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Organic Polymers: Past and the Present

Jyotirmoy Sarma1, Subhasish Roy1, Bhaskar Sharma1, Fredy A. Madukkakuzhy1, Monjumoni Das2, and Pallabi Borah1

1Assam Don Bosco University, Department of Chemistry, Tapesia Gardens, Sonapur, Assam 782402, India

2Sibsagar College, Department of Chemistry, Joysagar, Assam 785665, India

1.1 Introduction and History of Polymers

The word polymer is derived from the Greek word "polumeros" where "Polus" means "many" and "meros" means "units." Henceforth polymers can be defined as the complex and giant molecules or "macromolecules" which are supposed to form by the combination of many small repeating molecules called monomers. Examples of some commercially important polymers and their practical applications have been highlighted in Table 1.1. The most practical distinguishing feature of polymer from its monomer is its huge difference in physical, chemical, and mechanical properties after the polymerization process occurs (Dorel 2008). For example, ethene is a gas but when they combine with each other via the polymerization process, a new class of compound, i.e., polyethene, is formed which differs from its monomer in terms of many physicochemical properties. Monomers being smaller have low molecular weight, while polymers being much larger have very high molecular weight. Compared to simple organic molecules, polymers aren't composed of identical molecules; hence, a polymer sample generally comprises chains of different lengths, which is why their molecular weight is always expressed as an average molecular weight. For instance, the HDPE (high-density polyethylene) molecules are all long-chain carbon chains, but the lengths generally vary by thousands of monomer units. Depending on the type of monomeric units, polymers may be of different types such as homopolymers where all the repeating units (s) are same and co-polymers which can be made up of two or more monomer species. For example, in case of homopolymers such as polythene the monomer unit is ethylene, in polyvinylchloride (PVC) the monomer unit is vinyl chloride. Important examples of co-polymers include polyethylene-vinyl acetate (PEVA), nitrile rubber, and acrylonitrile butadiene styrene (ABS) which are formed by the combination of more than one monomer.

Table 1.1 Some commercially important polymers and their uses.

Based on the type of backbone chain and composition, polymeric materials are classified into two types, viz. organic polymers and inorganic polymers (Currell and Frazer 1969; Gowarikar et al. 2022). Basically, organic polymers are made of carbon-carbon backbone skeleton (Peng et al. 2017), while inorganic polymers do not have carbon-carbon skeleton, rather they have a skeleton like Si-Si for polysilanes, Si-O for polysiloxanes, Si-N for polysilazanes, S-S for polysulfides, B-N for polyborazylenes, and S-N for polythiazyls (Seth 2020; Indra and Shrray 2015).

Cellulose is one of the most abundant organic polymers on Earth, and it is a linear polymer of as many as 10,000 dextro-glucose units joined with each other. Starch, belonging to carbohydrates, can be found in grains and potatoes. Starch is a polymer, also known as a polysaccharide because it is made from glucose as monosaccharide. Starch molecules include two types of glucose polymers, amylose and amylopectin. Amylopectin, being a major starch component, is found in many plants. Amylose belongs to a linear-chain polymer having around two hundred glucose molecules per molecule.

Based on the existence and method of formation, organic polymers may be further categorized as natural or synthetic, and interestingly both of them find equal attention in our day-to-day life. Natural organic polymers can be found in nature or living system and important examples of natural organic polymers include proteins or polypeptides, polynucleotides like DNA and RNA (DNA is a double-stranded polynucleotide chain, while RNA is a single-chain structure of polynucleotides), silk, wool, cellulose, natural rubber. Synthetic polymers are man-made polymers which are being synthesized in the industry or laboratory. Important examples of synthetic polymers include polyethylene (both low-density polyethylene - LDPE and high-density polyethylene - HDPE), polypropene (PP), polyacrylonitrile (PAN), polyaniline (PANI), polystyrene (PS), polyvinylchloride (PVC), tetrafluoroethene (Teflon), polyacetylene, nylon, thermoplastic polyurethane (TPU), and Bakelite. Most of the synthetic polymers possess enhanced lifetime and improved mechanical properties. However due to the absence of a functional group in most of them, they do not have some important physical and chemical properties which limit their synthetic utility in many practical applications. To address this problem post-synthetic functionalization and further modification of polymers have been done in recent times to achieve multinational properties.

During the fifteenth century, Christopher Columbus was involved in the discovery of rubber by isolating it from trees, and later Joseph Priestly observed that the material is helpful for erasing pencil marks on paper. This observation launched the rubber industry. Combining the latex of rubber tree with the morning glory plant juice in various proportions helped to achieve rubber's distinct properties for designing selective products like bouncing balls, various kinds of rubber bands, etc. To gain the advantages and properties of both natural and synthetic polymers, researchers across the globe were always in search for the development of improved semi-synthetic organic polymers. For example, vulcanization of rubber was introduced for enhancing the quality of natural rubber where a small amount of sulfur is added as a cross-linking agent which can enhance the quality and stability of rubber. Vulcanized rubber is comparatively stronger, elastic, more resistant to abrasion and temperature change, and most importantly inert with respect to chemicals and electric current as compared to untreated natural rubber (Brown and Poon 2005).

Likewise, natural resources polymeric materials like cellulose and proteins have been extensively used for making improved polymers via copolymerization techniques.

Henri Braconnot's Braconnot, Christian Schönbein, and coworkers in 1830s first developed the derivatives of cellulose for constructing novel semi-synthetic materials, known as celluloid and cellulose acetate. Later the term "polymer" was introduced in 1833 by Jöns Jakob Berzelius, even though Berzelius could not provide significant contribution for the development of modern polymer science.

In 1909, Leo Baekeland developed Bakelite from cheap and readily available chemicals such as phenol and formaldehyde which opened the door of emerging technology for designing many innovative polymeric products (Baekeland 1909; Wallace 1945).

In spite of the noteworthy developments in polymer synthesis, the molecular nature of polymers was not clear until the concept was introduced by Hermann Staudinger in the year 1922. Earlier formation of polymers was explained via aggregation theory proposed by Thomas Graham in 1861. According to Graham, cellulose and other polymers were supposed to have colloidal nature where aggregation of small molecules having smaller molecular masses was joined by some unknown intermolecular force. Later, Hermann Staudinger first anticipated that polymers...