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Biodegradable Polyesters: Synthesis, Properties, Applications

Chi Zhang

1.1 Historical Overview on the Origin of Polymer Science and Synthesis of Polyamides and Polyesters

There are some persons who stand out prominently in any review of the history of polyesters. As early as in 1911, Meyer [1] found that the saponification of polyesters of polyacids and polyalcohols and the hydrolysis of a glycerol ester proceeds in acid and alkaline solution, and, the hydrolysis can only be detected under certain conditions with heterogeneous saponification. However, he did not mention the molecular weights of the polyesters used. Hermann Staudinger (1881-1965) is recognized as the father of polymer chemistry because of his great contributions to polymer science (Figure 1.1).

Figure 1.1 Hermann Staudinger who was born on 23 March 1881 at Worms, Germany and died on 8 September 1965 at Freiburg, Germany [2].

In 1920, Staudinger reported [3] that polymerization processes in the wider sense are all processes in which two or more molecules combined to a result in a product of the same composition but with higher molecular weight. These high-molecular-weight compounds are produced by two basic polymerization processes: the first was called condensation polymerization(nowadays, termed step-growth polymerization) in which polymers are formed by stepwise reactions between functional groups of monomers, usually containing heteroatoms such as nitrogen or oxygen; and the second called chain-growth polymerization (or addition polymerization), which involves double or triple carbon-carbon bonds breaking and the linking together of the other molecules. In reactions, these unsaturated monomers are able to form repeating units where the main-chain backbone usually contains only carbon atoms. In his paper entitled "Polymerization" [3], Staudinger presented several reactions that form high-molecular-weight molecules by linking together a large number of small molecules. This new concept was the base for "macromolecular chemistry" in the history of science. Staudinger noticed that the natural fibers were not comprised of small molecules as imagined earlier [4] and he gave an early formal awareness of "small" differing from "big" molecules. Thanks to his pioneering concept, in the past 90 years, the molecular architectures of synthetic polymers and biopolymers have become clearly known and most synthetic polymers can be designed, characterized, and tailored with high precision. But in his time, his concepts about macromolecules were doubted by the scientific community, although he presented key experimental evidence to support the existence of high-molecular-weight polymers. Most scientists were very reluctant to accept the existence of large compounds with molecular weights exceeding 5000 Da. Instead, micelle-type aggregates, as observed for soap molecules, were considered to account for the unusual properties of such materials. Moreover, some scientists were convinced that the size of a molecule could never exceed the size of the unit cell, as measured by X-ray crystallography [5]. A well-known effort was that of a natural rubber, which was selected as the model system by Staudinger because Carl Harries and Rudolf Pummerer had suggested that natural rubber consisted of aggregated small cyclic polyisoprene units via "partial valencies" associated with the double bonds. Such aggregates should have been destroyed when the double bonds were removed by hydrogenation. Staudinger's hydrogenation experiments showed that hydrogenated rubber was very similar to normal unsaturated rubber, indicating the existence of high-molecular-weight polymers whether the double bonds were hydrogenated or not. During the late 1920s [6-9], Staudinger provided additional evidence based on viscometry to confirm that molecular weights remained unchanged during chemical modification of polymers. Staudinger continued to encounter very strong opposition from leading organic chemists for nearly two decades. For instance, Heinrich Wieland, 1927 Nobel laureate in chemistry, wrote to Staudinger, "Dear colleague, drop the idea of large molecules; organic molecules with a molecular weight higher than 5000 do not exist. Purify your products, such as rubber, then they will crystallize and prove to be low molecular compounds!" [5].

Staudinger continued the promotion of his concepts of polymer sciences, despite his colleagues' mistrust of many of his methods and results. He eloquently defended his ideas against all attacks using his ingenuity, persistence, and pronounced enthusiasm. By the end of the 1920s and during the 1930s, Staudinger's macromolecular concept found increasing acceptance by other chemists. Finally, on 10 December 1953, Staudinger was rewarded for his concept of macromolecules and his prolonged effort to establish the science of large molecules, when he was awarded the Nobel Prize for chemistry [5].

