Organic Chemistry I & II
Organic Chemistry I & II

Organic Chemistry I & II

Lead Author(s): Steven Forsey

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Organic Chemistry I & II is designed for instructors who want an active, dynamic, and understandable approach to support their own efforts in the classroom. This ever-evolving textbook includes auto-graded questions, videos and approachable language in order to make difficult concepts easier to understand and implement.

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Steven Forsey, “Organic Chemistry”, Only one edition needed

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Solomons et al., “Organic Chemistry”, 12th Edition

Wiley

David R. Klein, “Organic Chemistry”, 3rd Edition

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Customizable

Ability to revise, adjust and adapt content to meet needs of course and instructor

All-in-one Platform

Access to additional questions, test banks, and slides available within one platform

Pricing

Average price of textbook across most common format

Top Hat

Steven Forsey, “Organic Chemistry”, Only one edition needed

Up to 40-60% more affordable

Lifetime access on any device

McGraw-Hill

Carey & Giuliano, “Organic Chemistry”, 10th Edition

$219

Hardcover print text only

Wiley

Solomons et al., “Organic Chemistry”, 12th Edition

$301

Hardcover print text only

Wiley

David R. Klein, “Organic Chemistry”, 3rd Edition

$301

Hardcover print text only

Always up-to-date content, constantly revised by community of professors

Constantly revised and updated by a community of professors with the latest content

Top Hat

Steven Forsey, “Organic Chemistry”, Only one edition needed

McGraw-Hill

Carey & Giuliano, “Organic Chemistry”, 10th Edition

Wiley

Solomons et al., “Organic Chemistry”, 12th Edition

Wiley

David R. Klein, “Organic Chemistry”, 3rd Edition

In-book Interactivity

Includes embedded multi-media files and integrated software to enhance visual presentation of concepts directly in textbook

Top Hat

Steven Forsey, “Organic Chemistry”, Only one edition needed

McGraw-Hill

Carey & Giuliano, “Organic Chemistry”, 10th Edition

Wiley

Solomons et al., “Organic Chemistry”, 12th Edition

Wiley

David R. Klein, “Organic Chemistry”, 3rd Edition

Customizable

Ability to revise, adjust and adapt content to meet needs of course and instructor

Top Hat

Steven Forsey, “Organic Chemistry”, Only one edition needed

McGraw-Hill

Carey & Giuliano, “Organic Chemistry”, 10th Edition

Wiley

Solomons et al., “Organic Chemistry”, 12th Edition

Wiley

David R. Klein, “Organic Chemistry”, 3rd Edition

All-in-one Platform

Access to additional questions, test banks, and slides available within one platform

Top Hat

Steven Forsey, “Organic Chemistry”, Only one edition needed

McGraw-Hill

Carey & Giuliano, “Organic Chemistry”, 10th Edition

Wiley

Solomons et al., “Organic Chemistry”, 12th Edition

Wiley

David R. Klein, “Organic Chemistry”, 3rd Edition

About this textbook

Lead Authors

Dr. Steven Forsey, Ph.D.University of Waterloo

Steven Forsey is currently a Professor at University of Waterloo, teaching a variety of organic chemistry courses to Chemistry, Science, Chemical Engineering, Nanotechnology and distance education students. He received his Ph.D. (2004) for Synthetic Organic Chemistry from University of Waterloo, Ontario. He is a recipient of the Excellence of Science Teaching Award and has acted as the Teaching Fellow for the Department of Chemistry since 2016.

Contributing Authors

Felix NgassaGrand Valley State University

Neil GargUCLA

Jennifer ChaytorSaginaw Valley State University

Greg DomskiAugustana College

Christian E. MaduCollin Community College

Christopher NicholsonUniversity of West Florida

Franklin OwEast Los Angeles College, UCLA

Robert S. PhillipsUniversity of Georgia

Grigoriy SeredaUniversity of South Dakota

Simon E. LopezUniversity of Florida

Brannon McCulloughNorthern Arizona University

Jason JonesKennesaw State University

José BoquinAugustana College

Stephanie BrouetSaginaw Valley State University

Explore this textbook

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Chapter 27: Synthetic Polymers

Synthetic polymers are a very versatile group of materials. They are used in many household items, including but not limited to paints, flooring, adhesives, and rubber. [1]

