Monomers link together by two basic methods:
- addition polymerization and
- condensation polymerization
There are many monomer molecules. Here are some examples:
Each of these monomer molecules seems very different, but they do have some common features.
Actually, these monomers have two distinctive features:
carbon-carbon double bonds and
side groups
The highlighted areas show the side groups on these monomer molecules. These groups give the polymer chain some of its properties.
The double bond, however, is the vital feature that allows these monomers to form the long polymer chains.
Two other functional groups you may see in polymerization reactions are the amine group, , and the alcohol group, . Below are examples of monomers containing these functional groups:
N
otice that each of these monomers contains two functional groups. That is, they are difunctional. It is their difunctionality that allows them monomers to form long polymer chains.
As we move on to polymerization, remember what you read about monomers:
The first group of monomers had
carbon-carbon double bonds and
side groups
The second group of monomers had
functional groups and
two functional groups each
INTRODUCTION TO POLYMERS
Polymer, substance consisting of large molecules that are made of many small, repeating units called monomers. The number of repeating units in one large molecule is called the degree of polymerization. Materials with a very high degree of polymerization are called high polymers. Polymers consisting of only one kind of repeating unit are called homopolymers. Copolymers are formed from several different repeating units.
Most of the organic substances found in living matter, such as protein, wood, chitin, rubber, and resins, are polymers. Many synthetic materials, such as plastics, fibres (see Nylon; Rayon), adhesives, glass, and porcelain, are also largely polymeric substances.
STRUCTURE OF POLYMERS
Polymer Structures
Polymers consist of large molecules that are made up of smaller repeating units, or monomers. Polymers containing only one type of monomer are known as homopolymers, whereas polymers combining more than one type of monomer are known as copolymers. This diagram provides examples of various polymer structures.
Polymers can be subdivided into three or four structural groups. The molecules in linear polymers consist of long chains of monomers joined by bonds, like beads on a necklace. Typical examples are polyethene, polyethenol—formerly called polyvinyl alcohol—and PVC (polyvinyl chloride, or polychloroethene).
Branched polymers have side chains that are attached to the chain molecule itself. Branching can be caused by impurities or by the presence of monomers that have several reactive groups. Chain polymers composed of monomers with side groups that are part of the monomers, such as polyphenylethene (polystyrene) or polypropene, are not considered branched polymers.
In cross-linked polymers, two or more chains are joined together by side chains. With a small degree of cross-linking, a loose network is obtained that is essentially two dimensional. High degrees of cross-linking result in a tight three-dimensional structure. Cross-linking is usually caused by chemical reactions. An example of a two-dimensional cross-linked structure is vulcanized rubber, in which cross-links are formed by sulphur atoms. Thermosetting plastics are examples of highly cross-linked polymers; their structure is so rigid that when heated they decompose or burn rather than melt.
SYNTHESIS
Two general methods exist for forming large molecules from small monomers: addition polymerization and condensation polymerization. In the chemical process called addition polymerization, monomers join together without the loss of atoms from the molecules. Some examples of addition polymers are polyethene, polypropene, polyphenylethene, polyethenyl ethanoate, and polytetrafluoroethylene (Teflon).
In condensation polymerization, monomers join together with the simultaneous elimination of atoms or groups of atoms. Typical condensation polymers are polyamides, polyesters, and certain polyurethanes.
In 1983 a new method of addition polymerization called group transfer polymerization was announced. An activating group within the molecule initiating the process transfers to the end of the growing polymer chain as individual monomers insert themselves in the group. The method has been used for acrylic plastics; it should prove applicable to other plastics as well.
One form of polymer is plastics.
Plastics, organic polymeric materials (those consisting of giant organic molecules) that are plastic—that is, they can be formed into desired shapes through extrusion, moulding, casting, or spinning. The molecules can be either natural—including cellulose, wax, and natural rubber—or synthetic—including polyethene and nylon. The starting materials are resins in the form of pellets, powders, or solutions; from these are formed the finished plastics.
PLASTICS
Plastics are characterized by high strength-to-density ratios, excellent thermal and electrical insulation properties, and good resistance to acids, alkalis, and solvents. The giant molecules of which they consist may be linear, branched, or cross-linked, depending on the plastic. Linear and branched molecules are thermoplastic (soften when heated), whereas cross-linked molecules are thermosetting (harden when heated).
Plastics, organic polymeric materials (those consisting of giant organic molecules) that are plastic—that is, they can be formed into desired shapes through extrusion, moulding, casting, or spinning. The molecules can be either natural—including cellulose, wax, and natural rubber—or synthetic—including polyethene and nylon. The starting materials are resins in the form of pellets, powders, or solutions; from these are formed the finished plastics.
Plastics are characterized by high strength-to-density ratios, excellent thermal and electrical insulation properties, and good resistance to acids, alkalis, and solvents. The giant molecules of which they consist may be linear, branched, or cross-linked, depending on the plastic. Linear and branched molecules are thermoplastic (soften when heated), whereas cross-linked molecules are thermosetting (harden when heated).
Variety of Plastics
The versatility of plastics, which can be easily moulded and shaped, is demonstrated by the variety of objects made from the material. Plastics are synthetic resins with molecules that are polymers, or large organic chains. Plastics are extremely durable and light. Small organic molecules, called monomers, are combined to form resinous polymers. These are moulded or extruded to make plastic articles.
