Formation of Polymers

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        Polymers are formed by causing small units (mon-omers) to chemically bond together and build up into long polymeric chains. The two principal methods used to cause this bonding of monomers to occur are chain-growth polymerization or addition polymerization and stepwise polymerization or condensation polymerization. Each of these polymerization methods will be described in detail and compared and contrasted.

2.6.1. Chain-Growth Polymerization or Addition Polymerization

        The addition polymerization mechanism, sometimes called the chain-reaction mechanism, proceeds by several sequential steps, whose essential features are illustrated in Figure 2.20. The steps for addition polymerization are the following:

        Step 1. Introduce the monomer containing a carbon-carbon double bond into the reaction vessel. (The simplest molecule containing a carbon-carbon double bond is ethylene, which was also depicted in Figures 2.12 and 2.13 and will be used as the example to depict addition polymerization in Figure 2.20.) A high concentration of monomer is usually introduced. (Because the monomer is a gas, the concentration is usually increased by increasing the pressure of the monomer inside the reaction vessel. Monomers can also be liquids and solids, although the latter are less common.)

Figure 2.20 Addition polymerization steps (Hs omitted for simplicity).

        Step 2. Inject an initiator (usually a small amount is sufficient) into the reaction vessel such that the initiator mixes with the monomer. An initiator is any material that will start the polymerization reaction. In the most common case, the reaction is started by the formation of free radicals, that is, molecules that contain an unpaired electron. Peroxide molecules are common initiators for addition polymerization because when heated, they break apart to form free radicals. Peroxide molecules contain two oxygens that are bonded by a single bond and two organic functional groups that are bonded to each of the oxygens. Heated peroxides break apart at the oxygen-oxygen single bond to form the free radicals. These peroxide free radicals are unstable
(because of the unshared electrons) and immediately try to join with some nearby electron to form a covalent bond. (Note that initiators are sometimes called catalysts. This is an improper use of the term because catalysts by definition are not "used up" in the reaction, but initiators are reactants in the polymerization process and are used up as the reaction proceeds. Nevertheless, the use of catalyst to describe these initiators is widespread. In this text, however, a distinction between initiator and catalyst will be made and only when a catalyst meets the strict definition of not being a reactant will that term be used.)

        Step 3. A convenient source of relatively available electrons is the π-bond in the carbon-carbon double bond of the monomer. As previously discussed, π-bond electrons are quite far removed from the center of the carbon-carbon bond. These electrons are, therefore, rather loosely held and readily available for interaction with the free radicals. This interaction of the peroxide free radical and the π-bond is illustrated in Figure 2.20a. This step is sometimes called the initiation step.

        Step 4. The peroxide free radical extracts one of the two electrons in the π-bond, thus breaking the π-bond and forming a new bond connecting the peroxide with one of the carbons previously having the π-bond. The other electron in the π-bond is now unpaired. It is, therefore, a free radical. This new free radical moves to the other carbon, which previously had the π-bond. This sequence is shown in Figure 2.20b.

        Step 5. This new free radical is also unstable and will try to form a bond. A convenient source of relatively available electrons is the π-bond of another monomer molecule. (This is especially true at high monomer concentrations.) This interaction is depicted in Figure 2.20c.

        Step 6. The new free radical extracts an electron from the π-bond of the new monomer molecule and forms a new bond that links the atom on the growing chain with the new monomer atom. The π-bond in the new monomer is broken by this step and a new free radical is formed. This sequence results in a lengthening of the chain by the length of the new monomer (two carbons in this case) and the creation of a reactive site at the distant end of the chain. This step is referred to as the propagation step (See Figure 2.20d.)

        Step 7. This process of creation of a free radical, attack on a π-bond, formation of a bond with a new monomer, and creation of a new free radical can continue as long as a monomer is available to react, provided some other readily available electron source does not interfere with the process by reacting with the newly formed free radical. The chain can, therefore, become very long, often growing to several thousand units. The chain quickly becomes so long that the presence of the peroxide on the end becomes insignificant in the total picture (except for some analytical techniques that examine the end groups). At this point, the polymer is usually represented simply by the repeating unit, that is, by the unit that is added to the chain in each step. This unit is usually the monomer, but without the π-bond because it has been lost in the polymerization process. In its place, two bonds have been formed, one on each of the carbons that originally held the π-bond. This representation is given in Figure 2.20e. The n represents the total number of units in the chain. The name of the polymer (made by this method) is usually the name of the monomer preceded by the prefix "poly."

        Step 8. Eventually the polymer chain must be ended. Several reactions can result in chain termination. One of the simplest is that the carbon free radical meets another free radical (either peroxide or carbon). In this case, the two unpaired (free radical) electrons join together, forming a covalent bond between them. If two carbon free radicals join, the chains will be combined. Another chain termination mechanism is the combining of the free radical with some other electron-rich molecule. This molecule could be a contaminant in the reaction vessel or could be a molecule intentionally introduced to stop (quench) the polymerization reaction. This step is called the termination step and is the last step in the addition polymerization process.

