P
etroleum is the primary raw material for most polymers. When crude oil is distilled, the valuable petroleum products are removed, leaving behind less valuable, higher-molecular-weight compounds that are heated over a catalyst. As a result, the large hydrocarbons are broken into smaller molecules through a process called cracking. These smaller molecules serve as the initial building blocks for the monomers that are converted to polymers. One of the most common monomers, ethylene, is also obtained from natural gas.Because of the cost of crude oil and the environmental impact of refi ning it, signifi cant effort is being placed into fi nding commercially viable alter-native sources for polymer precursors. Corn, barley, soybeans, and wood have all been identifi ed as renewable sources of polymer precursors, but large-scale commercial production is currently hindered by high costs and low yields.
Whatever the source of the precursor materials, most polymers are made through one of two reaction schemes: addition polymerization and condensation polymerization.
| Addition Polymerization | One of the two most common reaction schemes used to create polymers, involving three steps: initiation, propagation, and termination. Also called chain growth polymerization and free-radical polymerization.
| Condensation Polymerization | Formation of a polymer that occurs when two potentially reactive end groups on a polymer react to form a new covalent bond between the polymer chains. This reaction also forms a by-product, which is typically water.
5.2 | Types of Polymers 161 Table 5-1 Physical Properties of Commercial Polymers
Polymer Category Tm(°C) Tg(°C)
PMMA Acrylic 265–285 105 1.19 55–76 2400–3400
PBO Aramid n/a n/a 1.54 5800 180,000
Nylon 6,6 Nylon 255 n/a 1.14 90 3400
PET Polyester 245–265 80 1.4 172 4275
LDPE Polyolefi n 110 n/a 0.92 10.3 166
HDPE Polyolefi n 130–137 n/a 0.94–0.97 19–30 800–1400
PP Polyolefi n 164 220 0.903 35.5 1380
Viscose Rayon n/a n/a 1.5 28–47 9.7
PVDC HVTP 160 24 1.17 34.5 517
PS HVTP 180 74–110 1.04 46 2890
PVC HVTP 175 81 1.39 55 2800
Polyisoprene Elastomer 40 263 0.970 17–25 1.3
Polybutadiene Elastomer n/a 2110 to 295 1.01 18–30 1.3
Polychloroprene Elastomer n/a 245 1.32 25–38 0.52
Values from Polymer Handbook, 4th edition, J. Brandrup, E. Immergut, and E. Grulke, eds. (Hoboken, NJ: John Wiley & Sons, 1999).
| Cracking |
Process of breaking large organic hydrocarbons into smaller molecules.
5.3 ADDITION POLYMERIZATION
A
ddition polymerization, which is also known as free-radical polymerization or chain growth polymerization, begins with a vinyl monomer, like the one shown in Figure 5-21. The groups that surround the double-bonded carbon can be different, and the identity of the monomer changes accordingly. If all four atoms are hydrogen, the vinyl monomer is ethylene. If one hydrogen atom is replaced by a benzene ring, the monomer is styrene. More than one hydrogen atom can be replaced with other atoms. A vinyl monomer with one chlorine atom would be vinyl chloride; two chlorine atoms would make the monomer vinylidene.Regardless of the specifi c vinyl monomer, all addition polymerization reac-tions occur in three steps:
1. Initiation 2. Propagation 3. Termination
Figure 5-22 illustrates the complete reaction using a styrene monomer. The initiation step can be induced by heat, radiation (including visible light), or
| Vinyl Monomer | Double-bonded organic
molecule used to begin addition polymerization.
| Initiation | First step in the process of polymerization, during which a free radical is formed.
HO OH 2HO•
Free Radical Reacts with First Monomer
C
Propagation (Repeats Until Termination)
Propagation (Repeats Until Termination) Primary Termination
Figure 5-21 Vinyl Monomer (Ethylene)
Figure 5-22 Reaction Sequence for the Addition Polymerization of Styrene
the addition of a chemical (generally hydrogen peroxide). The purpose of the initiator is to induce the formation of a highly reactive unpaired electron called afree radical. Once formed, the free radical attacks the double bond, forming a new bond and transferring the free radical to the end of the chain. If we consider the chemical initiation of the addition polymerization of styrene using hydrogen peroxide, the H—O—O—H spontaneously decomposes into a pair of O—H
·
radicals, which are then able to attack the double bond of a styrene molecule.
During the second stage, propagation, the newly formed styrene radical can attack the double bond in another styrene molecule, again breaking the double bond, transferring the radical to the end, and adding one more mol-ecule to the growing chain.