Contents 

27.1 Introduction to Synthetic Polymers
27.2 Polymer Nomenclature
     27.2.1 Naming Vinyl Polymers
     27.2.2 Naming Non-Vinyl Polymers
27.3 Classification of Polymers
     27.3.1 Classifications Based on Molecular Structure
          27.3.1.1 Homopolymers and Copolymers
          27.3.1.2 Classification According to Functional Groups
               27.3.1.2.1 Vinyl Polymers
               27.3.1.2.2 Non-Vinyl Polymers
                    27.3.1.2.2.1 Polyethers
                    27.3.1.2.2.2 Polyesters
                    27.3.1.2.2.3 Polycarbonates
                    27.3.1.2.2.4 Polyamides
                    27.3.1.2.2.5 Polyurethanes
          27.3.1.3 Classification According toPolymer Formation Mechanism
                27.3.1.3.1 Chain Growth (Addition) Polymerization
                     27.3.1.3.1.1 Radical Polymerization
Keeping it Real
                     27.3.1.3.1.2 Cationic Polymerization
                     27.3.1.3.1.3 Anionic Polymerization
Keeping it Real
                     27.3.1.3.1.4 Coordination-Insertion Polymerization
                27.3.1.3.2 Step-Growth (Condensation) Polymerization
                27.3.1.3.2.1 Polyethers
                27.3.1.3.2.2 Polyesters
                27.3.1.3.2.3 Polycarbonates
                27.3.1.3.2.4 Polyamides
                27.3.1.3.2.5 Polyurethanes
     27.3.4 Industrial Classifications
          27.3.4.1 Plastics
          27.3.4.2 Fibers
          27.3.4.3 Elastomers
          27.3.4.4 Coatings and Adhesives
27.4 Physical Properties of Polymers
     27.4.1 Thermal Properties of Polymers
     27.4.2 Mechanical Properties
     27.4.3 Factors Affecting the Physical Properties of Polymers
          27.4.3.1 Molecular Weight/Chain Length
          27.4.3.2 Intermolecular Forces
          27.4.3.3 Stereochemical Configuration 
          27.4.3.4 Polymer Branching
27.5 Polymer Recycling
     27.5.1 Polymer Recycling Codes
     27.5.2 Methods of Polymer Recycling
          27.5.2.1 Reuse
          27.5.2.2 Depolymerization
          27.5.2.3 Biodegradable Polymers

Learning Objectives

  • Understand that polymers are comprised of repeat units derived from the monomer(s) that reacted to form the polymer.
  • Understand the differences between biopolymers and synthetic polymers.
  • Be able to name commonly encountered polymers.
  • Be able to classify polymers as homopolymers or alternating, random, block, or graft copolymers.
  • Be able to draw the mechanism of cationic, radical, and anionic addition polymerization reactions.
  • Be able to draw the mechanism for common step-growth/condensation polymerization reactions.
  • Be able to decide, based on monomer structure, which method (cationic or anionic) addition polymerization will be most effective.
  • Be able to predict the products of condensation polymerization reactions.
  • Working from polymer and byproduct structure, suggest reasonable monomeric starting material(s) for polymer synthesis.
  • Explain the difference between chain-growth and step-growth polymerization and predict which one a monomer is likely to undergo.
  • Be able to distinguish between branched and linear polymers and understand how branching affects polymer properties.
  • Be able to classify vinyl polymers as syndiotactic, isotactic or atactic.
  • Understand how the stereochemical configuration of polymers affects polymer properties.
  • Understand how intermolecular forces and cross-linking affect polymer properties.
  • Understand the properties of crystalline and amorphous polymers/regions.
  • Know the definition of Tm and Tg.
  • Know the difference in properties between thermoplastics, elastomers, fibers and thermosetting resins.
  • Know common recycling codes, methods for polymer recycling and how polymer structure affects method of recycling.
  • Be able to classify polymers as vinyl polymers, polyesters, polycarbonates, polyamides, polyethers or polyurethanes.
  • Understand the differences between coordination-insertion polymerization and cationic, anionic and radical polymerization of alkene monomers.

