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Basic Types POLYMERIZATION Sacramento, Isaiah Paul G. BSChE-5 Engr. Elaine G. Mission 11/19/13

Polymerization Types

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  • Basic Types

    POLYMERIZATION

    Sacramento, Isaiah Paul G. BSChE-5 Engr. Elaine G. Mission 11/19/13

  • Polymerization: Basic Types 1

    Table of Contents

    ADDITION-CONDENSATION 2

    ADDITION POLYMERIZATION 2 CONDENSATION POLYMERIZATION 3

    CHAIN-STEP 4

    CHAIN-GROWTH POLYMERIZATION 4 FREE RADICAL 5 GENERAL MECHANISM OF A CHAIN-GROWTH POLYMERIZATION 6 STEP-REACTION POLYMERIZATION 8 GENERAL MECHANISM OF A STEP-REACTION POLYMERIZATION 8 RING-OPENING POLYMERIZATION 12 GENERAL MECHANISMS OF RING-OPENING POLYMERIZATION 13

  • 2 ChE Elective II: Polymer Engineering

    Polymerization Polymerization is defined as the process of converting a monomer or a mixture of monomers into a polymer. (IUPAC, 1996). It is a process where monomer molecules are chemically reacted to form polymer chains. (Young, 1987).

    Basic Types of Polymerization

    Different systems for classification of polymerization types exist. The mostly used are the ones established by Wallace Hume Carothers (1929) and by Paul John Flory (1953).

    Addition-Condensation

    W. H. Carothers (1929) was an American chemist, inventor, researcher and the leader of organic chemistry from DuPont. He was credited for the invention of nylon (Hermes, 1996).

    Carothers suggested the classification of polymerization (and polymers) based on the stoichiometry of the reaction or the difference in composition between the monomer and the resulting polymer. These are Addition and Condensation Polymerization. (Odian, 2004).

    Addition Polymerization According to Carothers (1929), an addition polymerization is a polymerization that results in the polymer product only. Addition Polymers are polymers formed without losing small molecules from the monomers. The end product has a repeating unit whose composition is the same as the monomer used.

    In addition polymerization, hydrocarbons with double bonds or alkenes

    react with each other. The double bond is broken and new covalent bond is formed between two monomers. (Need, 2013).

    Figure 1. Monomer and Polymer.

    Figure 2. Wallace Hume Carothers

    Figure 3. Addition Polymerization of Polyethylene.

  • Polymerization: Basic Types 3

    Condensation Polymerization Carothers (1929) defined condensation polymerization as a polymerization that results in the formation of a molecule with a low molecular weight other that the polymer product. Condensation Polymers are formed with loss of small molecules like water from the monomer. In order for condensation polymerization to proceed, two hydrogen-containing functional groups must be present. (Need, 2013).

    Figure 4. Condensation Polymerization of Nylon 6,6.

    Figure 5. Typical Condensation Polymers.

  • 4 ChE Elective II: Polymer Engineering

    Chain-Step P. J. Flory (1953) was an American chemist and notable for his works in the field of polymers and he won a Nobel Prize for Chemistry in 1974. He was with W. H. Carothers at Du Pont (Morris, 1986).

    He made a revision of Carothers work. In his great work Principles of Polymer Chemistry, he classified polymerization according to the mechanism of the polymerization processes. These are Chain-Growth and Step-Reaction Polymerization. (Odian, 2004).

    Chain-Growth Polymerization Chain-growth polymerization or simply chain polymerization occurs by successive addition of monomer molecules to the reactive end of a growing polymer chain. Usually it is the polymerization of vinyl monomers such as ethylene, propene, and styrene.

    Figure 7. Examples of Vinyl Monomers.

    To begin the chain-growth reaction, an initiator or catalyst is required. Right from the start, a product with high molecular weight is already produced along with the decrease in quantity of the monomer slowly with time. This type of polymerization is generally fast, irreversible and moderately to highly exothermic. (Odian, 2004). In other literature, addition polymerization and chain-growth polymerization are used synonymously with each other. According to Billmeyer (1984), an addition or chain-reaction polymerization involves chain reactions in which the chain carrier

    Figure 6. Paul John Flory

  • Polymerization: Basic Types 5

    may be an ion or a reactive substance called a free radical (or simply radical). A free radical is usually formed by the decomposition of a relatively unstable material called an initiator. The free radical is capable of reacting to open the double bond of a vinyl monomer and add to it, still leaving an unpaired electron. Within a short amount of time of about a few seconds or less, many more monomers add successively to the growing chain. Finally two free radicals react to annihilate each others growth activity and form one or more polymer molecules.

    Figure 8. Commercially Important Vinyl Polymers Prepared by Free Radical Polymerization.

    Free Radical

    A free radical is an atom, molecule, or ion that has unpaired electrons. When an orbital has at least one unpaired electron, and/or the atom, molecule, or ion have an odd number of electrons; then it is considered as a radical. Because of the unpaired valence electrons, radicals are highly chemically reactive toward other substances or even to themselves. They often polymerize when they come in contact with each other.

