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Learn about polymers, large molecules made of repeating units, their structure, classifications like thermoplastics, and examples of everyday polymers like nylon and cellulose.
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A large molecule that is made up of many smaller, repeating units is called a polymer • A polymer forms when hundreds or thousands of these small individual units, which are called monomers, bond together in chains. • The monomers that bond together to form a polymer may all be alike, or they may be different. • The properties of a polymer are different from those of the monomers that formed it.
Polymers are everywhere, making fabrics such as nylon and polyester, plastic wrap and bottles, rubber bands, and many more products you see every day.
Balls, uniforms, artificial turf, bandages used to wrap sprains, and nets used in hoops and goals are usually made of synthetic polymers. • Sports activities would be different today without synthetic polymers.
Proteins, DNA, the chitin exoskeletons of insects, wool, silky spiderwebs and moth cocoons, and the jellylike sacs that surround salamander eggs are polymers that are synthesized naturally. • Living cells are efficient polymer factories.
The strong cellulose fibers that give tree trunks enough strength and rigidity to grow hundreds of feet tall are formed from monomers of glucose, which is a sweet crystalline solid.
Structure of Polymers • If you examine the structure of a polymer, you can identify the repeating monomers that formed it. • Because polymer molecules are large, they are commonly represented by showing just a piece of the chain.
The piece shown must include at least one complete repeating unit. • Look carefully at the structure of a segment of a cellulose molecule shown.
Cellulose is a polymer found in the cell walls of plant cells such as those of wood, cotton, and leaves. It is responsible for giving plants their structural strength • Notice that the ring parts of the molecule are all identical. • These are the monomer units that combine to form the polymer. • Glucose is the name of the monomer found in cellulose.
Another natural plant polymer that is formed from glucose monomers is starch. • Examine the structure of starch to see if you can find the difference between starch and cellulose.
Both cellulose and starch are made from only glucose monomers. The difference between them is the way that these monomers are bonded to each other.
Polymer ChemistryClassification of Polymers • The most common way of classifying polymers is to separate them into three groups - thermoplastics, thermosets, and elastomers. The thermoplastics can be divided into two types - those that are crystalline and those that are amorphous.
Thermoplastics Molecules in a thermoplastic are held together by relatively weak intermolecular forces so that the material softens when exposed to heat and then returns to its original condition when cooled. Thermoplastic polymers can be repeatedly softened by heating and then solidified by cooling - a process similar to the repeated melting and cooling of metals.
Most linear and slightly branched polymers are thermoplastic. All the major thermoplastics are produced by linear polymerization. • Thermoplastics have a wide range of applications because they can be formed and reformed in so many shapes. Some examples are food packaging, insulation, automobile bumpers, and credit cards.
Thermoplastics (80%) • No cross links between chains. • Weak attractive forces between chains broken by warming. • Change shape - can be remoulded. • Weak forces reform in new shape when cold.
Plastics • Although the terms plastic and polymer are often used synonymously, not all polymers are plastics. • Plastics are polymers that can be molded into different shapes. • After a polymer has formed, it must be heated enough to become liquefied if it is to be poured into a mold. • After pouring, the plastic will harden if it is allowed to cool.
Thermoplastic materials are easy to recycle because each time they are heated, they can be poured into different molds to make new products.
Crystaline polymers • Highly crystalline polymers are rigid, high melting, and less affected by solvent penetration. Crystallinity makes a polymers strong, but also lowers their impact resistance. As an example, samples of polyethylene prepared under high pressure (5000 atm) have high crystallinities (95 - 99%) but are extremely brittle.
Crystalline polymers • Areas in polymer where chains packed in regular way. • Both amorphous and crystalline areas in same polymer. • Crystalline - regular chain structure - no bulky side groups. • More crystalline polymer - stronger and less flexible.
Cold-drawing • When a polymer is stretched a ‘neck’ forms. • What happens to the chains in the ‘neck’? • Cold drawing is used to increase a polymers’ strength.