1.1.1 Synthesis of Polyamides

Wallace Carothers (1896-1937) who is another great polymer giant must not be neglected here. In the 1930s, Carothers formally reported laboratory and theoretical studies for the condensation polymerization and made synthetic polymers of Nylon and glycol esters [10, 11] (Figure 1.2). Among the discoveries of synthetic polymers, Carothers' most significant achievement was the synthesis of Nylon. Nylon is a generic term for a family of synthetic polymers known generically as aliphatic polyamides, first produced in 1935 at DuPont's research facility at the DuPont Experimental Station led by Carothers. As one of the most popular thermoplastics, Nylon has different chemical structures and trade names such as nylon-6,6, nylon-6, nylon-6,9, and nylon-11.

Figure 1.2 Wallace Carothers who was born on 27 April 1896 in Burlington, Iowa, United States and died on 29 April 1937 at Philadelphia, Pennsylvania, United States [12].

There are two typical approaches for the synthesis of nylon. The first approach includes combining molecules with an acid (COOH) group on each end are reacted with two chemicals that contain amine (NH2) groups at each end. This process creates nylon-6,6, made of hexamethylene diamine with six carbon atoms and adipic acid. The second approach consists of a compound having an acid at one end and an amine at the other and this compound is polymerized to form a chain with repeating units of (-NH-[CH2]n-CO-)x. For instance, nylon-6 is made from a single six-carbon substance called caprolactam. In this equation, if , then nylon-6 is the assigned name (this may also be referred to as polymer). The numerical suffix specifies the number of carbons donated by the monomers: the diamine first and the diacid next. The most common nylon polymer is nylon-6,6 which refers to the one in which the diamine (hexamethylene diamine) and the diacid (adipic acid) each donate six carbons to the polymer chain. It is difficult to get equimolar proportions between the two involved monomers exactly correct, and deviations can lead to chain termination at molecular weights less than a desirable 10 000 Da. To overcome this problem, a crystalline, solid "nylon salt" can be formed at room temperature, using an exact 1 : 1 equimolar ratio of the acid and the base to neutralize each other. Heated to 285 °C, the salt reacts to form nylon polymer. However, it is impossible to spin the chains into yarn at above 20 000 Da. To overcome this difficulty, some acetic acid must be added to react with a free amine end group during polymer propagation to limit the molecular weight. The general reaction is

Two molecules of water are given off and the nylon is formed. Its properties are determined by the R and R´ groups in the monomers. In nylon-6,6, and alkanes, but one also has to include the two carboxyl carbons in the diacid to get the number it donates to the chain. In Kevlar, both R and R´ are benzene rings.

In early discovery of polymers, of course there were a lot of arguments on the basics of polymer science and unclear concepts and terms. Carothers criticized Berzelius [10, 13] about his misleading concept on polymers as the term polymers indicated the presence of the same atoms in the same proportions. Carothers described a "condensation" that requires as starting materials compounds in which at least two functional groups are present in the same molecule (e.g., hydroxy acids, HO-R-COOH, might lead to polyesters, HO[RCOO]xRCO-; and amino acids to polyamides, NH2[RCONH]yRCO-.) [10]. In addition, he suggested the possible condensation products via intra- or interpolymerization with bifunctional monomers as limited atomic ring or high-molecular-weight chains [10]. In 1929, Carothers and Arvin [14] prepared some esters by heating a variety of acids and 5% excess of glycol for about 3 h at 175-185° and then at 200-250° and 0.2 mmHg for 3 h. Some solid esters had molecular weights ranging from 2300 Da to the highest of 5000 Da as polymers and in the final chemical structure of the polyesters it was assumed that HO groups were present at each end of the chain because of the presence of one more molecule of glycol than that of acid. Carothers, who analyzed the hydroxy-acids of the series, found that HO(CH2)xCOOH might condense with them, but in most cases the...