Specialty: Methods and methodology of teaching chemistry

Ministry of Education of the Republic of Azerbaijan

Azerbaijan State Pedagogical University

Specialty:Methods and methodology of teaching chemistry

Course:I

Group:KIM2107M

Teacher:Faida İsayeva

Student:Ayla Aliyeva

Subject:English language

Topic:Synthetic Polymers

Bakı-2021

Synthetic Polymers

People have always used polymers. Prehistoric tools and shelters made from wood and straw derive their strength and resilience from cellulose, a biopolymer of glucose. Clothing made from the hides and hair of animals is made strong and supple by proteins, which are biopolymers of amino acids. After people learned to use fire, they made ceramic pottery and glass, using naturally occurring inorganic polymers.

A polymer is a large molecule composed of many smaller repeating units (the monomers) bonded together. Today when we speak of polymers, we generally mean synthetic organic polymers rather than natural organic biopolymers such as DNA, cellulose, and protein, or inorganic polymers such as glass and concrete. The first synthetic organic polymer was made in 1838, when vinyl chloride was accidentally polymerized. Polystyrene was discovered in 1839, shortly after styrene was synthesized and purified. The discovery of polystyrene was inevitable, since styrene polymerizes spontaneously unless a stabilizer is added.

Also in 1839, Charles Goodyear (of tire and blimp fame) discovered how to convert the gummy polymeric sap of the rubber tree to a strong, stretchy material by heating it with sulfur. Vulcanized rubber quickly revolutionized the making of boots, tires, and rainwear. This was the first time that someone had artificially cross-linked a natural biopolymer to give it more strength and stability.

In fewer than 150 years, we have become literally surrounded by synthetic polymers. We wear clothes of nylon and polyester, we walk on polypropylene carpets, we drive cars with ABS plastic fenders and synthetic rubber tires, and we use artificial hearts and other organs made of silicone polymers. Our pens and computers, our toys and our televisions are made largely of plastics.

Articles that are not made from synthetic polymers are often held together or
coated with polymers. A bookcase may be made from wood, but the wood is bonded by a phenol-formaldehyde polymer and painted with a latex polymer. Each year, about 400 billion pounds of synthetic organic polymers are produced worldwide, mostly for use in consumer products. Large numbers of organic chemists are employed to develop and produce these polymers.

We discuss some of the fundamental principles of polymer chemistry. We begin with a survey of the different kinds of polymers, then consider the reactions used to induce polymerization. Finally, we discuss some of the structural characteristics that determine the physical properties of a polymer.

Classes of Synthetic Polymers

The two major classes of synthetic polymers are chain-growth polymers and step-growth polymers. Chain-growth polymers result from the rapid addition of one monomer at a time to a growing polymer chain, normally with a reactive intermediate (cation, radical, or anion) at the growing end of the chain. Chain-growth polymers are usually addition polymers, which result from monomers adding together without the loss of any molecules. Monomers for chain-

growth polymerization are commonly alkenes, and polymerization involves successive additions across the double bonds. Poly(vinyl chloride), widely used as a synthetic leather, is a chain-growth addition polymer.

In a step-growth polymerization, any two monomers having the correct functionality can react with each other, or two polymer chains can combine. Most step-growth polymers are condensation polymers, bonded by some kind of condensation (bond formation with loss of a small molecule) between the monomers or the polymer segments. The most common condensations involve the formation of amides and esters. Dacron polyester is an example of a step-growth condensation polymer.

Many alkenes undergo chain-growth polymerization when treated with small amounts of suitable initiators. The products are addition polymers, resulting from repeated additions across the double bonds of the monomers. Table 26-1 shows some of the most common addition polymers, all made from substituted alkenes. The chain-growth mechanism involves addition of the reactive end of the growing chain across the double bond of the monomer. Depending on the monomer and the initiator used, the reactive intermediates may be free radicals, carbocations, or carbanions. Although these three types of chain- growth polymerizations are similar, we consider them individually.

Free-Radical Polymerization

Free-radical polymerization results when a suitable alkene is heated with a radical initiator. For example, styrene polymerizes to polystyrene when it is heated to 100 °C in the presence of benzoyl peroxide. This chain-growth polymerization is a free-radical chain reaction. Benzoyl peroxide cleaves when heated to give two carboxyl radicals, which quickly decarboxylate to give phenyl radicals.