27.1 Introduction to Synthetic Polymers

Chemistry instructors often point out that “chemistry is everywhere”; this is especially true when it comes to polymers. Much of your body is made of polymers, such as proteins and carbohydrates. The instructions for protein synthesis are encoded in polymers (DNA) and many of the chemical reactions essential to life are catalyzed by polymers (enzymes). All of the aforementioned are examples of biopolymers; these generally have precise lengths and functions, and are synthesized through cellular processes. On the other hand, synthetic polymers are, for the most part, prepared in laboratories and industrial plants using many of the basic organic reactions that you have already studied. Your clothes, car, water bottle, smartphone, house and food containers are constructed wholly, or in part, from polymeric materials; in most of the developed world, not a day goes by where a person does not benefit from synthetic polymers.

Figure 27.1. Polyethylene milk jugs. [2]​

Polymers, or macromolecules, are constructed by linking many (102–105) smaller molecules (monomers) together via covalent bonds. For example, polyethylene (used to make plastic grocery bags and translucent plastic bottles) is prepared by polymerizing ethene monomers (Figure 27.1). Besides the fact that polymers are made from smaller molecules, the major, practical difference between polymer molecules and small molecules is in terms of their physical properties. Whereas most small organic molecules are gases, liquids, or brittle crystalline solids under ambient conditions, polymers are generally solids, with useful mechanical properties (e.g., strength, resistance to stretch) under the same conditions. This difference in behaviors is largely due to the greater surface area of polymer chains, which results in stronger intermolecular forces and entanglements between chains that make it difficult to separate the chains from one another (Figure 27.2).

Figure 27.2. Entangled polymer chains.


27.2 Polymer Nomenclature

Polymer science is a relatively new area of chemical study. It is also unique in that it was developed largely in the industrial sector rather than in an academic setting. Because of these factors, the naming of polymers has two distinct conventions: one developed by industry and one developed by IUPAC. Having two systems of nomenclature can cause confusion for those new to the field of polymer chemistry, but since the industrial convention is the most commonly used, especially in North America, we will limit our discussion to this method.

27.2.1 Naming Vinyl Polymers

A polymer prepared from monomers that exchange their C=C double bond for a C–C single bond is called a vinyl polymer after the vinyl groups present in the monomers. Polyethylene (Figure 27.1) is an example of a vinyl polymer. More vinyl polymers are shown in Table 27.1. Notice that the name of these polymers comes from adding “poly” to the name of the monomer. Again, notice that some of the polymer names are derived from non-IUPAC names for the monomers. The parentheses in the figure represent a repeat unit of the polymer. 

Table 27.1. Common vinyl polymers.

27.2.2 Naming Non-Vinyl Polymers

As we will see later in this chapter, vinyl polymers are prepared using different methods than those used for preparing polymers not derived from vinyl monomers. Naming non-vinyl polymers can be a bit more complicated than naming vinyl polymers since a greater variety of repeat unit structures are possible.

In some cases, the polymer is named after the monomer, as is the case for vinyl monomers (e.g., poly(ethylene glycol); Figure 27.3).

Figure 27.3. Polyethylene glycol.

In some cases, the polymer is named after the repeat unit structure (e.g., poly(ethylene terephthalate); Figure 27.4).

Figure 27.4. Poly(ethylene terephthalate).


Since the systematic IUPAC nomenclature is not often used for polymers, it is best to be familiar with the names for commonly encountered, non-vinyl polymers and to name new polymers using the analogy of the common examples summarized in Table 27.2.

Table 27.2. Common non-vinyl polymers.



Q27.1 - Level 1

What is the name of the polymer below?

question description
A

Poly(1-pentene)

B

Poly(isopropyl butene)

C

Poly(1-hexene)

D

Poly(4-methyl-1-pentene)


Q27.2 - Level 1

What is the name of the polymer below?

question description
A

Poly(ethylene glycol)

B

Poly(ethylene oxide)

C

Poly(propylene oxide)

D

Poly(propylene glycol)


Q27.3 - Level 1

What is the name of the polymer below?

question description
A

Poly(methylene terephthalate)

B

Poly(terephthalic acid)

C

Poly(terephthalic ester)

D

Poly(propylene terephthalate)


27.3 Classification of Polymers

As discussed above, polymer chemistry was largely developed in industrial settings where the end use of the polymer is considered paramount. However, as the importance of polymeric materials to humanity became clearer, academic chemists adopted a systematic approach to the study of polymer chemistry. Whereas the industrial sector was primarily interested in polymers as commodity materials, academic chemists were more interested in understanding the fundamental mechanisms of polymer formation and behavior. As such, several different classification schemes have been developed; some based on polymer structures, some based on polymer properties and uses and some based on the mechanism by which a polymer forms.

