The raw materials used to produce them are not limitless, and the way you dispose of them can also lead to environmental problems. Synthetic polymers are an incredibly versatile group of compounds -- so versatile, in fact, you can find them in all sorts of unexpected places. The methyl 2-cyanopropenoate in your superglue polymerizes to make a tough, solid film; RTV silicone hardens when dried to make gaskets for use in cars. The nylon in stockings and ropes, the polyesters in clothes, the polyethylene in shopping bags, the PVC in plumbing and the rubber in your car tires are just a few more examples of synthetic polymers in your everyday life.
Society uses synthetic polymers because many of them have highly desirable properties: strength, flexibility, resistivity, chemical inertness and so forth. ABS is found in objects as diverse as car bumpers and camera cases. Or take polystyrene, which is easily molded to make items like plastic forks. Polystyrene foam, better known as Styrofoam, is a fantastic thermal insulator popular as beverage containers used in restaurants.
Instead, it is a specialized, broad and unique discipline that could cover some parts of chemistry and several other scientific fields as well. The fields of science have always become very active when research groups trained in one specialized field turn their interests to a related field. This has always been and in the future, will be especially true in polymer research works. The requirement in polymer is the application of ideas and chemistry knowledge and techniques to complex materials and macromolecules.
This is a fundamental task, and it demands the very best ways that chemistry could provide. Perhaps polymer chemistry, more than any other research field, crosses over and cuts the traditional lines of all branches of chemistry, biology, physics, material, engineering, pharmacy, and even medicine. And, a newcomer to polymer science requires enough ability to mix together the vast knowledge from all aforementioned fields.
Therefore, this editorial has been written to show the very significant and unforgettable roles of polymers in human life. Professor Hassan Namazi received his PhD in natural polymer chemistry from University of Dalhousie, Canada in with a minor in dendrimer and carbohydrate chemistry.
He joined to the University of Tabriz as the staff member in and now serves as full professor. He has published 4 books and scientific papers in highly ranked journals. In , he won an award and honor diploma from Iranian Academy of Medical Sciences as the top distinguished researcher in Iran.
He has also received many academic awards and national medals of merit for outstanding research activities in , , and National Center for Biotechnology Information , U. Journal List Bioimpacts v. Published online Jun Author information Article notes Copyright and License information Disclaimer. Received Jun 1; Accepted Jun Non-commercial uses of the work are permitted, provided the original work is properly cited. Pectin is a long chain polymer composed of pectic acid and pectinic acid molecules see structure below.
Because these acids are sugars, pectin is called a polysaccharide. It is obtained from citrus peels and the remains of apples. The pectin chains form a network because some of the segments of the pectin chains join together by crystallization to form a three-dimensional network in which water, sugar and other materials are held. Formation of a gel is caused by physical or chemical changes that tend to decrease the solubility of the pectin and this favors the formation of small localized crystals.
The most important factor which influences pectin's tendency to gel is temperature. When cooling a hot solution containing pectin, the movement of the molecules is decreased and their tendency to combine into a gel network is increased. This ability makes pectin a good thickener for many food products, like jellies and jams. Just as bacteria and archaea, fungi are also excellent decomposers [ 59 ]. In addition to their high degradative potential, many fungi have remarkable capacity to expand on the substrate surface, surrounding it with their hyphae, which release extracellular enzymes close enough to achieve the substrate [ 59 ].
PHAs are biodegradable in windrow composting, soil or marine sediments. The enzymes which are involved in the degradation of PHAs are depolymerases, hydrolases which may be intra- or extracellular and endo- or exoenzymes The enzymes may be classified as intra- or extracellular according their action inside or outside the cell, and also as endo- or exoenzymes, according their action inside or at the end of the substrate molecule.
They are usually induced enzymes whose expression is repressed in the presence of other carbon sources such as glucose and organic acids. PCL is a biodegradable polyester obtained from raw materials originating from petroleum, through ring opening polymerization of the lactone with suitable catalysts Figure 2.
