17-06-2013, 03:19 PM
General Chemistry
[attachment=55558]
POLYMERS
Polymers are substances whose molecules have high molar masses and are composed of a large number of
repeating units. There are both naturally occurring and synthetic polymers. Among naturally occurring polymers are
proteins, starches, cellulose, and latex. Synthetic polymers are produced commercially on a very large scale and have
a wide range of properties and uses. The materials commonly called plastics are all synthetic polymers.
Polymers are formed by chemical reactions in which a large number of molecules called monomers are joined
sequentially, forming a chain. In many polymers, only one monomer is used. In others, two or three different
monomers may be combined. Polymers are classified by the characteristics of the reactions by which they are formed.
If all atoms in the monomers are incorporated into the polymer, the polymer is called an addition polymer. If some
of the atoms of the monomers are released into small molecules, such as water, the polymer is called a condensation
polymer. Most addition polymers are made from monomers containing a double bond between carbon atoms. Such
monomers are called olefins, and most commercial addition polymers are polyolefins. Condensation polymers are
made from monomers that have two different groups of atoms which can join together to form, for example, ester
or amide links. Polyesters are an important class of commercial polymers, as are polyamides (nylon).
POLYETHYLENE
Polyethylene is perhaps the simplest polymer, composed of chains of repeating –CH2– units. It is
produced by the addition polymerization of ethylene, CH2=CH2 (ethene). The properties of polyethylene
depend on the manner in which ethylene is polymerized. When catalyzed by organometallic compounds at
moderate pressure (15 to 30 atm), the product is high density polyethylene, HDPE. Under these conditions,
the polymer chains grow to very great length, and molar masses average many hundred thousands. HDPE is hard,
tough, and resilient. Most HDPE is used in the manufacture of containers, such as milk bottles and laundry
detergent jugs. When ethylene is polymerized at high pressure (1000–2000 atm), elevated temperatures
(190–210EC), and catalyzed by peroxides, the product is low density polyethylene, LDPE. This form of
polyethylene has molar masses of 20,000 to 40,000 grams. LDPE is relatively soft, and most of it is used in
the production of plastic films, such as those used in sandwich bags.
POLYPROPYLENE
This polymer is produced by the addition polymerization of propylene, CH2=CHCH3 (propene). Its
molecular structure is similar to that of polyethylene, but has a methyl group (–CH3) on alternate carbon
atoms of the chain. Its molar masses falls in the range 50,000 to 200,000 grams. Polypropylene (PP) is
slightly more brittle than polyethylene, but softens at a temperature about 40EC higher. Polypropylene is
used extensively in the automotive industry for interior trim, such as instrument panels, and in food packaging, such
as yogurt containers. It is formed into fibers of very low absorbance and high stain resistance, used in clothing and
home furnishings, especially carpeting.
POLYSTYRENE
Styrene, CH2=CH–C6H5, polymerizes readily to form polystyrene (PS), a hard, highly transparent
polymer. The molecular structure is similar to that of polypropylene, but with the methyl groups of polypro-
pylene replaced by phenyl groups (–C6H5). A large portion of production goes into packaging. The thin,
rigid, transparent containers in which fresh foods, such as salads, are packaged are made from polystyrene.
Polystyrene is readily foamed or formed into beads. These foams and beads are excellent thermal insulators and are
used to produce home insulation and containers for hot foods. Styrofoam is a trade name for foamed polystyrene.
When rubber is dissolved in styrene before it is polymerized, the polystyrene produced is much more impact resistant.
This type of polystyrene is used extensively in home appliances, such as the interior of refrigerators and air conditioner
housing. [For more information about this polymer, see Chemical Demonstrations: A Handbook for Teachers of
Chemistry, by Bassam Z. Shakhashiri, Volume 1 (1983), page 241.]
POLYVINYL CHLORIDE
Polymerization of vinyl chloride, CH2=CHCl (chloroethene), produces a polymer similar to
polyethylene, but having chlorine atoms at alternate carbon atoms on the chain. Polyvinyl chloride (PVC)
is rigid and somewhat brittle. About two-thirds of the PVC produced annually is used in the manufacture
of pipe. It is also used in the production of “vinyl” siding for houses and clear plastic bottles. When it is
blended with a plasticizer such as a phthalate ester, PVC becomes pliable and is used to form flexible articles such
as raincoats and shower curtains.
POLYETHYLENE TEREPHTHALATE
Polyethylene terephthalate (PET), or polyethylene terephthalic ester (PETE), is a condensation
polymer produced from the monomers ethylene glycol, HOCH2CH2OH, a dialcohol, and dimethyl
terephthalate, CH3O2C–C6H4–CO2CH3, a diester. By the process of transesterification, these monomers form
ester linkages between them, yielding a polyester. PETE fibers are manufactured under the trade names of
Dacron and Fortrel. Pleats and creases can be permanently heat set in fabrics containing polyester fibers, so-called
permanent press fabrics. PETE can also be formed into transparent sheets and castings. Mylar is a trade name for a
PETE film. Transparent 2-liter carbonated beverage bottles are made from PETE. (The opaque base on some bottles
is generally made of HDPE.) One form of PETE is the hardest known polymer and is used in eyeglass lenses.