    Figure 9. Free Radical and Active Center.

  • 6 ChE Elective II: Polymer Engineering

    General Mechanism of a Chain-Growth Polymerization There are three major steps in the process: (1) initiation of the chain, (2) propagation of the chain as the monomers add to the reactive site, (3) and the termination of the reactive site to give completed macromolecules.

    Initiation Using either an initiator or a catalyst does the initiation of the chain. Initiator functions different from catalysts though. An initiator becomes incorporated into the polymer chain, usually at one end, and is consumed during the reaction, while a catalyst does not get incorporated into a specific, fixed location on the chain.

    Figure 10. Example of Free Radical Initiators.

    This step begins when an initiator decomposes into free radicals in the presence of monomers. The lack of stability of carbon-carbon double bonds in the monomer makes them susceptible to reaction with the unpaired electrons in the radical. In this reaction, the active center of the radical "grabs" one of the electrons

  • Polymerization: Basic Types 7

    from the double bond of the monomer, leaving an unpaired electron to appear as a new active center at the end of the chain. Addition can occur at either end of the monomer. Different types of initiation exist, and chain-growth polymerization can be classified depending on its type of initiation: (1) Radical Polymerization, (2) Coordination Polymerization, and (3) Ionic Polymerization. In Radical Polymerization, initiation starts by generating free radicals using techniques such as thermal decomposition, high-energy radiation, or redox reactions. Usually, free radicals are produced by thermal decomposition of molecules containing weak bonds e. g. peroxides (OO) or azo compounds (N=N). The formed radicals then react with the monomers. In Coordination Polymerization, usually transition-metal catalysts are used. The most important catalysts for coordination polymerization are the so-called Ziegler-Natta or Ziegler catalysts, and Phillips catalysts, both are effective for alkene polymerization. Ziegler catalysts combine transition metal compounds such as titanium and vanadium with organometallic compounds. Phillips catalyst is a catalyst system developed by Phillips Petroleum Co., where chromium oxide supported on silica is the most important constituent. This catalyst is limited only to the production of polyethylene. Ionization of the initiating species is utilized in the ionic polymerization.

    Propagation Next step is the propagation of the chain. Rapid growth of the chain occurs as the monomers react with the active center on the chain and then generating a new active center. In free radical polymerization, the entire propagation reaction usually takes place within a fraction of a second. Thousands of monomers are added to the chain within this time. The entire process stops when the termination reaction occurs. For example, if X were a methyl group, the monomer would be propylene and the polymer, polypropylene.

    Figure 11. Propagation Reaction of Propylene into Polypropylene.

    Termination In theory, the propagation reaction could continue until the supply of monomers is exhausted. However, this outcome is very unlikely because most often the propagation of the polymer chain is stopped by a termination reaction.

  • 8 ChE Elective II: Polymer Engineering

    Chain termination occurs when two active centers come in close contact and react with each other by combination or disproportionation. Both reactions yield completed macromolecules that no longer propagate chains.

    Combination occurs when the two chain-ends approach along their lines of centers and a carbon-carbon bond is reformed between them or free electrons from two growing chains that join and form a single chain. The following diagram depicts combination, with the symbol (R) representing the rest of the chain.

    Figure 12. Combination Reaction Mechanism.

    Disproportionation occurs when the reactive center on one chain takes away a hydrogen atom from the carbon atom neighboring the reactive center of a second molecule. Then that neighboring chain reforms a carbon-carbon double bond at its end in place of the missing hydrogen.

    Figure 13. Disproportionation Reaction Mechanism.

    Step-Reaction Polymerization Step-reaction or step-growth polymerization involves the reaction between the functional groups (HO, HOOC, etc.) of any two molecules of monomers or polymers. By repeated reaction, long chains are gradually produced. Mostly but not always, they are stoichiometrically condensation polymers, meaning they lose small molecules like water in the reaction. (Billmeyer, 1984).

    General Mechanism of a Step-Reaction Polymerization According to Odian (2004), step-growth polymerization usually proceeds by the reaction between two functional groups, for example, hydroxyl and carboxyl groups, or isocyanate and hydroxyl groups.

    Figure 14. Generic Representation of a Step Polymerization (white dots are monomers and black chains represents oligomers and polymers

  • Polymerization: Basic Types 9

    Figure 15. Examples of Stepwise Polymerizations.

    In step-growth polymerization, the bi-functional or multifunctional monomers react to form first dimers, then trimers to tetramers and they proceed to react with themselves; longer oligomers and eventually long chain polymers are formed. It proceeds by a relatively slow increase in molecular weight of the polymer; molecular weight increasing with time thus, the conversion. The monomer disappears early in the reaction far before the production of any polymer of sufficiently high molecular weight. (Odian, 2004).

    Figure 16. Monomers Used for Stepwise Reactions.

