FIGURE 7-5 The structure of the phthalic acid plasticizer di(2-ethylhexyl)phthalate.
DEHP might affect development of their offspring." The concern arose from rodent studies in which juvenile rats that ingested DEHP had lower testicular weight, testicle degeneration, and reduced sperm counts in comparison to controls. From these results, the possibility was raised that DEHP might be leached from vinyl surgical tubing by fluid running through it to infants with serious medical problems. In addition the report led many toy manufacturers to stop making toys with plastics that contained phthalate plasticizers. While there are few expensive or non toxic plasticizers that can replace phthalates in vinyl polymers, there are plastics that can replace vinyl in medical devices. DEHP is also a potential carcinogen.
Phthalates are abundant in the environment because they are used extensively (10 x 109 lbs produced worldwide in 1999) and because they are degraded at a moderate rate by microorganisms that use a variation on the oxidative pathway illustrated in reaction (7-24).
The bulk of the monomers used in polymer synthesis are synthesized directly or indirectly from petroleum. Therefore, our present dependence on plastic materials is ultimately a dependence on sources of crude oil.
Polymers may be formed from monomers in chain reaction or step reaction polymerization. The chain reaction process is illustrated by the free-radical polymerization of ethylene or its derivatives. The specific example of the polymerization of acrylonitrile to Orlon is given in reactions (7-26)-(7-30).
7.3.1 Polymer Synthesis
CN CN CN CN
2ROCH2CH(CH2CH)mCH2CH-CN CN CN
ROCH2CH(CH2CH)mCH2CH-CHCH2(CHCH2)mCHCH2OR CN CN CN CN CN CN
ROCH2CH(CH2CH)bCH = CH + CH2CH2(CHCH2)„CHCH2OR
In free-radical polymerizations, a small amount of initiator, such as a peroxide, is added to the monomer to start the polymerization. The initiator decomposes to form radicals (7-26), which add to the monomer in the chain-initiating step (7-27). Further addition of monomer takes place in the chain-propagating step (7-28). Polymer growth stops in the chain-terminating steps, which require the reaction of two radicals such as the combination and disproportionation reactions shown in (7-29) and (7-30).
Ionic processes may also be involved in chain reaction polymerization. Cationic polymerization of vinyl monomers is initiated by acids. The synthesis of propene oligomers from propene by this process has already been discussed (Section 126.96.36.199). Anionic polymerization is often initiated by strong bases such as butyllithium (CH3CH2CH2CH^ Li+). In these chain reaction polymerization processes, the molecular weight of the final polymer is dependent on the maintenance of the active species, be it the radical, cation, or anion formed initially. In step reaction polymerization, monomers and oligomers react to form polymers. The formation of a nylon illustrates this process:
HOC(CH2)4COH + NH2(CH2)6NH2 2 » HOC(CH2)4CNH(CH2)6NH2
NH2(CH2)6NH II II -^^-- NH2(CH2)6NHC(CH2)4CNH(CH2)6NH2 (7-31)
continued addition of monomers and reaction . II II .
- -|-C(CH2)4CNH(CH2)6NH}-b between oligomers already formed a Nylon
The polyester poly(ethylene terephthalate) (PET), (Dacron or Terylene) is prepared in a similar fashion from ethylene glycol and terephthalic acid (reaction 7-32). These step reaction polymerizations are examples of condensation polymerizations. A small molecule, water here, is eliminated in each step. Many of the biodegradable polymers are aliphatic polyesters prepared by step reaction condensation reactions as shown earlier (Figure
Paper is composed mainly of cellulose, a polymer made up of ^-glucose units linked through oxygen atoms. Paper is generally produced by the chemical removal of lignin, resins, and other components of wood to leave cellulose fibers (pulp), which is compacted to produce the paper sheet. Mechanical grinding can also be used to facilitate the removal of the lignin, but this yields a lower quality paper. The kraft pulping process hydrolyzes the lignin with a sodium hydroxide-sodium sulfide solution, producing organic sulfur compounds and other organic material in what is called black liquor. This can be a considerable source of pollution, but most of it is treated and recycled for reuse of the reactants. The sulfite process, used for high-quality papers, uses calcium and magnesium sulfites produced from SO2 and the corresponding carbonate.