        Still another possibility is that the carbon free radical could extract an electron from a bond other than a π-bond. This is far less likely to occur because the electrons in other bonds are generally much more tightly held. However, it can happen, especially at high temperatures, when all electrons become excited and more active. When this occurs, the most likely place for the free radical to extract another electron would be from a carbon-hydrogen bond in another polymer chain. Should this happen, a bond between the two carbon atoms in the two chains would be formed and a hydrogen free radical would be released. The carbon-carbon bond would result in a connection between the two chains that would then constitute two different branches of one chain. This process is, therefore, called branching (illustrated in Figure 2.21). The released hydrogen free radical would be available to react with other π-bond electrons or for any other reaction common for free radicals. It could, for instance, terminate some other chain. Other reactions that result in termination are also possible. These are, in general, of less importance than those already discussed.

        Many different monomers can be polymerized by addition polymerization. All of them must have a carbon-carbon double bond but can have several side chains attached to the carbons containing the double bond. Several of the most important are listed in Figure 2.22.

Figure 2.21 Branching reaction in addition to polymerization.

Figure 2.22 Common monomers that polymerize using addition polymerization.

        Initiation of the addition polymerization reaction can also occur with materials other than peroxides. Almost any material that forms free radicals or that attracts an electron from the π-bond will work. Both positive and negative ions (charged atoms or molecules) work in some cases. Ultraviolet light, X-rays, and other energy sources can also cause free radicals to form, either directly with the π-bond or with an intermediate atom (such as oxygen).

        Another important consideration in addition polymerization is the use of a true catalyst. As indicated previously, catalysts are materials that promote the effectiveness of a chemical reaction but are not consumed in the reaction itself. A typical catalyst for addition polymerization is a metal, such as titanium, platinum, or various metal-organic molecule combinations that form a solid surface within the reaction vessel. These metals attract the growing polymer chain and hold it in a particular orientation, thereby facilitating the chain- lengthening reaction with the monomer, which is also attracted by the catalyst. In some cases, the holding of the growing polymer in a specific orientation can permit the formation of polymers with specific orientations of any side groups that may be attached to the double-bond carbons. A well-known catalyst system of this type is called the Ziegler-Natta catalyst, named after the developers of the system. These catalysts (1) promote the polymerization, (2) can permit specific orientation reactions to occur, and (3) can give the added benefit of initiating the reactions. Another important group of catalysts, called the metallocenes, has been developed recently. These catalysts, like the Ziegler-Natta catalysts, force polymerization into specific spatial arrangements (stereoregularity) of the atoms. The advantage of the metallocenes is that the shape of the catalyst can be changed at the molecular level, thus permitting a wide variety of polymers to be carefully and precisely polymerized to specific spatial arrangements. In some cases, this capability allows polymers that were previously only noncrystalline to form crystalline structures. Furthermore, some properties of the polymers can be significantly enhanced by the use of metallocene catalysts.

2.6.2. Step-Growth Polymerization or Condensation Polymerization

        The condensation polymerization mechanism, sometimes called the step-growth polymerization mechanism, proceeds by several steps that are quite different from the steps involved in addition polymerization. Before describing the specific steps in condensation polymerization, some concepts regarding the reactions of functional groups (containing active reaction sites) should be considered, because these form the basis of condensation polymerization. The most fundamental of these basic concepts is the reaction between functional groups that is illustrated in Figure 2.23.

        Each step in a condensation polymerization involves a reaction between dissimilar functional groups that are part of monomer molecules. These functional groups are specifically chosen from the many available in organic chemistry to react in the desired way and to give the properties desired in the resulting material. (Figure 2.14 lists several functional groups used to form commercially important polymers.)

        The most common type of reaction in forming condensation polymers involves the formation of a new covalent bond between the functional groups and the simultaneous formation of a small molecule, such as water, which is a by-product of the reaction. (The joining of two molecules with the formation of a small by-product molecule is called condensation and is the process from which the polymerization method takes its name.) For instance, an alcohol can react with an acid to form an ester with water as the condensate; or an amine can react with an acid to form an amide, again with water as the by-product. These reactions are represented in Figure 2.23.

Figure 2.23 Reaction between dissimilar functional groups, which is the basis of condensation or step-growth polymerization reactions.

        For polymerization to occur, each monomer molecule must have two reactive functional groups (otherwise the monomers would react once and then stop, with no chain being formed). Monomers
that have two groups are bifunctional. In most monomers the reactive sites are on either side of a short chain of carbon atoms. Some typical examples of bifunctional monomers are given in Figure 2.24.