It has good resistance to water and organic solvents. PCL is a polymer stable against abiotic hydrolysis, which occurs slowly with molecular weight decrease. Its melting temperature is low, as its viscosity, facilitating its thermal processing. PCL may present spherulitic structure.
It is a soft and flexible polymer, that may be used in blends with other biodegradable polymers, such as starch. A major global manufacturer is Solvay Capa, 5, t per year. Some applications are foamed food trays, bags, bioabsorbable medical items, replacement of gypsum in the treatment of broken bones, etc.
PCL may be degraded by many microorganisms, including bacteria and fungi, that are spread by soils and water bodies [ 56 ]. However, an initial stage of abiotic hydrolysis appears to be necessary [ 61 ]. The rates of hydrolysis and biodegradation depend on molecular weight and crystallinity [ 40 ]. Pronounced biodegradation occurs with molecular weights below about 5, g-mol Abiotic and biotic degradation take place preferentially in the amorphous phase.
Enzymes from the two major classes of excreted esterases - lipases and cutinases - are able to degrade PCL and its blends [ 62 ]. Biodegradation causes surface erosion, without reduction of molecular weight [ 54 ]. PLA is an aliphatic polyester, derived from renewable resources, e. It is a polymer produced from lactic acid Figure 2 , which is obtained from the fermentation of various carbohydrate species: glucose, maltose and dextrose from corn or potato starch; sucrose from beet or sugar cane; and lactose from cheese whey [ 63 ].
The lactic acid monomer may be obtained by fermenting carbohydrate crops such as corn, sugar cane, cassava, wheat and barley, being eventually converted to lactide by means of a combined process of oligomerization and cyclization, with the use of catalysts. Mitsui used a solvent based process to remove water azeotropically in the condensation polymerization process.
Neste has obtained high molecular weight PLLA i. All the others use the dimer lactide process, with lactide ring opening polymerization. In the process using lactides, the additional step of dimerization of lactic acid increases production costs, but improves the control of molecular weight and end groups of the final polymer [ 38 ].
Through the stereochemical control of lactic acid ratio of D- and L- optical isomers , one can vary the crystallinity of PLA and also rate of crystallization, transparency, physical properties and even the biodegradation rate. DL-PLA is used when it is important to have a homogeneous dispersion of the active species in the single-phase matrix, such as in devices for controlled release of drugs in the same manner that PLAGA copolymers.
L-PLA is preferred for applications where mechanical strength and toughness are required, such as in sutures and orthopedic appliances. The mechanical properties are somewhat higher than those of polyolefins in general. PLA is a hard material, similar in hardness to acrylics as methyl methacrylate.
Because of its hardness, PLA fractures along the edges, resulting in a product that cannot be used. To overcome these limitations, PLA must be compounded with other materials to adjust the hardness [ 65 ]. The low glass transition temperature see Table 2 is the reason for the limited resistance of PLA to heat, making PLA inadequate for hot drink cups, for example.
PLA is suitable for frozen food or for packages stored at ambient temperatures. It is a polymer with consolidated use in the medical area, due to its biocompatibility and biodegradability in the human body. PLA-based resins may be modified to adapt to many applications, from disposable food-service items to sheet extrusion, and coating for paper [ 40 ].
The abiotic degradation of PLA takes place in two stages: a diffusion of water through the amorphous phase, degrading that phase; and b hydrolysis of crystalline domains, from the surface to the center [ 61 ]. The ester linkages are broken randomly. A semicrystalline material such as poly L-lactate presents a hydrolysis rate much lower than that from an amorphous material, such as poly D,L-lactate , with half-lives of, respectively, one or a few years, and a few weeks.
The hydrolysis is self-catalyzed by the acidity of the resulting carboxylic groups [ 66 ]. PLA can not directly be degraded by microorganisms, but requires first abiotic hydrolytic degradation, so that the microorganisms mainly bacteria and fungi, which form biofilm can metabolize the lactic acid and its oligomers dissolved in water.
