of repeating molecular units joined through biologically mediated reactions. Like synthetic polymers, they consist of long-chain molecules whose properties depend on molecular weight, chain structure, and intermolecular interactions. However, their formation typically occurs under ambient temperatures and pressures through enzyme-controlled processes, and their composition is limited largely to elements commonly available in biological systems, especially carbon, hydrogen, oxygen, and nitrogen. From a materials standpoint, natural polymers represent some of the earliest structural and functional materials used in engineering applications, long preceding the development of modern plastics.
Natural polymers are commonly classified according to their chemical building blocks. Polysaccharides, such as cellulose and starch, are composed of repeating sugar units and serve primarily structural and energy-storage roles in plants. Proteins are polymers of amino acids and exhibit a wide range of mechanical behaviors, from the stiffness of collagen to the extensibility of elastin and silk. Natural elastomers, most notably natural rubber, consist of long, flexible hydrocarbon chains that provide significant elasticity with limited crosslinking. Nucleic acids, including DNA and RNA, are also polymers, though their principal function is informational rather than structural. In each case, the macroscopic properties of the material are governed not only by the chemistry of the repeating unit, but also by chain orientation, crystallinity, and higher-order organization.
Because they are derived from renewable biological sources and are generally susceptible to biological degradation, natural polymers are of continued interest in modern materials engineering and manufacturing. Many traditional engineering materials, including wood, leather, and natural fibers, are polymeric in nature, and contemporary bio-based polymers and composites frequently draw on these same molecular architectures. An understanding of natural polymers therefore provides both historical context and a conceptual basis for analyzing synthetic polymers, particularly with respect to structure–property relationships, processing behavior, and environmental performance.