27.3.1 Classifications Based on Molecular Structure

27.3.1.1 Homopolymers and Copolymers

One of the simplest distinctions between polymer types is the distinction between homopolymers and copolymers. Homopolymers are prepared via the polymerization of only one monomer, whereas copolymers are prepared via the polymerization of two or more monomers (Figure 27.5).

Figure 27.5. Homopolymers and copolymers.

Homopolymers can be further classified as being branched or linear. For example, depending on the conditions used to polymerize ethene, either branched or linear polyethylene may be formed (Figure 27.6). As we will see later, branched and linear polymers often have significantly different physical properties and end uses. In the case of polyethylene, the branched homopolymer is quite flexible and prone to stretching; thus, this material is used to make plastic grocery bags, whereas the linear homopolymer is used to make rigid plastic items like water bottles.

Figure 27.6. Linear and branched polyethylene.

Copolymers can be further classified as random, alternating or block copolymers. As implied by the name, random copolymers result when two or more monomers are polymerized together and the monomeric subunits are randomly distributed throughout the polymer chain (Figure 27.5). Alternating copolymers result when two monomers are polymerized and the monomeric subunits alternate in a regular, repeating fashion. Block copolymers have extended sequences in which identical monomeric subunits are linked together (blocks).

For questions 27.4–27.7 below, the monomers are ethylene and propylene.

Q27.4 - Level 1

Classify the polymer below.

question description
A

Homopolymer

B

Block copolymer

C

Alternating copolymer

D

Random copolymer


Q27.5 - Level 1

Classify the polymer below.

question description
A

Homopolymer

B

Block copolymer

C

Alternating copolymer

D

Random copolymer


Q27.6 - Level 1

Classify the polymer below.

question description
A

Homopolymer

B

Block copolymer

C

Alternating copolymer

D

Random copolymer


Q27.7 - Level 1

Classify the polymer below.

question description
A

Homopolymer

B

Block copolymer

C

Alternating copolymer

D

Random copolymer


27.3.1.2 Classification According to Functional Groups

27.3.1.2.1 Vinyl Polymers

There are many common vinyl polymers, but these polymers can have a diverse set of functional groups and, hence, vastly different properties and uses.

Polymers derived from alkenes are often referred to as polyolefins (olefin is an archaic name for alkenes). Examples include polyethylene, polypropylene and polystyrene (Table 27.1). These polymers are composed only of carbon and hydrogen and, therefore, the primary intermolecular force acting between the polymer chains is the London dispersion force. Thus, these polymers are non-polar and hydrophobic and have effective barrier properties toward water. Polyolefins tend to be rigid, durable materials that resist degradation due to the strength of their C–C and C–H bonds; however, their properties can vary depending on the molecular weight of the polymer and stereochemical configuration about the polymer backbone. Generally speaking, low molecular weight polyolefins have poor mechanical properties because the strength of London dispersion forces depends on the surface area of a molecule; for example, polyethylene behaves like a waxy solid until it reaches a molecular weight of about 10,000 g/mol, whereas ultra-high molecular weight polyethylene (UHMWPE) is a very hard and durable material used in applications such as hip and knee replacements.

Haloalkenes can also be polymerized to form halogenated polyolefins. Two very common polymers in this class are polyvinyl chloride (PVC) and polytetrafluoroethylene (PTFE; TeflonTM) (Figure 27.7). PVC is a highly rigid material and most modern plumbing fixtures are made from this material. PVC may be made flexible by adding small molecules called plasticizers, which disrupt the regular packing of PVC chains. PTFE is commonly used as a non-stick coating on cookware since the high electronegativity of the fluorine atoms lead to low polarizability and, hence, weak London dispersion forces.

Figure 27.7. Non-olefinic vinyl polymers.


A third common class of vinyl polymers is the polyacrylates, which are derived from the polymerization of acrylic acid and its derivatives. Acrylic acid can be polymerized to form polyacrylic acid; this polymer can be deprotonated with NaOH to form sodium polyacrylate, which is extremely hydroscopic and is used as an absorbent material in disposable diapers (Figure 27.8). Other common polyacrylates and their uses may be found in Table 27.3.

Figure 27.8. Preparation of sodium polyacrylate​.


Table 27.3. Common polyacrylates.