Abiotic hydrolysis takes place at temperatures above the glass transition temperature, i. Thus PLA is fully biodegradable in composting conditions of municipal waste plants, although it may need a few months to several years to be degraded under conditions of home composting, soil or oceans [ 35 , 63 , 67 ]. Furthermore, the PLA degrading microorganisms are not widespread in the environment [ 20 , 61 , 67 ].
The polymer passes the tests of compostability, provided that the thickness of the parts do not exceed around mm. The extracellular enzymatic degradation consists of two steps: a the enzyme is adsorbed on the polymer surface, through its binding site; and b ester bonds are cleaved through the catalytic site of the enzyme [ 61 ].
The polymer chain ends are attacked preferentially. The biodegradation rate is a function of the crystallinity and the content of L-monomers [ 68 ]. Some enzymes proteases that may degrade PLA are proteinase K, pronase and bromelain. It is the simplest linear polyester, consisting only of a methylenic group between the ester linkages.
It may be synthesized in a way quite similar to that of PLA, by the ring opening polymerization of glycolide, that is the cyclic dimer of glycolic acid. Glycolate is copolymerized with lactate in order to obtain a copolymer with appropriate stiffness and elongation capacity known as PLAGA.
The biodegradation of the PGA is usually faster than that of PLA, although an initial stage of abiotic hydrolysis appears to be necessary, since the polymer has a phase in the crystalline state and another in the amorphous glassy state [ 61 ].
PGA and its copolymers with lactic acid have very important medical applications: body absorbable sutures; ligaments reestablishment, through resorbable plates and screws; drugs of controlled release; grafting of arteries; etc. Although the homopolymers PGA and D-PLA are not biodegradable, copolymers of glycolic acid and D-lactic acid, which may still contain L-lactic acid, are usually biodegradable by lipase enzymes [ 67 ].
The degradation of PGA seems to follow the same steps of PLA: diffusion of water into the amorphous region, with degradation and erosion; hydrolytic attack of the crystalline region; and biodegradation of monomers and oligomers dissolved in water. The aliphatic-aromatic polyesters have petrochemical origin, and are generally produced through traditional polycondensation reactions.
They consist of aliphatic chain segments residues of 1,4-butanediol, and of adipic or succinic acid , which provide flexibility, toughness, and biodegradability and aromatic segments residues of terephthalic acid and 1,4-butanediol , which impart mechanical strength and rigidity. PET, an aromatic polyester, decomposes very slowly in recalcitrant aromatic oligomers [ 69 ]. The degradation of the aliphatic-aromatic polymers may be oxidative, hydrolytic and enzymatic.
The hydrolytic degradation occurs in presence of water, and is self-catalyzed by the acidity of the carboxylic acids, being more intense inside the part. The enzymatic hydrolysis uses non-specific enzymes, such as hydrolases and lipases, produced by an enormous variety of organisms, especially the mycelium-forming microorganisms fungi and actinomycetes [ 69 ]. The amorphous regions are degraded preferentially over the crystalline regions, both chemically and biologically.
Interestingly, there is an inverse relationship between the melting temperature of the polyester and its rate of biodegradation, indicating that the crystalline characteristics are a very important factor in its biodegradability [ 69 , 70 ]. The polyesters that behave like elastomers at the degradation temperature, undergo enzymatic degradation from the moment in which they are placed in the disposal environment, showing surface erosion.
The polyesters that behave like glass at the degradation temperature are enzymatically degraded only at the end of the degradation process, from the by-products of the preliminary abiotic degradation. Although the aliphatic-aromatic polyesters present high degradability in industrial composting, their rate of degradation in soil and water bodies is much lower, and their degradability under anaerobic conditions is even lower [ 69 ].
Some applications are the same typical for LDPE: transparent blown films, mulch films for agriculture, films for package and bags, and also blown bottles, filaments, injection moulded and thermoformed products [ 69 ].