The pulp, especially from the kraft process, contains oxidized organic compounds that produce a brown color and must be bleached. Traditionally, this has required chlorine to produce the actual bleaching agent employed, chlorine dioxide (Section 11.5.1). Paper manufacturing thus has been a heavy user of chlorine and sodium hydroxide, and paper plants have often operated their own chloralkali plants with their associated environmental problems (Section 11.5.1). Recent practice is to replace the chlorine-based bleach with an alternative process that employs oxygen as the bleaching agent,
Most paper products contain various additives—fillers such as clay or titanium dioxide, resins to improve wet strength, sizings, coloring, and so on. These differ depending on the type of paper and add to the complications of paper recycling (Section 16.7.2).
About 108 million tons of plastics was produced in the world in 1996, with 24% of the production in the United States, 25% in western Europe, 9% in Japan, and 25% in the rest of Asia. In the same year western Europe discarded 19.3 million tons of plastic waste; 10% of this waste was recycled, 14% was used for energy, and the rest was placed in landfills. In 1990 25 million tons of plastics was manufactured in the United States, while during the same time period 16 million tons was placed in landfills (this does not include about 1.8 million tons of tires also placed in landfills). Only about 0.4 million tons of the plastic was reused or recycled. Plastic material constitutes about 10% by weight and 20% by volume of the material in landfills, with paper being the principal constituent at 32% by weight and by volume. As discussed in Section 16.2, relatively little decomposition takes place in landfills, so very little of this plastic material disappears with time.
The plastic that is not placed in landfills, recycled, used for energy, or incinerated, is scattered either on land or in the sea. These materials, which have been designed to be strong and stable, persist for a long time. For example, it is estimated that the six-pack plastic strap made of high-density polyethylene has a lifetime of 450 years in the sea. It is estimated that 6 million tons of garbage is dropped in the oceans each year by boats. The amount of plastic material in this garbage has not been estimated, but it includes commercial fishing nets, fishing line, ropes, bags, lids, six-pack rings, disposable diapers, gloves, tampon applicators, bottles, plastic foam, and pellets to name a few.
The principal concern associated with plastic materials scattered on land is one of aesthetics, not toxicity or harm to ecosystems. Plastic articles dumped in oceans are not aesthetically pleasing when they wash up on beaches, but of even greater importance is their danger to marine life. It has been estimated that plastics kill or injure tens of thousands sea birds, seals, sea lions, and sea otters each year, and hundreds of whales, porpoises, bottlenose dolphins, and sea turtles. A large portion of the reported fatalities are due to the entanglement of birds and mammals in the driftnets of commercial fishing boats. These nets, up to 30 km long, entangle seals, bottlenose dolphins, porpoises, and birds. For example, the 4-6% annual decline in the population of the northern fur seal (20,000-40,000 seals), which live in the Bering Sea off Alaska, is attributed to entanglement in drift nets. Most of these drift nets are in use and cannot be classified as a disposal problem, but free-floating drift nets, which have broken loose or have been discarded and continue to trap sea life, are an environmental problem. The most dramatic victims are whales that are unable to dive and feed because they are entangled in large piece of drift net. Dying, beached whales wrapped in drift nets have been found on both the east and west coasts of the United States.
Other plastic items also endanger sea life. For example, seal pups play with plastic items floating in the sea. If pups happen to stick their heads through plastic collars, as they grow the plastic will sever their neck arteries or strangle them. Of perhaps even greater danger is the ingestion of plastic items by some marine life. A floating plastic bag can look like a tasty jellyfish to a sea turtle. When turtles consume a lot of plastic it blocks their intestines, preventing them from assimilating food. Sea birds, which often mistake plastic pellets for fish eggs or other marine food, also suffer from intestinal blockage if they consume large quantities of plastic. The use of biodegradable polymers will help to reduce the level of plastic in the ocean, but extensive research will be necessary to develop polymers that not only are biodegradable but also have the strength and other desirable properties of the nonbiodegradable polymers in use today. Moreover, after such materials have been developed, the consumer will have to be convinced to purchase biodegradable products even though they will slowly degrade and therefore may have a shorter useful life than the corresponding nonbiodegradable product.