The new polystyrene radical can continue attacking the double bonds in styrene monomers, each time growing by one monomer length. The polymer continues to grow as long as it reacts with more monomer.
The fi nal stage, termination, occurs when two free radicals react with each other and end the polymerization reaction. Two distinct types of free radicals are present in the system: growing polystyrene chains and unreacted initiator radicals. When a growing polymer radical with M monomer units added reacts with another growing radical with N monomer units, mutual termination occurs, resulting in a completed polymer chain with M1 N units.
Alternatively, the growing polystyrene radical of length M can combine with a primary free radical from the hydrogen peroxide resulting in primary termination. In this case, a fi nished polymer with M monomer units results.
Three points about addition polymerization must be considered:
1. The free radical may form on either the substituted or the unsubsti-tuted side of the vinyl monomer (except when ethylene is the mono-mer). Thus, the new monomer may add head to head only, head to tail only, or a blend of the two, depending on the relative stabilities of the free radicals.
2. Although the polymer may have thousands (or millions) of identical monomer units added, there will be only two end groups. In this case, both will be —OH groups. The end groups have little signifi cance on the mechanical properties of most polymers but can be used to deter-mine the number of chains formed through titration.
3. Many different chains are reacting at the same time, and whether they react with another vinyl monomer or with a free radical is probabi-listic. Therefore, a distribution of chain lengths will form, as shown in Figure 5-23. Controlling that distribution is a major challenge for polymer scientists and engineers.
5.4 CONDENSATION POLYMERIZATION
T
he alternative to addition polymerization is condensation polymeriza-tion, which sometimes is referred to as step-growth polymerization.Unlike addition polymerization, condensation polymerization does not require sequential steps or any initiation. In condensation, potentially reac-tive functional groups on the ends of molecules react. A new covalent bond forms between the functional groups, and a small molecule (usually water) is
| Free Radical |
Molecule containing a highly reactive unpaired electron.
| Propagation | Second stage of the
polymerization process during which the polymer chain begins to grow as monomers are added to the chain.
| Termination |
Final step in the polymerization process, which causes the elongation of the polymer chain to come to an end.
| Mutual Termination | One of the two different types of termination in the polymerization process.
During this type of
termination, the free radicals from two different polymer chains join to end the propagation process.
| Primary Termination | Last step in the polymerization process, which occurs when the free radical of a polymer chain joins with the free radical on an end group.
| End Groups | Two substituents found at both ends of a polymer chain, which have little to no effect on mechanical properties.
| Step-Growth Polymerization | Formation of a polymer that occurs when two potentially reactive end groups on a polymer react to form a new covalent bond between the polymer chains. This reaction also forms a by-product, which is typically water. Also known as condensation polymerization.
5.4 | Condensation Polymerization 163
Mass
DPN
Figure 5-23 Distribution of Chain Lengths from Addition Polymerization
formed as a by-product. Functional groups are specifi c arrangements of atoms that cause an organic compound to behave in predictable ways. Most chemical reactions occur at functional groups. Figure 5-24 shows the functional groups frequently found in polymers. The most common condensation polymerizations occur between an acid and an alcohol, such as the reaction between terephthalic acid and ethylene glycol to form PET and water, as shown in Figure 5-25.
The polymer still contains potentially active functional groups on each end that are capable of reacting with another acid, alcohol, or with another growing chain. The polymer continues to grow until fi nally providing a polymer of the form
| Functional Groups | Specifi c arrangements of atoms that cause organic compounds to behave in predictable ways.
| Homopolymer | Polymer that is made up
of a single repeat unit.
| Quenching | Terminating a condensation
polymerization reaction by adding a material with only one functional group.
Figure 5-24 Functional Groups Found in Polymers
C
along with 21n 2 12 water molecules. PET is called a homopolymer because it has a single repeat unit. The functional group from the glycol will react only with an acid, while the functional group from the acid will react only with an alcohol.
This ensures that the monomer always will add A-B-A-B-A-B. Theoretically, con-densation polymerization could continue until all of the available monomer has reacted to form a single giant polymer chain. In reality, the polymerization reac-tion reaches some equilibrium, and, as the chain grows larger, stearic hindrance inhibits further growth. Again, a distribution of chain lengths will result from the polymerization process. In some cases the reaction can be terminated by adding a material with only one functional group. This termination is called quenching the reaction. In addition to acids and alcohols, acids and amines also experience condensation polymerizations. Nylon 6,6 is formed by the condensation reac-tion of adipic acid and hexamethylene diamine, for example.