Poly butylene adipate-co-terephthalate PBAT : Some of its main applications are films mulch, containers, bags , filaments, thermoformed and injection moulded products, and blown bottles. It may also be mixed with thermoplastic starch [ 69 , 71 ]. Poly butylene succinate-co-terephthalate PBST : Some of its main applications are blown films, filaments, blown and injection moulded containers, thermoformable cups and trays, paper coating, etc.
PBST has good mechanical properties, reasonable processing and biodegrades slowly [ 69 , 72 ]. There is not a standard test of biodegradability of universal validity. For the hydro-biodegradable and inherently biodegradable polymers, it is common the use of patterns for testing compostability. These are standards for biodegradation in the special conditions found in industrial composting, that require short timescales and rapid CO 2 emissions.
There are also standards for biodegradation in soils and aquatic environments but they are less used. Finally, the third group of biodegradable polymers consists of oxo-biodegradable materials, that is, those that need to undergo the chemical process of oxidation combination with oxygen, which leads to breakage of the molecules before they become biodegradable. In general, all the traditional widely used plastic materials are oxo-biodegradable, that is, over time undergo oxidative degradation, what leads to the breakdown of their molecules into smaller fragments, highly oxygenated, capable of being biodegraded.
However the time scale to complete degradation and biodegradation is too long: it takes several decades [ 25 , 26 , 73 ]. To accelerate this process, organic salts such as stearates of transition metals such as iron, manganese and cobalt are added, so reducing the time required for degradation and biodegradation to some years [ 74 ].
Such additives are known as pro-oxidants or pro-degrading. Until now, these salts have shown no toxicity to animals, plants or microorganisms, being rather micronutrients to them.
To this group belong lignin, lignocellulose, natural rubber and polybutadiene without the need of pro-oxidant additives as well as traditional plastics, such as polyethylene, polypropylene, polystyrene and PET, all these formulated with pro-oxidant additives. Oxo-biodegradable polymers are the polymer materials that present in their formulation pro-oxidant and antioxidant additives, so as to provide a planned period of useful life, after which the materials begin to degrade oxidatively, the residues being inherently biodegradable.
It is also possible that the polymer contains pro-oxidant chemical structures, such as double bonds and atoms susceptible to attack by free radicals.
The oxidation process is called peroxidation, and occurs through a free radical mechanism [ 1 ]. The first step is the formation of a free radical in the polymer i.
Then the polymer radical formed may capture an oxygen molecule, generating a peroxide radical, which after capturing a hydrogen atom bound to the polymer will form a hydroperoxide bond. The hydroperoxide decomposes over time, generating an alkoxy and a hydroxyl radical. The decomposition can be accelerated by about 10 2 times with the use of catalysts based on organic salts of Fe, Mn, Co, etc [ 27 , 28 ].
These salts also help to carry oxygen to the polymer molecules. The hydroxyl radical may capture hydrogen atoms, generating new macrorradicais. The alkoxy radical can recombine, generating a ketone group, or breaking the molecule, generating a new radical. The ketone group is susceptible to degradation by UV, which can cause rupture of the chain, by the mechanisms of Norish I and II.
The free radical reactions involving organic polymer and oxygen generate many different molecules, which may contain the groups hydroxyl, carbonyl, ether, ester, carboxyl, etc. Therefore, the final product from the polymer abiotic degradation generally consists of small and strongly oxygenated molecules, capable of crossing the cell wall if present and membrane, and that are metabolyzed in the cytoplasm of microorganisms with the help of the available enzymes, which depend on the chemical structure of the oligomers and the genetic potential of the organism [ 4 ].
The antioxidant additives present in the formulation of a polymer resin may have the function of protecting it against degradation by deactivating the free radicals formed primary antioxidants or by decomposing hydroperoxides formed secondary antioxidants. The former are useful during the service life of the material at ambient temperatures, whereas the latter are most useful when processing the material at elevated temperatures [ 75 ]. Molecular weight reduction is generally a consequence of oxidative degradation being e.