A goal of the plastics industry is to design biodegradable polymers that can be substituted for materials that are difficult to collect and reuse. These include water-soluble polymers, polymers designed for use for fishing or other marine applications that end up in the marine environment, and polymers employed with other materials such as coatings on cups, diapers, and sanitary products. Polymers that tend to be used only once such as six-pack plastic straps, plastic cutlery, and hospital waste should also be degradable. Microorganisms use degradable polymers for their growth, and thus the polymer will be converted to compounds that can be utilized in the natural cycle of life on earth. As noted already, these polymers will break down only where there is abundant molecular oxygen, so the designation "biodegradable" does not apply to the anaerobic environment in a landfill.
7.3.4 Environmental Degradation of Polymers
Many hydrocarbon polymers have chemical reactivity similar to that of high boiling petroleum fractions and, as a consequence, these compounds are very persistent in the environment (Section 7.2.4). Until recently, the goal of the polymer chemist was to design a polymer that degrades very slowly in the environment. Each new polymer was tested extensively to be sure that it did not break down rapidly in its particular use. For example, a material designed for use in vinyl siding for homes would be tested for its resistance to degradation by oxygen, water, and sunlight. Some products made from vinyl polymers appear to degrade in the environment, but this is mainly due to the loss of their phthalate ester plasticizer, which results in a change of the mechanical properties of the polymer. They vinyl products become brittle and crack, but the actual degradation of the the polymer proceeds more slowly.
Polymers will be degraded if they absorb sunlight at wavelengths greater than that of the radiation that is not absorbed by stratospheric ozone (290 nm: see Section 5.2.3). Since many polymers do not have functionality that results in light absorption at wavelengths greater than 290 nm, direct photolysis is not usually a major degradative pathway. Indirect photochemical processes, such as oxidation by photochemically generated singlet oxygen (Sections 5.2.2 and 6.4) can also result in polymer degradation.
The peroxides formed by polymer processing may trigger photochemical polymer degradation. Mechanical shearing of polymers during processing results in radical formation as a result of chain breaking [reaction (7-33)], and these radicals react with molecular oxygen to form hydroperoxides, as shown in reaction (7-34) and (7-35).
Hydroperoxides absorb light at wavelengths greater than 290 nm and are cleaved to hydroxyl radicals and alkoxy radicals with a quantum yield close to one, according to reaction (7-36).
The radicals react further as follows to form alcohols:
Alternatively, hydroperoxides cleave to aldehydes or ketones [reaction (7-38)], and these undergo further photochemical chain scission as outlined next in Section 188.8.131.52.
ROOH —Vv—> R'(CO)R", R'"(CO)CHs (7-38)
The same radical processes may be triggered by impurities in the polymers that form peroxides on irradiation in the presence of oxygen. For example, iron(III) and titanium(IV) present in a polymer initiate peroxide formation from molecular oxygen. Exposure to the ozone, formed as a result of air pollution (Section 5.3), also leads to peroxide formation. Antioxidants, which react with free radicals, are added to many polymers to slow their rate of degradation.
