chemistry of industrial polymers

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.

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 T_g, which intersects the amorphous and semicrystalline curves at points f and b. In the rubbery state above T_g, 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.

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When brought to still higher temperatures, polymer molecules eventually begin to flow past one another. The polymer reaches its melting temperature (T_m 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.

Copolymers and polymer blends

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When a single monomer is polymerized into a macromolecule, the product is called a homopolymer—as shown in Figure 3A, with polyvinyl chloride as the example. Copolymers, on the other hand, are made from two or more monomers. Procedures have been developed to make copolymers in which the repeating units are distributed randomly (Figure 3B), in alternating fashion (Figure 3C), in blocks (Figure 3D), or as grafts of one monomer block onto the backbone chain of another (Figure 3E). In the figures the molecular structure of each type is shown schematically, along with the chemical structure of the representative polymer and its monomer repeating units. Such structural variety affords the polymer manufacturer considerable latitude in tailoring polymers to satisfy a diversity of applications.

In the industrial marketplace, polymers are blended to modify their properties in much the same way that metals are alloyed. The blended polymers may or may not dissolve in one another; most, in fact, do not. Where they are miscible, the properties of the homogeneous blend are often a weighted average of those of the individual polymers, although sometimes a synergistic relationship is exhibited that leads to improved properties.

In the case of immiscible polymer blends, a variety of strategies have been developed to keep the separate phases together when the blends are subjected to stress. One is to synthesize two or more interlocking network polymers—an arrangement referred to as an interpenetrating polymer network (IPN). Another strategy is to add block or graft copolymers formed from monomers of the immiscible polymers in order to improve adhesion at the boundaries between the polymer phases. In this technique interfacial adhesion is strengthened because of the natural affinity of the individual blocks for their respective homopolymers. Industrial products include both homogeneous and heterogeneous polymer blends.

Chain-growth polymerization

Free-radical initiation

Chain-growth polymerization reactions require the presence of an initiator, a compound that reacts with the monomer to form another reactive compound, which begins the linking process. The most widely used initiators are compounds such as peroxides that break down to an unstable species called a radical (or free radical). A radical is a reactive compound that contains an unpaired electron; in chemical formulas it is commonly given the generic designation R · . As shown in the reaction diagram below, the most commonly used peroxide initiator, benzoyl peroxide, can produce benzoyloxy radicals by cleaving at an oxygen-oxygen bond. The pair of benzoyloxy radicals thus produced may initiate a polymer chain, or they may break down further to yield carbon dioxide and yet a new initiator, a phenyl radical:

The first step in polymerization involves addition of the initiator radical (R · ) to the monomer to form a new radical having the unpaired electron on a carbon atom, as can be seen in the polymerization of ethylene (CH₂=CH₂):

The new radical then adds to a second ethylene molecule:

Ethylene molecules are added successively to the chain until very little ethylene is left. At this point the chain is terminated, either by a combination of two chains

or by a disproportionation reaction involving the transfer of a hydrogen atom from one of the growing chains to the other:

The structure enclosed in brackets, ―[CH₂―CH₂―]_n, is the repeating unit of the polymer chain. The number of repeating units, n, varies according to the length of the polymer chain or, in other words, the molecular weight. Because polymer chains do not all terminate at the same length, reference is normally made to a polymer’s average molecular weight.

The polymer produced by reactions such as that outlined above is named by adding the prefix “poly-” to the monomer name—in this case, polyethylene. A monomer name that contains more than one word can be enclosed in parentheses—e.g., poly(vinyl chloride)—although in industrial usage the parentheses are often omitted. (This article follows common industrial usage by omitting the parentheses.) Abbreviations are commonly used for polymer names, such as HDPE for high-density polyethylene or PVC for polyvinyl chloride.

Because growing polyethylene chains are very flexible, the radical at the chain end may curl around and abstract a hydrogen atom from a CH₂ group at some point in the middle of the chain, thus forming a new radical site from which chain growth continues. This reaction, shown in Figure 4, is referred to as backbiting or, more technically, chain transfer. The result is a polymer chain with the branched structure of low-density polyethylene (LDPE), also shown in Figure 1B. Chain-transfer reactions may also occur intermolecularly.

If an atom larger than a hydrogen atom—for example, chlorine (Cl)—is attached to one of the carbon atoms, the initiator radical adds preferentially to the other carbon:

This selectivity results from the increased crowding afforded by the chlorine atom as well as from a stabilizing effect of the chlorine atom on the radical. Subsequent radical additions to the monomer, vinyl chloride, proceed the same way, with the result that polyvinyl chloride contains chlorine atoms predominately on alternate carbon atoms:

This type of reaction, termed head-to-tail polymerization, is characteristic of most vinyl monomers, regardless of the type of initiator employed.

Organometallic catalysis

In the early 1950s the German chemist Karl Ziegler discovered a method for making almost entirely linear HDPE at low pressures and low temperatures in the presence of complex organometallic catalysts. (The term catalyst may be used with these initiators because, unlike free-radical initiators, they are not consumed in the polymerization reaction.) In the Ziegler process the polymer chain grows from the catalyst surface by successive insertions of ethylene molecules, as shown in Figure 5. When polymerization is complete, the polymer chains detach from the catalyst surface. A great variety of complex organometallic catalysts have been developed, but the most commonly used are formed by combining a transition metal compound such as titanium trichloride, TiCl₃, with an organo-aluminum compound such as triethylaluminum, Al(CH₂CH₃)₃.