Oxidative degradation also causes the incorporation of oxygen atoms in the chains and the rise of double bonds. The residue from abiotic degradation of a plastic material is no longer plastic, but a complex mixture of unsaturated and oxygenated oligomers, showing some hydrophilicity and being biodegradable by a large number of genera of naturally occurring microorganisms [ 25 , 26 , 28 ].
Characteristics of oxo-biodegradable polyethylene films subjected to weathering for different periods of time [ 28 ]. The increase in crystallinity can be explained by the higher freedom of motion of smaller polymer chains, that could be rearranged in more crystalline structures [ 28 ]. The rates of biodegradation of the residues from oxo-biodegradable materials are usually lower than those of most hydro-biodegradable materials.
It generally takes a few years to quantitative biodegradation of the oxo-bio materials, depending on resin type, environmental conditions and formulation of additives used. After a certain level of oxidative degradation, biofilms can be observed on the oxidized polymer residues [ 25 - 27 ].
These biofilms mainly consist of fungi and bacteria, although archaea, algae and protozoa may also be present. The oxo-bio materials may be recycled with conventional polymer materials, provided that the resins still contain antioxidant additives in concentration sufficient to prevent oxidative degradation during processing and service life. Some people consider that the residues of oxo-biodegradable polymers contain toxic metals, but so far there is no evidence of toxicity of them to plants or animals.
Instead, the cations of Fe, Co and Mn are micronutrients, acting as cofactors of enzymes. At very high concentrations, these cations may damage the plants, even because they increase the osmotic pressure of the environment and may dehydrate the root cells.
Standards for slowly biodegradable products are a challenge, because it becomes very difficult to simulate in a laboratory the biodegradation that takes place in real environments. In laboratory microcosms, the environment is isolated, without the possibility of free mass exchange with the surroundings. So there may be accumulation of metabolites produced by microorganisms, to the point where they become toxic and disrupt microbial growth.
Thus, it is possible that only the beginning of biodegradation can be observed, but this does not mean that biodegradation would be interrupted in real conditions. It is not possible to provide a specific timescale in a general standard for oxo-biodegradable polymers, as distinct from a standard for industrial composting because the conditions found in industrial composting are specific and the conditions found in the open environment are variable.
Also, the time taken for oxo-biodegradable plastic to commence and complete the processes of degradation and biodegradation can be varied.
They usually require three test levels: 1 abiotic degradability decrease of molecular weight and mechanical properties, low gel formation ; 2 biodegradability biofilm formation, release of CO 2 , tested with the residue obtained at level 1 ; and 3 ecotoxicity to animals, plants and microorganisms of the residues from levels 1 and 2.
A highly sensitive method for measuring low rates of biodegradation was developed by Chiellini et al. In order to maintain a good compromise among biodegradability, chemical and physical properties, and costs, mixtures or blends of polymers of any of the three groups mentioned above i.
Some biodegradable polymers are very rigid and brittle e. The mixture blend of two or more different polymers may lead to a blend of interesting intermediate mechanical properties. The processability of the final material may also be improved, included here the resistance to chemical decomposition during processing, among other features.
Some polymer materials are produced wholly or partly from renewable the term renewable here is very limited, as was previously explained raw materials. Some examples are listed below. Braskem - green polyethylene : is the traditional polyethylene, but derived from ethylene produced with ethanol from sugar cane.
DuPont - PTT - poly trimethylene terephthalate : one of its monomers, 1,3-propanediol, is obtained from corn or sugar beets. In addition, the PP cap is slightly smaller. The proportion of raw materials obtained from fossil fuels oil, coal and gas and obtained from plants can be found through analysis of the proportion of carbon to carbon present in the polymer, since fossil fuels contain virtually no carbon whose half-life is about years see ASTM D The environmental impacts produced by conventional polymers on the planet are now clearly observed.
As a consequence, these materials, especially plastic bags, have suffered many attacks in several countries, and alternative solutions to their use have been encouraged.
However, to date, no definitive and universal solution for the replacement of conventional polymer materials has emerged.
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