        The reactive ends are identical on both ends of each of these monomers, which are therefore called symmetric bifunctional monomers. These are the usual, but not the only kind of monomers that can be used for condensation polymerization. (The use of nonsymmetrical monomers will be illustrated in appropriate sections later in this text.) The general mechanism for the condensation reaction was represented in Figure 2.23; a complete step-by-step illustration of a condensation reaction to form a specific product is useful and is shown in Figure 2.25. The particular condensation polymerization reaction illustrated is of great historical and commercial significance. The reaction shown is used to make nylon, which launched the use of condensation polymerization as a method for making polymers. This reaction uses two symmetric bifunctional monomers—hexamethylene diamine (a monomer with two amine groups on either side of six carbons) and adipic acid (a monomer with two acid groups on either side of four carbons). The resulting material is a polyamide, usually known as nylon (6/6), where the numerical designation in the name represents the number of carbons in each monomer; in this case each monomer has six carbons. For simplicity, the hexamethylene diamine monomer will be referred to as "monomer A" and the adipic acid monomer will be called "monomerB." The steps in the condensation polymerization process are as follows:

        Step 1. The two different monomers (A and B), each having two active sites, are introduced into the reaction vessel. The materials are heated and vigorously stirred or agitated to facilitate the reaction. (No initiator or catalyst is normally needed in condensation reactions.)

Figure 2.24 Typical molecules having two functional groups, which are, therefore, bifunctional.

        Step 2. One end of monomer A reacts with one end of monomer B and a new bond is formed linking the two monomers, which begins the polymer chain. A molecule of water condensate is formed as a by-product, as shown in Figure 2.25a. The reaction occurs because of the inherent tendency of acids and amines to react. (Reactions of some functional groups occur without the creation of a by-product molecule, although these re actions are far less common.)

        Step 3. The beginning polymer chain has active functional groups on each of its ends. (This is because both monomers had two active sites before they reacted.) In the case shown in Figure 2.25, one end of the beginning polymer chain is an amine group and the other end is an acid group. The amine group can react with one of the adipic acid monomers (monomer B) and the acid group on the other end can react with one of the diamine monomers (monomer A). The reaction of the acid end of the beginning polymer with monomer A is illustrated in Figure 2.25b. In this reaction monomer A is added to the beginning polymer, which extends the chain and forms a longer polymer. Again a molecule of water is created as a by-product. Note that this reaction step is not dependent upon any previous reaction step, in contrast to the chain-reaction mechanism of addition polymerization, where a previous step had to create a new free radical. For this reason, this reaction method is called stepwise polymerization. The water is removed by a special distillation process as the reaction proceeds.

Figure 2.25 Condensation reaction steps to form a polyamide (nylon 6/6).

        Step 4. The longer polymer chain still has reactive groups at both ends. An amine can react with the acid monomer (monomer B) or an acid end can react with an amine monomer (monomer A) to produce new bonds and add to the chain to create an extended polymer, as shown in Figure 2.25c. Water is again made as a by-product. This process of reaction with an ever longer polymer can proceed, in theory, until all the monomers have reacted. Long polymer chains can be formed but are typically not as long as with addition polymerization. Although the actual dimensional length of the chain is very small (because atoms are so small), it is significant that the material now has thousands of bonded units and the collection of these units into a polymer chain has significantly altered the properties of the material as compared to the original monomers.

        Step 5. Not only can monomers react with each other and with growing chains, two growing chains can react with each other. This would happen if the amine end of one polymer were to encounter and react with the acid end of another polymer. If this happens, the length of the polymer chain can grow very rapidly because the process joins already created long chains instead of adding a single monomer. Eventually, the chains will become so long that further movement enabling the reactive sites on the polymers to come into close proximity for bonding is no longer possible. The reaction is then terminated simply by cooling, which further reduces the movement and slows the tendency of the functional groups to react. The polymerization reaction can also be terminated by adding a material that has only one active end. This is called quenching the reaction and is done after the polymers have reached the desired length. The quenching material is said to form end-caps.

When the polymer chains become very long, the end groups are relatively unimportant as far as polymer properties are concerned, and the polymers are best represented by a generalized polymer structure showing the repeat unit, that is, the smallest unit that reflects the basic structure of the polymer. In the case of a condensation polymer, this repeat unit is a combination of the two monomers that were used to form the polymer, as indicated in Figure 2.25d. These polymers are named by using the prefix "poly" with the name of the new type of functional group created. For instance, the reaction between an amine and an acid creates an amide, and so the polymers are called polyamides. The name nylon is just a common name for polyamides.

In some cases the by-product is not water, but it is almost always a small, stable molecule. An example of a by-product other than water is the reaction between phosgene and bisphenol A to form polycarbonate. Hydrochloric acid (HCl) is the byproduct.