27.3.1.2.2 Non-Vinyl Polymers

27.3.1.2.2.1 Polyethers

Polyethers are made of repeat units containing C–O–C linkages. This very broad class of polymers has a diverse set of properties and applications ranging from adhesives to engineering plastics. Examples of commonly encountered polyethers may be found in Table 27.4.

Table 27.4. Common polyethers.

27.3.1.2.2.2 Polyesters

Polyesters contain –C(=O)–O– linkages and, like polyethers, have a great diversity of structures and, hence, properties. The most commonly encountered polyester is poly(ethylene terephthalate) (PET), which is prepared via the reaction between ethylene glycol and terephthalic acid (Figure 27.9) and has been in use since the 1940s. Most soft drink bottles are made of PET. Another commonly encountered polyester is poly(lactic acid) (PLA), which has the benefit of being biodegradable and biorenewable; it may be found in certain food/drink containers and eating utensils and is marketed under the trade name GreenwareTM. Other commonly encountered polyesters and their uses are described in Table 27.5.

Figure 27.9. Preparation of poly(ethylene terephthalate).


Table 27.5. Common polyesters.


27.3.1.2.2.3 Polycarbonates

Polycarbonates have similar structures to polyesters except that they bear carbonate linkages (O–C(=O)–O–). The most commercially successful polycarbonate is called poly(bisphenol A carbonate) (BPA-PC); often simply called polycarbonate, it is prepared through the reaction of bisphenol A and phosgene (Figure 27.10). BPA-PC is marketed under the trade name LexanTM and owing to its high impact strength is used in applications such as bulletproof glass and safety helmets.

Figure 27.10. Preparation of polycarbonate.

27.3.1.2.2.4 Polyamides

Polyamides bear amide linkages (–C(=O)–NH–) and are capable of hydrogen bonding. As a result of their ability to hydrogen bond, polyamides have very high tensile strengths (resist stretching) and have higher melting points than polyesters. The most commonly encountered polyamides are nylons, with nylon-6 and nylon-66 being the most prevalent (Figure 27.11). These polymers find uses mainly as fibers in applications from clothing to ropes. In fact, during World War II, when most of the silk in the United States was requisitioned to make parachutes for the military, stockings were manufactured from nylon and this polymer has displaced silk in many applications to this day due to its lower cost.

Figure 27.11. Commercially important nylons.

Another less common, but important polyamide is poly(p-phenylene terephthalamide) or KevlarTM (Figure 27.12). Because of the strong hydrogen bonding, rigid backbone structure, and π-stacking interactions between the benzene rings, the polymer chains in this polyamide pack quite closely and this results in a material of incredible strength and durability. Kevlar is used in lightweight bulletproof vests, in the construction of high performance automobile and bicycle tires and in numerous other applications where high strength and durability are important.

Figure 27.12. Kevlar.

27.3.1.2.2.5 Polyurethanes

Polyurethanes are also sometimes called polycarbamates after the carbamic acid linkages they contain (–N(C=O)O–). Polyurethanes are typically synthesized via a reaction between diisocyanates and diols (Figure 27.13). They find use as insulators, upholstery, sponges and coatings. SpandexTM is another commonly encountered material containing a polyurethane. This polymer is actually a diblock copolymer with a flexible polyether segment and a rigid polyurethane segment (Figure 27.14). The combined properties of these two segments are responsible for Spandex’s ability to stretch without tearing.

Figure 27.13. Preparation of polyurethanes.


Figure 27.14. Spandex​​.


Q27.8 - Level 1

Classify the following polymer according to its functional group.

question description
A

Vinyl polymer

B

Polyether

C

Polyester

D

Polyamide

E

Polycarbonate

F

Polyurethane


Q27.9 - Level 1

Classify the following polymer according to its functional group.

question description
A

Vinyl polymer

B

Polyether

C

Polyester

D

Polyamide

E

Polycarbonate

F

Polyurethane


Q27.10 - Level 1

Classify the following polymer according to its functional group.

question description
A

Vinyl polymer

B

Polyether

C

Polyester

D

Polyamide

E

Polycarbonate

F

Polyurethane


Q27.11 - Level 1

Classify the following polymer according to its functional group.

question description
A

Vinyl polymer

B

Polyether

C

Polyester

D

Polyamide

E

Polycarbonate

F

Polyurethane


27.3.1.3 Classification According to Polymer Formation Mechanism

There are two broad categories of polymer formation: chain growth or addition polymerization and step-growth or condensation polymerization (Figure 27.15). As the name implies, chain growth polymers form by sequential addition of monomer units to the end of a growing chain. In step-growth polymerization, small molecules combine randomly to form larger and larger molecules; these larger molecules can combine with each other to make even larger molecules and this process continues until the reactive end groups are capped. Step-growth polymerization is also referred to as condensation polymerization since, in many cases, a byproduct is formed, such as in the formation of poly(ethylene glycol) (PEG) where, along with the polymeric product, water is also formed (Figure 27.16).