A phenyl radical adds to styrene to give a resonance-stabilized benzylic radical. This reaction starts the growth of the polymer chain. Each propagation step adds another molecule of styrene to the growing chain. This addition takes place with the orientation that gives another resonance-stabilized benzylic radical.

Chain growth may continue with addition of several hundred or several thousand styrene units. The length of a polymer chain depends on the number of additions of monomers that occur before a termination step stops the process. Strong polymers with high molecular weights result from conditions that favor fast chain growth and minimize termination steps. Eventually the chain reaction stops, either by the coupling of two chains or by reaction with an impurity (such as oxygen) or simply by running out of monomer.

Cationic Polymerization

Cationic polymerization occurs by a mechanism similar to the free-radical process,except that it involves carbocation intermediates. Strongly acidic catalysts are used to initiate cationic polymerization. BF₃ is a particularly effective catalyst, requiring a trace of water or methanol as a co-catalyst. Even when the reagents are carefully dried, there is enough water present for the first initiation step of the mechanism shown in Mechanism.

Cationic polymerization requires relatively stable carbocation intermediates.

A major difference between cationic and free-radical polymerization is that the cationic process needs a monomer that forms a relatively stable carbocation when it reacts with the cationic end of the growing chain. Some monomers form more stable intermediates than others. For example, styrene and isobutylene undergo cationic polymerization easily, while ethylene and acrylonitrile do not polymerize well under these conditions. Figure compares the intermediates involved in these cationic polymerizations.

Anionic Polymerization

Anionic polymerization occurs through carbanion intermediates. Effective anionic polymerization requires a monomer that gives a stabilized carbanion when it reacts with the anionic end of the growing chain. A good monomer for anionic polymerization should contain at least one strong electron-withdrawing group such as a carbonyl group, a cyano group, or a nitro group. The following reaction shows the chain-lengthening step in the polymerization of methyl acrylate. Notice that the chain-growth step of an anionic polymerization is simply a conjugate addition to a Michael acceptor

Anionic polymerization is usually initiated by a strong carbanion-like reagent such as an organolithium or Grignard reagent. Conjugate addition of the initiator to a monomer molecule starts the growth of the chain. Under the polymerization conditions, there is no good proton source available, and many monomer units react before the carbanion is protonated. Mechanism 26-3 shows a butyllithium-initiated anionic polymerization of acrylonitrile to give Orlon.

Chain-growth polymerization of alkenes usually gives a head-to-tail bonding arrangement, with any substituent(s) appearing on alternate carbons of the polymer chain. This bonding arrangement is shown here for a generic polyalkene. Although the polymer backbone is joined by single bonds (and can undergo conformational changes), it is shown in the most stable all-anti conformation.

The stereochemistry of the side groups (R) in the polymer has a major effect on the polymer’s properties. The polymer has many chirality centers, raising the possibility of millions of stereoisomers. Polymers are grouped into three classes, according to their predominant stereochemistry. If the side groups are generally on the same side of the polymer backbone, the polymer is called isotactic (Greek, iso, meaning “same,” and tactic, meaning “order”). If the side groups generally alternate from one side to the other, the polymer is called syndiotactic (Greek, meaning “alternating order”). If the side groups occur randomly on either side of the polymer backbone, the polymer is called atactic (Greek, meaning “no order”). In most cases, isotactic and syndiotactic polymers have enhanced strength, clarity, and thermal properties over the atactic form of the polymer. Figure 26-3 shows these three types of polymers.

Plasticizers

In many cases, a polymer has desirable properties for a particular use, but it is too brittle- either because its glass transition temperature is above room temperature or because the polymer is too highly crystalline. In such cases, addition of a plasticizer often makes the polymer more flexible. A plasticizer is a nonvolatile liquid that dissolves in the polymer, lowering the attractions between the polymer chains and allowing them to slide by one another. The overall effect of the plasticizer is to reduce the crystallinity of the polymer and lower its glass transition temperature .

A common example of a plasticized polymer is poly(vinyl chloride). The common atactic form has a of about 80 °C, well above room temperature. Without a plasticizer, “vinyl” is stiff and brittle. Dibutyl phthalate (see the structure at left) is added to the polymer to lower its glass transition temperature to about 0 °C. This plasticized material is the flexible, somewhat stretchy film we think of as vinyl raincoats, shoes, and even inflatable boats. Dibutyl phthalate is slightly volatile, however, and it gradually evaporates. The soft, plasticized vinyl gradually loses its plasticizer and becomes hard and brittle.