[attachment=55558]
POLYMERS
Polymers are substances whose molecules have high molar masses and are composed of a large number of
repeating units. There are both naturally occurring and synthetic polymers. Among naturally occurring polymers are
proteins, starches, cellulose, and latex. Synthetic polymers are produced commercially on a very large scale and have
a wide range of properties and uses. The materials commonly called plastics are all synthetic polymers.
Polymers are formed by chemical reactions in which a large number of molecules called monomers are joined
sequentially, forming a chain. In many polymers, only one monomer is used. In others, two or three different
monomers may be combined. Polymers are classified by the characteristics of the reactions by which they are formed.
If all atoms in the monomers are incorporated into the polymer, the polymer is called an addition polymer. If some
of the atoms of the monomers are released into small molecules, such as water, the polymer is called a condensation
polymer. Most addition polymers are made from monomers containing a double bond between carbon atoms. Such
monomers are called olefins, and most commercial addition polymers are polyolefins. Condensation polymers are
made from monomers that have two different groups of atoms which can join together to form, for example, ester
or amide links. Polyesters are an important class of commercial polymers, as are polyamides (nylon).
POLYETHYLENE
Polyethylene is perhaps the simplest polymer, composed of chains of repeating –CH2– units. It is
produced by the addition polymerization of ethylene, CH2=CH2 (ethene). The properties of polyethylene
depend on the manner in which ethylene is polymerized. When catalyzed by organometallic compounds at
moderate pressure (15 to 30 atm), the product is high density polyethylene, HDPE. Under these conditions,
the polymer chains grow to very great length, and molar masses average many hundred thousands. HDPE is hard,
tough, and resilient. Most HDPE is used in the manufacture of containers, such as milk bottles and laundry
detergent jugs. When ethylene is polymerized at high pressure (1000–2000 atm), elevated temperatures
(190–210EC), and catalyzed by peroxides, the product is low density polyethylene, LDPE. This form of
polyethylene has molar masses of 20,000 to 40,000 grams. LDPE is relatively soft, and most of it is used in
the production of plastic films, such as those used in sandwich bags.
POLYPROPYLENE
This polymer is produced by the addition polymerization of propylene, CH2=CHCH3 (propene). Its
molecular structure is similar to that of polyethylene, but has a methyl group (–CH3) on alternate carbon
atoms of the chain. Its molar masses falls in the range 50,000 to 200,000 grams. Polypropylene (PP) is
slightly more brittle than polyethylene, but softens at a temperature about 40EC higher. Polypropylene is
used extensively in the automotive industry for interior trim, such as instrument panels, and in food packaging, such
as yogurt containers. It is formed into fibers of very low absorbance and high stain resistance, used in clothing and
home furnishings, especially carpeting.
POLYSTYRENE
Styrene, CH2=CH–C6H5, polymerizes readily to form polystyrene (PS), a hard, highly transparent
polymer. The molecular structure is similar to that of polypropylene, but with the methyl groups of polypro-
pylene replaced by phenyl groups (–C6H5). A large portion of production goes into packaging. The thin,
rigid, transparent containers in which fresh foods, such as salads, are packaged are made from polystyrene.
Polystyrene is readily foamed or formed into beads. These foams and beads are excellent thermal insulators and are
used to produce home insulation and containers for hot foods. Styrofoam is a trade name for foamed polystyrene.
When rubber is dissolved in styrene before it is polymerized, the polystyrene produced is much more impact resistant.
This type of polystyrene is used extensively in home appliances, such as the interior of refrigerators and air conditioner
housing. [For more information about this polymer, see Chemical Demonstrations: A Handbook for Teachers of
Chemistry, by Bassam Z. Shakhashiri, Volume 1 (1983), page 241.]
POLYVINYL CHLORIDE
Polymerization of vinyl chloride, CH2=CHCl (chloroethene), produces a polymer similar to
polyethylene, but having chlorine atoms at alternate carbon atoms on the chain. Polyvinyl chloride (PVC)
is rigid and somewhat brittle. About two-thirds of the PVC produced annually is used in the manufacture
of pipe. It is also used in the production of “vinyl” siding for houses and clear plastic bottles. When it is
blended with a plasticizer such as a phthalate ester, PVC becomes pliable and is used to form flexible articles such
as raincoats and shower curtains.
POLYETHYLENE TEREPHTHALATE
Polyethylene terephthalate (PET), or polyethylene terephthalic ester (PETE), is a condensation
polymer produced from the monomers ethylene glycol, HOCH2CH2OH, a dialcohol, and dimethyl
terephthalate, CH3O2C–C6H4–CO2CH3, a diester. By the process of transesterification, these monomers form
ester linkages between them, yielding a polyester. PETE fibers are manufactured under the trade names of
Dacron and Fortrel. Pleats and creases can be permanently heat set in fabrics containing polyester fibers, so-called
permanent press fabrics. PETE can also be formed into transparent sheets and castings. Mylar is a trade name for a
PETE film. Transparent 2-liter carbonated beverage bottles are made from PETE. (The opaque base on some bottles
is generally made of HDPE.) One form of PETE is the hardest known polymer and is used in eyeglass lenses.