Figure 27.15. Step-growth vs. chain growth polymerization. Note A only reacts with B and vice versa.


Figure 27.16. Condensation polymerization of propylene glycol.

27.3.1.3.1 Chain Growth (Addition) Polymerization

Chain growth polymerization is the most common method for the polymerization of vinyl monomers. There are four main types of addition polymerization mechanisms. Three of these (cationic, anionic, and radical) mechanisms you are already familiar with from previous chapters. The fourth mechanism, coordination–insertion polymerization, requires the use of a transition metal catalyst. 

27.3.1.3.1.1 Radical Polymerization

Many vinyl polymers are prepared via radical polymerization in part because there is no need for a counter anion to stabilize the reactive chain, which simplifies the polymerization process. Two common vinyl polymers formed via radical polymerization mechanisms are polystyrene (PS; StyrofoamTM) and polytetrafluoroethylene (PTFE; TeflonTM). Much like the radical reactions you learned about earlier in this course, radical polymerizations have an initiation, propagation, and termination sequence (Figure 27.17). Consider the example of styrene polymerization. In the initiation sequence, an initiator (usually a peroxide like benzoyl peroxide) is used to generate an alkoxide radical, which then adds to the π-bond of the vinyl monomer. In the case of styrene, a stable benzylic radical is formed: this is the propagating species. In the propagation sequence, the reactive chain end reacts with one styrene monomer at a time and regenerates the benzylic radical so that another monomer may be enchained. As the amount of monomer decreases and the relative concentration of radical species increases, termination processes occur via radical coupling reactions, thereby ending the polymerization process.


Figure 27.17. Radical polymerization of styrene.


Teflon

A famous polymer you have probably heard of is polytetrafluoroethylene (Teflon), which is found in wires, cables, and the coating on non-stick cooking pans.

The polymerization reaction used to synthesize polytetrafluoroethylene is a radical chain-growth polymerization.

Teflon pan. [3]​

Teflon can also be found on coated guitar strings, made by ELIXIR. These strings are used by many musicians!

Image of John Paul Jones ​ [4], Miranda Lambert [5], Wayne Sermon. [6]


27.3.1.3.1.2 Cationic Polymerization

Cationic polymerizations are mechanistically similar to the electrophilic addition reactions of alkenes that you saw earlier in this course. In a cationic polymerization an electrophilic initiator is used to react with the π-bond of the vinyl monomer. The resultant anion balances the positive charge of the carbocation formed during the initiation step to maintain electric neutrality. The carbocation then reacts with a monomer via electrophilic addition; this process is repeated over and over again as the polymer chain grows. Chain growth is terminated by adding an excess amount of a nucleophile that irreversibly reacts with the carbocation at the chain end (for example, water). Polyisobutylene (Figure 27.18) is one of the commercially important polymers that is formed via cationic polymerization and is used in adhesives, sealants, oils, and moisture barriers. Cationic polymerizations work most efficiently with monomers that bear electron-donating groups that can stabilize the electron-deficient carbocation; 1,1-disubstituted alkenes, with their electron-donating alkyl substituents, readily undergo cationic polymerizations as do vinyl ethers since their alkoxy groups can stabilize the carbocation via resonance (Figure 27.19).

Figure 27.18. Cationic polymerization of isobutene​.


Figure 27.19. Monomers amenable to cationic polymerization.


Q27.12 - Level 2

Which monomer is most effectively polymerized via cationic polymerization?


Q27.13 - Level 2

Which monomer is most effectively polymerized via cationic polymerization?


Q27.14 - Level 2

Which monomer is most effectively polymerized via cationic polymerization?