  • 10

    ChE Elective II: Polymer Engineering

    The easiest way to visualize the mechanism of a step-growth polymerization is a group of people reaching out to hold their hands to form a human chain each person has two hands (= reactive sites). There also is the possibility to have more than two reactive sites on a monomer: In this case branched polymers are produced. Step polymerization does not require an initiator and the end remains active thus, no termination.

    Figure 17. Typical Step-Reaction Polymers.

    Comparison between Chain and Step Reaction

    Here is a summary of comparison between chain and step polymerization:

    Chain Step

    1. Only species with active centers add monomer units.

    1. Any two potentially reactive end groups can react.

    2. Monomer concentration decreases steadily.

    2. Monomer depletion occurs very rapidly.

    3. High molecular weight polymer forms at once.

    3. Polymer molecular weight increases slowly with time.

    4. The concentration of reacting chains is usually low compared to the non-reacting monomer and polymer.

    4. Any size species can react with another and many chains are reacting at one time.

  • Polymerization: Basic Types 11

    Addition-Condensation & Chain-Step

    Because most of the reactions of chain-growth polymerization fall in the addition polymerization category; and step-reaction polymerization in the condensation polymerization category, the terminologies are often used in synonymous with each other (addition & chain, condensation & step). But that is not always the case. Thats why it must be clear of which is which. The two groups of terminologies are referring to two different classification systems, which are stated in the earlier parts of this document. Thats why one must be careful in using these terms. Here are some of the exceptions:

    Step polymerization by addition of alcohols to diisocyanates to form polyurethanes:

    Chain polymerization (ring opening of heterocycle) with loss of CO2 to form

    polypeptide:

    5. Some monomer remains even at long reaction times.

    5. Rapid loss of monomer early in the reaction.

    6. Different steps operate at different stages of mechanism (i.e. initiation, propagation, and termination.)

    6. Similar steps repeated throughout reaction process.

    7. Molar mass of backbone chain increases rapidly at early stage and remains approximately the same throughout the polymerization.

    7. Average molecular weight increases slowly at low conversion and high extents of reaction are required to obtain high chain length.

    8. Chains not active after termination. 8. Ends remain active meaning no termination.

    9. Initiator is required. 9. No initiator necessary.

  • 12

    ChE Elective II: Polymer Engineering

    Ring-Opening Polymerization Ring-opening polymerization is another mode of polymerization in addition to step and chain, consisting of a sequence of initiation, propagation, and termination. The terminal ends acts as a reactive center and further cyclic monomers join to form a larger polymer chain through ionic propagation. This is the polymerization of cyclic monomers such as cyclic ethers, acetals, amides (lactams), esters (lactones), and siloxanes. (Odian, 2004). The polymerization of these compounds has some aspects of both chain and step polymerization as far as kinetics and mechanism are concerned. It is proceeding by addition of monomer to growing chain molecules like chain-growth polymerization, though it never adds larger units. As in step polymerization, the polymer molecules increase continuously in molecular weight throughout the reaction. (Billmeyer, 1984).

    Figure 18. Commercially Important Polymers Prepared by Ring-Opening Polymerization.

  • Polymerization: Basic Types 13

    General Mechanisms of Ring-Opening Polymerization Ring-opening polymerization is initiated by opening of the ring to form an initiator species, which is either an ion or a neutral molecule like water. But most of these cyclic compounds polymerize by ionic chain mechanisms in the presence of strong acids or bases when water and alcohols are excluded. These polymerizations are often very rapid. (Billmeyer, 1984)

    References:

    Odian, G. (2004). Principles of Polymerization 4th Ed. New York: McGraw-Hill Book Co.

    Billmeyer, F. W. Jr. (1984) Textbook of Polymer Science 3rd Ed. Canada: John Wiley & Sons, Inc.

    Treloar, R. G. (1970). lntroduction to Polymer Science. New York: Springer-Verlag.

    "Glossary of basic terms in polymer science (IUPAC Recommendations 1996)". Pure and Applied Chemistry. Definition 3.1, p. 2305.

    Young, R. J. (1987) Introduction to Polymers. Chapman & Hall. International Union of Pure and Applied Chemistry, et al. (2000) IUPAC Gold

    Book, Polymerization. Hermes, M. (1996). Enough for One Lifetime. Wallace Carothers, Inventor of

    Nylon. Chemical Heritage Foundation.

    Morris, Peter J. T. (1986). Polymer Pioneers: A Popular History of the Science

    and Technology of Large Molecules p. 70-73. Philadelphia: Center for History

    of Chemistry.

    Polym-react. Retrieved from:

    http://www.che.iitb.ac.in/faculty/sm/CL442/notes/POLYM-REACT.pdf

    Polymer Synthesis. Retrieved from:

    http://plc.cwru.edu/tutorial/enhanced/files/polymers/synth/synth.htm

    Need (2013). Polymers. Retrieved from:

    http://chemistry.need.org/curriculum/polymers