184.108.40.206 Photodecomposition Triggered by Carbonyl Groups
Carbonyl groups that are present in polymers as a result of the decomposition of peroxides or are incorporated into the polymer during synthesis absorb UV light at 300-325 nm and initiate reactions leading to the cleavage of the polymer backbone. Two processes can occur, called Norrish type I and Norrish type II. In the type I reaction, bond breaking proceeds by a radical pathway (7-39), while a concerted electron shift occurs in the type II reaction (7-40):
II hn II II
Many polymers contain small amounts of carbonyl groups that are formed by the decomposition of peroxides, which initiate the slow decomposition of the polymer. For example, unstabilized 2.5-mil-thick films of polyethylene fragment in less than 90 days in the summer Texas sun. It should be noted that this polyethylene is only "fragmented," not totally broken down. Polymers have been prepared in which carbonyl groups were purposely incorporated into the chain to facilitate their extensive photochemical degradation. Polystyrene cups in which 1% of the styrene units also contained a ketone grouping broke down to a wettable powder after standing outside in the sun for 3 weeks. Currently a copolymer of ethylene and carbon monoxide is being used to make the plastic strap that holds beverage cans in a six-pack because it undergoes photochemical decomposition in sunlight. This addresses the problem of wildlife getting caught in this strap as well as its general lack of environmental aesthetics.
Polyesters, polyamides, and polyurethanes undergo slow hydrolytic degradation to form the corresponding acid and amine or alcohol. This random process does not usually result in the loss of the strength of the polymer.
However, the rate of cleavage increases with time because the carboxyl and amine or alcohol groups that are formed serve as catalysts to cleave the ester or amide groupings tethered in their vicinity. These catalyze chain scission and loss in the strength of the polymer. The extent of chemical cleavage is also dependent on the physical properties of the polymer. Polymers with a high degree of crystallinity are less susceptible to cleavage than amorphous ones. The crystalline phase is less accessible to penetration by water, so there is less chance that hydrolysis will occur. Biodegradation, to be discussed next, often involves an enzymatic acceleration of the hydrolytic process.
Since biodégradation requires metabolism by microorganisms present in the environment, most of the biodegradable polymers contain functional groups that are subject to attack by microbial enzymes. This means that these polymers contain ester or amide groups that can be hydrolyzed or linear chains that can be oxidatively cleaved by the process described for linear surfactants (Section 7.2.4). A major problem is the inability of microorganisms to ingest high polymers through their cell walls; it is generally not possible for a microorganism to ingest a molecule that has a molecular weight greater than about 500 Da. Some microorganisms that degrade polyesters secrete esterases, enzymes that catalyze the hydrolysis of ester groups, which break the polymer into smaller fragments that can then be ingested. As noted earlier, polyesters also undergo slow hydrolysis at the ester bond in the environment, which will provide smaller, ingestible fragments. Other processes of chemical degradation may also provide fragments small enough to be biodegraded.
Synthetic polymers in commercial use that undergo biodegradation in the presence of molecular oxygen were listed in Figure 7-4. These constituted about 2% of the total U.S. plastics production in 1992 and are mainly polyesters. Poly(ethylene glycols) are prepared from ethylene oxide by a process similar to that outlined for nonylphenol ethoxylates (Section 220.127.116.11). Poly (ethylene terephthalate) (Figure 7-3), is a polyester manufactured in large amounts that does not undergo biodegradation, apparently because its aromatic rings do not bind in the catalytic sites of the microbial esterases.
Poly(p-hydroxybutyrate) (PHB) and the copolymer of p-hydroxybutyrate and p-hydroxyvalerate (PHBV) (Figure 7-4) are unique in that they are synthesized by microorganisms. These polymers are prepared and stored in the cell in granules. When grown under conditions where nitrogen is a limiting nutrient, the microorganisms produce 30-80% of their cell weight as granules that contain mainly PHB. The genes for the enzymes required to produce PHB have been cloned and expressed in E. coli and in a plant.
PHB is a brittle polymer that may be used in plastic soft drink bottles but is difficult to mold because its melting point and decomposition point are almost the same. It has been possible to induce the microorganisms to prepare the copolymer PHVB by adding some ^-hydroxyvalerate to their growth medium. The PHVB formed is more malleable than PHB and has physical properties similar to those of polypropylene. Now manufactured in Great Britain, it is used for making products that are marketed on the basis of their "environmentally friendly" or "green" image (e.g., shampoos, razors, writing pens). In 1999 it was reported that a genetically engineered plant (oilseed rape) produced PHVB directly. This process is not economically feasible at present, but it does suggest the possibility of the future preparation of polymers using renewable resources.