Soon after Ziegler made his discovery, the Italian chemist Giulio Natta and his coworkers discovered that Ziegler-type catalysts could polymerize propylene, CH₂=CHCH₃, to yield a polymer having the same spatial orientation for all the methyl (CH₃) groups attached to the polymer chain:

Because all the methyl groups are located on the same side of the chain, Natta called the polymer isotactic polypropylene. With vanadium-containing catalysts, Natta was also able to synthesize polypropylene containing methyl groups oriented the same way on alternate carbons—an arrangement he called syndiotactic:

Isotactic and syndiotactic polymers are referred to as stereoregular—that is, polymers having an ordered arrangement of pendant groups along the chain. A polymer with a random orientation of groups is said to be atactic. Stereoregular polymers are usually high-strength materials because the uniform structure leads to close packing of the polymer chains and a high degree of crystallinity. The catalyst systems employed to make stereoregular polymers are now referred to as Ziegler-Natta catalysts. More recently, new soluble organometallic catalysts, termed metallocene catalysts, have been developed that are much more reactive than conventional Ziegler-Natta catalysts.

In addition to ethylene and propylene, other vinyl monomers used commercially with Ziegler-Natta catalysts are 1-butene (CH₂=CHCH₂CH₃) and 4-methyl-1-pentene (CH₂=CHCH₂CH[CH₃]₂). A copolymer of ethylene with 1-butene and other 1-alkene monomers is also produced, which exhibits properties similar to those of LDPE, but it can be made without the high temperature and pressure needed to make LDPE. The copolymer is referred to as linear low-density polyethylene (LLDPE).

Step-growth polymerization

Step-growth polymerization typically takes place between monomers containing functional groups that react in high yield to form new functionalities. Examples of such functional groups are carboxylic acids, which react with alcohols to form esters and with amines to form amides:

Here R and R′ represent two different organic molecular groups.

When monomers containing two of one type of functional group react with monomers containing two of another, linear polymers are formed. One commercially important example is the reaction of the dicarboxylic acid terephthalic acid (containing two CO―OH groups) with the dialcohol ethylene glycol (containing two OH groups) to form polyethylene terephthalate (PET), a polyester:

Another important reaction is that of adipic acid (containing two CO―OH groups) with 1,6-hexamethylenediamine (containing two NH₂ groups) to form polyhexamethylene adipamide, also called nylon 6,6:

All the step-growth reactions outlined above yield a by-product, water. Other reactions not shown yield different by-products—for example, hydrochloric acid. Because of this loss of compounds during the polymerization process, reactions of this type are often called condensation reactions. Not all step-growth reactions are condensation reactions, however; some do not yield any by-product. One example is the reaction between benzene-1,4-diisocyanate and ethylene glycol to form a polyurethane:

Monomers containing more than two functional groups yield network polymers. An example is glyptal, a polyester formed from a reaction of phthalic anhydride with the trialcohol glycerol:

Bulk polymerization

Bulk polymerization is carried out in the absence of any solvent or dispersant and is thus the simplest in terms of formulation. It is used for most step-growth polymers and many types of chain-growth polymers. In the case of chain-growth reactions, which are generally exothermic, the heat evolved may cause the reaction to become too vigorous and difficult to control unless efficient cooling coils are installed in the reaction vessel. Bulk polymerizations are also difficult to stir because of the high viscosity associated with high-molecular-weight polymers.

Solution polymerization

The conducting of polymerization reactions in a solvent is an effective way to disperse heat; in addition, solutions are much easier to stir than bulk polymerizations. Solvents must be carefully chosen, however, so that they do not undergo chain-transfer reactions with the polymer. Because it can be difficult to remove solvent from the finished viscous polymer, solution polymerization lends itself best to polymers that are used commercially in solution form, such as certain types of adhesives and surface coatings. Polymerization of gaseous monomers is also conducted with the use of solvents, as in the production of polyethylene illustrated in Figure 6.

Suspension polymerization

In suspension polymerization the monomer is dispersed in a liquid (usually water) by vigorous stirring and by the addition of stabilizers such as methyl cellulose. A monomer-soluble initiator is added in order to initiate chain-growth polymerization. Reaction heat is efficiently dispersed by the aqueous medium. The polymer is obtained in the form of granules or beads, which may be dried and packed directly for shipment.

Emulsion polymerization

One of the most widely used methods of manufacturing vinyl polymers, emulsion polymerization involves formation of a stable emulsion (often referred to as a latex) of monomer in water using a soap or detergent as the emulsifying agent. Free-radical initiators, dissolved in the water phase, migrate into the stabilized monomer droplets (known as micelles) to initiate polymerization. The polymerization reaction is not terminated until a second radical diffuses into the swelling micelles, with the result that very high molecular weights are obtained. Reaction heat is effectively dispersed in the water phase.

The major disadvantage of emulsion polymerization is that the formulating of the mix is complex compared with the other methods, and purification of the polymer after coagulation is more difficult. Purification is not a problem, however, if the finished polymer is to be used in the form of an emulsion, as in latex paints or adhesives. (Emulsion polymerization is illustrated in Figure 1 in the article surface coating.)

Gas-phase polymerization

This method is used with gaseous monomers such as ethylene, tetrafluoroethylene, and vinyl chloride. The monomer is introduced under pressure into a reaction vessel containing a polymerization initiator. Once polymerization begins, monomer molecules diffuse to the growing polymer chains. The resulting polymer is obtained as a granular solid.