What are the methods of polymerization

Linear, branched, and network

Polymers are manufactured from low-molecular-weight compounds called monomers by polymerization reactions, in which large numbers of monomer molecules are linked together. Depending on the structure of the monomer or monomers and on the polymerization method employed, polymer molecules may exhibit a variety of architectures. Most common from the commercial standpoint are the linear, branched, and network structures. The linear structure, shown in Figure 1A, is illustrated by high-density polyethylene (HDPE), a chainlike molecule made from the polymerization of ethylene. With the chemical formula CH2=CH2, ethylene is essentially a pair of double-bonded carbon atoms (C), each with two attached hydrogen atoms (H). As the repeating unit making up the HDPE chain, it is shown in brackets, as . A polyethylene chain from which other ethylene repeating units branch off is known as low-density polyethylene (LDPE); this polymer demonstrates the branched structure, in Figure 1B. The network structure, shown in Figure 1C, is that of phenol-formaldehyde (PF) resin. PF resin is formed when molecules of phenol (C6H5OH) are linked by formaldehyde (CH2O) to form a complex network of interconnected branches. The PF repeating unit is represented in the figure by phenol rings with attached hydroxyl (OH) groups and connected by methylene groups (CH2).

Branched polymer molecules cannot pack together as closely as linear molecules can; hence, the intermolecular forces binding these polymers together tend to be much weaker. This is the reason why the highly branched LDPE is very flexible and finds use as packaging film, while the linear HDPE is tough enough to be shaped into such objects as bottles or toys. The properties of network polymers depend on the density of the network. Polymers having a dense network, such as PF resin, are very rigid—even brittle—whereas network polymers containing long, flexible branches connected at only a few sites along the chains exhibit elastic properties.

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Amorphous and semicrystalline

Polymers exhibit two types of morphology in the solid state: amorphous and semicrystalline. In an amorphous polymer the molecules are oriented randomly and are intertwined, much like cooked spaghetti, and the polymer has a glasslike, transparent appearance. In semicrystalline polymers, the molecules pack together in ordered regions called crystallites, as shown in Figure 2. As might be expected, linear polymers, having a very regular structure, are more likely to be semicrystalline. Semicrystalline polymers tend to form very tough plastics because of the strong intermolecular forces associated with close chain packing in the crystallites. Also, because the crystallites scatter light, they are more opaque. Crystallinity may be induced by stretching polymers in order to align the molecules—a process called drawing. In the plastics industry, polymer films are commonly drawn to increase the film strength.

At low temperatures the molecules of an amorphous or semicrystalline polymer vibrate at low energy, so that they are essentially frozen into a solid condition known as the glassy state. In the volume-temperature diagram shown in Figure 2, this state is represented by the points e (for amorphous polymers) and a (for semicrystalline polymers). As the polymer is heated, however, the molecules vibrate more energetically, until a transition occurs from the glassy state to a rubbery state. The onset of the rubbery state is indicated by a marked increase in volume, caused by the increased molecular motion. The point at which this occurs is called the glass transition temperature; in the volume-temperature diagram it is indicated by the vertical dashed line labeled Tg, which intersects the amorphous and semicrystalline curves at points f and b. In the rubbery state above Tg, polymers demonstrate elasticity, and some can even be molded into permanent shapes. One major difference between plastics and rubbers, or elastomers, is that the glass transition temperatures of rubbers lie below room temperature—hence their well-known elasticity at normal temperatures. Plastics, on the other hand, must be heated to the glass transition temperature or above before they can be molded.

When brought to still higher temperatures, polymer molecules eventually begin to flow past one another. The polymer reaches its melting temperature (Tm in the phase diagram) and becomes molten (progressing along the line from c to d). In the molten state polymers can be spun into fibres. Polymers that can be melted are called thermoplastic polymers. Thermoplasticity is found in linear and branched polymers, whose looser structures permit molecules to move past one another. The network structure, however, precludes the possibility of molecular flow, so that network polymers do not melt. Instead, they break down upon reheating. Such polymers are said to be thermosetting.