A polyethylene containing 5-20% starch granules is used in the manufacture of shopping bags that are claimed to be biodegradable. The starch granules are degraded by fungi and bacteria. Biochemical degradation of the starch granules from the plastic film enhances its permeability to other rea-ctants. Other additives such as transition metals may also be added to the polyethylene to enhance the photodegradation of the polyethylene via hydroperoxides (Section 18.104.22.168). It is not clear whether all the polyethylene is eventually completely degraded to biomolecules like acetate (Section 7.2.4).
The environmental problems associated with nonbiodegradable surfactants have been solved with the design of biodegradable replacements. A potential problem with biodegradable surfactants is the eutrophication of lakes due to the buildup of organics resulting from the microbial degradation process. Since, however, the principal cause of most lake eutrophication is runoff of organic waste (manure) and fertilizer from farms, the role of surfactants in this problem appeare to be minor.
Polymers derived from petroleum will continue to predominate into the twenty-first century. It is likely that new technology will be developed to recycle a greater proportion of the materials. The extent of the recycling may depend on whether "cradle to grave" responsibility is mandated for the company that initially synthesizes the polymer. Not all synthetic polymers will be used in a way that permits efficient recycling. Research in progress suggests that it may be possible to devise biodegradable polymers for these uses as long as they can be subjected to decomposition in an aerobic environment such as that present in composting. Although progress has been made in the synthesis of biodegradable polymers, these materials cost more and, in general, their properties are not as useful as those prepared from petroleum feedstocks.
This is a rapidly developing area, so it appears likely that biodegradable polymers will assume an increasingly larger percentage of the polymer market.
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7.1. (a) What structural units are cleaved from a soap when it decays in a sewage disposal plant or in the environment? Illustrate by equations the cleavage of one unit from the salt of octadecanoic acid [CH3(CH2 )^COOH]. (b) Identify the agent(s) in a sewage plant or environment responsible for this degradation.
(c) Why were some surfactants slow to break down in a sewage plant and in the environment?
7.2. (a) Surfactants and some hydrocarbons are metabolized by certain microorganisms in the environment. Write one or more reaction(s) to illustrated the overall process by which one fragment is cleaved by these microorganisms from hydrocarbon with the formula CH3(CH2)8CH3 and from surfactant of general formula CH3(CH2)16 CH2OSO3-(b) Explain with words and/or equations why the following compound is resistant to microbial degradation. Be specific in your answer.
CH3 CH3 CH3
7.3. Outline the steps in the free-radical polymerization of styrene.
7.4. (a) Which of each of the following pairs of compounds degrades more rapidly in the environment? CH3(CH2)8CH3 and CH3(CH2)48CH3, polyethylene and polyhydroxybutyrate, branched-chain nonylbenze-nesulfonate and linear nonylbenzenesulfonate, «-decane and a linear hydrocarbon of formula C50H102.
(b) Give one reason for your answer based on the structure of the compound.
(c) Use chemical equations to illustrate the first step by which each of the more environmentally reactive compounds degrades in the environment.
7.5. Chemists have devised synthetic polymers with a wide array of uses. Some are stronger than steel and many are much less expensive than paper.
(a) What problems are associated with the disposal of these polymers on land and in the ocean?
(b) Many polymers, even the so-called degradable ones, don't breakdown in these environments. Why not?
(c) What problems are associated with recycling synthetic polymers?
7.6. Materials constructed of the following polymers were thrown away in a forest by slovenly campers.
(a) Which of them will have been degraded by environmental in a year's time? Explain your answers.
(b) Give the reaction pathway by which degradation occurred for those that did break down (« = 1000 in every example).
Polyacrylonitrile CH-^— Polydichlorostyrene
Polyacrylonitrile CH-^— Polydichlorostyrene
(c) Write the reaction pathway for the polymers that decay at a significant rate in the environment.
7.7. Of the garbage that is dumped in the ocean by ships mainly the plastic ware (bottles, spoons, cups, etc.) ends up on beaches. Why aren't the other items present in garbage (paper, food, etc.) also washed up on the beaches?
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