Amorphous Polymers Polymer chains with branches or irregular pendant groups cannot pack together regularly enough to form crystals. These polymers are said to be amorphous
Amorphous regions of a polymer are made up of a randomly coiled and entangled chains. They have been compared to a bucket containing a large number of entangled worms - each one 20-feet long and of 1/4-inch thickness. The worms are so tangled that an entire worm cannot slide past the others, but small portions of the worms can twist around within the mass.
Thermosets • A thermosetting plastic, or thermoset, solidifies or "sets" irreversibly when heated. Thermosets cannot be reshaped by heating. Thermosets usually are three-dimensional networked polymers in which there is a high degree of cross-linking between polymer chains. The cross-linking restricts the motion of the chains and leads to a rigid material
Thermosets are strong and durable. They primarily are used in automobiles and construction. They also are used to make toys, varnishes, boat hulls, and glues. A simulated skeletal structure of a network polymer with a high cross-link density is shown next.
Network Polymers Some polymers have cross-links between polymer chains creating three-dimensional networks. A high density of cross-linking restricts the motion of the chains and leads to a rigid material.A simulated skeletal structure of a network polymer with a high cross-link density is shown at the right.
Thermosets • Extensive cross-linking formed by covalent bonds. • Bonds prevent chains moving relative to each other. • What will the properties of this type of plastic be like?
Longer chains make stronger polymers. • Critical length needed before strength increases. • Hydrocarbon polymers average of 100 repeating units necessary but only 40 for nylons. • Tensile strength measures the forces needed to snap a polymer. • More tangles + more touching!!!
Elastomers Elastomers are rubbery polymers that can be stretched easily to several times their unstretched length and which rapidly return to their original dimensions when the applied stress is released.
Elastomers are cross linked, but have a low cross-link density. The polymer chains still have some freedom to move, but are prevented from permanently moving relative to each other by the cross-links.
To stretch, the polymer chains must not be part of a rigid solid - either a glass or a crystal. Rubber bands and other elastics are made of elastomers.
Polymerization Reactions • Polymerization is a type of chemical reaction in which monomers are linked together one after another to make large chains. • The two main types of polymerization reactions are addition polymerization and condensation polymerization.
Addition Reactions • In an addition reaction monomers that contain double bonds add onto each other, one after another, to form long chains. • The product of an addition polymerization reaction contains all of the atoms of the starting monomers.
Addition Reactions • The ethylene monomer contains a double bond, whereas there are none in polyethylene.
Addition Reactions • When monomers are added onto each other in addition polymerization, the double bonds are broken. • Thus, all of the carbons in the main chain of an addition polymer are connected by single bonds.
Addition polymerisation • Monomers contain C=C bonds • Double bond opens to (link) bond to next monomer molecule • Chain forms when same basic unit is repeated over and over. • Modern polymers also developed based on alkynes R-C C - R’
Chain-reaction polymerization, sometimes called addition polymerization, requires an initiator to start the growth of the reaction. The largest family of polymers 3, vinyl polymers, are produced by chain polymerization reactions. A good example is the free-radical polymerization of styrene, which is initiated by a free radical (R) that reacts with styrene. The compound that is formed still is a free radical, which can react again.
This reaction eventually leads to the formation of polystyrene
Condensation Reactions • In the second type of polymerization reactions, monomers add on one after another to form chains, as they do in an addition reaction. • However, with every new bond that is formed, a small molecule—usually water— is also formed from atoms of the monomers. • In such a reaction, each monomer must have two functional groups so it can add on at each end to another unit of the chain.
Condensation Reactions • This type of polymerization is called a condensation reaction because a portion of the monomer is not incorporated into the polymer but is split out—usually as water— as the monomers combine.
In a step-reaction polymerization reaction, sometimes called condensation polymerization, the polymer chains grow by reactions that occur between two molecular species.
Rubber • Another process that often occurs in combination with addition or condensation reactions is the linking together of many polymer chains. This is called cross-linking, and it gives additional strength to a polymer.