2.6.3. Comparison of Addition and Condensation Polymers

        The methods of addition polymerization and condensation polymerization differ in several key areas. These differences are summarized in Table 2.3.

        One difference between addition polymerization and condensation polymerization is the chain-growth mechanism. In addition polymerization, the polymer grows in a chain-reaction manner, that is, once started it is self-propagating. An initiator is normally used to start the reaction. In condensation polymerization, the polymer grows in a step-by-step manner, that is, the polymer chain increases in discrete steps. Initiators are not needed for condensation reactions to occur.

        Each of these two major types of polymerization is also characterized by the type of monomer used to form the polymer. In addition polymerization, the monomers have a carbon-carbon double bond, which is the single active reaction site. In condensation polymerization, each monomer must have two active sites that are functional groups but are not carbon-carbon double bonds. The typical pattern is for one condensation polymerization monomer to have two identical functional groups on each of its ends. This monomer is mixed with a different monomer that also has two identical functional groups on each of its ends, but not the same functional groups as the first monomer. For condensation polymerization to occur, the functional groups of the first monomer must react with the functional groups of the second monomer to form the polymer. Therefore, condensation polymerization generally requires that two types of monomers be present, whereas only one type of monomer need be present for addition polymerization. (Condensation polymerization can also be done when a single monomer has two different end groups that will mutually react. These are, however, rare.)

        In view of these differences, some monomers polymerize by the addition mechanism whereas others polymerize by the condensation mechanism. No polymer can be formed by both mechanisms, although a polymer may be formed by one mechanism and then enter into a later reaction that uses the other mechanism. (This is seen in crosslinking, to be discussed later.)

        The formation of a byproduct condensate is typical in a condensation polymerization but does not occur in addition polymerization reactions. Some minor products, such as very short-chain polymers, may be formed in addition polymerization, but these are the result of collisions between various free radicals formed during the course of the reaction rather than as a specific product from each polymerization step.

	Addition or Chain-Growth Polymerization	Condensation or Step-Growth Polymerization
Polymer growth mechanism	Chain reaction	Step-by-step reactions
Initiator needed	Yes	No
Type of monomer	Contains carbon-carbon double bond	Bifunctional (has reacting function groups on the ends)
Number of active sites (functional groups) per monomer	1	2
Number of different types of monomers needed to form polymer	1	2 (usually)
By-product formed	No	Yes (usually)
New type of bond formed	No	Yes
Basic representation (polymer repeat unit)	Monomer without the double bond and with bonds on either side	Two monomers joined together
Polymer chain characteristics	A few, long chains	Many, not very long chains
Branching	Possible	Unlikely
Name of polymer	Poly + name of monomer	Poly + name of new bond

Table 2.3 Characteristics of Addition and Condensation Polymerization Methods

        The polymer formed from addition polymerization can be represented by the monomer without the double bond and with two bonds extending on either side, thus indicating that a chain is present. The basic polymer formed from condensation polymerization can be represented by two monomers joined together as they would be in the polymer after the condensate has been extracted.

        Some differences in the polymers produced by the addition polymerization and condensation polymerization processes can be seen from an examination of their mechanisms. Because of the relatively small amount of initiator used in addition polymerization, only a few chains begin to grow. The chain-reaction mechanism proceeds very quickly, especially in the presence of a catalyst, andresults in a few very long chains in a short period of time. Eventually, the monomer is entirely combined into the chains and the reaction stops. In condensation polymerization, any two different monomers that meet can initiate a chain. Therefore, many chains are growing simultaneously. The growth of these chains depends upon their ability to encounter monomers or other chains and react effectively. Eventually, the monomer is essentially all reacted, but because these chains still have active ends, the growing chains can continue to react among themselves, thereby increasing the length of the chain. This chain combination continues until the chains become so long that further movement is difficult, even at the elevated temperatures normally used in the polymerization process, or until a quenching agent is added. The likelihood of very long chains is, therefore, more common with addition polymerization than with condensation, and the likelihood of having many polymer chains being formed is greater with condensation than with addition.

2.6.4. Polymerizations Other Than by Addition and Condensation Mechanisms

        Some polymerizations use reactions other than addition or condensation. For instance, a diisocyanate can react with a dialcohol (diol) to produce a polyurethane. No condensate is formed in this reaction, but some rearrangement of the atoms occurs when the bond between the monomers is formed. Atom rearrangement also occurs in the formation of acetal polymers from formaldehyde. Another method for making acetal polymers involves the opening of a trioxane ring. Epoxy resins also use the ring opening to create a reactive site, which can bond to other monomers and form a polymer.

        These and a few other methods have been used both experimentally and commercially to form polymers, but by far, the greatest number of polymers are formed by either the addition or the condensation polymerization method.