27.3.1.3.1.3 Anionic Polymerization

Anionic polymerizations are initiated by nucleophilic attack on a vinyl monomer, much like that seen previously in the mechanism of the Michael addition. As a result of a nucleophilic attack, a carbanion is formed and attacks another vinyl monomer, thereby enchaining it and extending the growing polymer chain. Anionic polymerizations are terminated by the addition of an electrophile, like water or an acid. Because a carbanion is present at the growing chain end, monomers with electron-withdrawing groups (e.g., NO2, carboxyl, CN, and aromatics) are readily polymerized via anionic methods. Depending on the stability of the incipient carbanion, the nucleophile may be relatively weak like water or, in the case of a less stable carbanion, a stronger nucleophile like the butyl anion may be required to initiate polymerization. One of the most common polymers produced via an anionic mechanism is polycyanoacrylate or SuperGlueTM (Figure 27.20). Because of the electron-withdrawing cyano and methyl ester groups, the resulting carbanion is quite stable and the polymerization of cyanoacrylate can be initiated by atmospheric water; this is why SuperGlueTM containers must be tightly capped once opened.

Figure 27.20. Anionic polymerization of cyanoacrylate (SuperglueTM).


27.15 - Level 2

Which monomer is most effectively polymerized via anionic polymerization?


Q27.16 - Level 2

Which monomer is most effectively polymerized via anionic polymerization?


Q27.17 - Level 2

Which monomer is most effectively polymerized via anionic polymerization?



Stuck on polymers? Here is another common one: super glue!

The major compound found in a bottle of super glue is called cyanoacrylate. Upon exposure to water vapor in the air, a polymerization reaction occurs to form a strong adhesive. Here is the mechanism for cyanoacrylate polymerization:

Step 1: Water attacks cyanoacrylate to form a nucleophilic enolate.

Step 2: The enolate attacks another cyanoacrylate molecule to form the ‘polymerization intermediate’.

Step 3: This process is repeated until all the cyanoacrylate is consumed to give the polymer.

Keeping it Real Q27.1


Keeping it Real Q27.1 - Level 2

Using the image above, predict the structure of the ‘polymerization intermediate’.

question description
A

a

B

b

C

c


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27.3.1.3.1.4 Coordination–Insertion Polymerization

In the 1950s, Karl Ziegler and Giulio Natta discovered that certain transition metal compounds (commonly group 4 or group 10 metals) were capable of catalyzing the polymerization of alkenes with high levels of control over chain length and, in the case of 1-alkenes, stereochemical configuration of the pendent groups of the polymer backbone. For their discovery, the pair were awarded the 1963 Nobel Prize in chemistry. The mechanistic details of Ziegler–Natta catalysis are quite complex compared with the other chain growth mechanisms described above and require a solid background in organometallic chemistry to fully comprehend. The basic mechanism is shown in Figure 27.21. The first step, coordination, involves the donation of the π-electrons from the alkene into an empty orbital of d-parentage of the transition metal. The second step, migratory insertion, involves the nucleophilic attack of a coordinated metal alkyl group at the more substituted carbon of the coordinated alkene; in this process, a new C–C σ bond is formed and a new M–C σ bond is formed while the C=C π-bond is broken and the original M–C σ bond is broken. After the migratory insertion step, a new vacant site is created and another alkene monomer can bind to the metal; thus, the process is repeated as the polymer chain grows. The steric environment around the metal center determines the stereochemical outcome of the coordination and insertion thereby controlling the stereochemical configuration of the pendent groups on the polymer backbone. Natta’s original catalyst system comprised of TiCl4- and AlMe3-produced isotactic polypropylene, where the pendent methyl groups were all oriented on the same side of the polymer backbone (see Figure 27.34). Recent advances in catalyst architecture have provided access to syndiotactic poly(alphaolefins). Atactic poly(alphaolefins) can be prepared with certain non-stereoselective catalysts or via radical polymerization. The tacticity of polymers plays a major role in determining their properties as we shall see in a later section.

Figure 27.21. Coordination–insertion polymerization. The empty box refers to a binding site for alkenes. The metal center is often bound to other molecules (represented by the diamond-shaped appendage).


27.3.1.3.2 Step-Growth (Condensation) Polymerization

The mechanisms of step-growth polymerization are almost as varied as the mechanisms for all of the reactions you have encountered in your study of organic chemistry since most organic reactions that are used to prepare step-growth polymers are energetically favorable (negative ΔG). We will focus on common methods for the formation of the five most frequently encountered non-vinyl polymers mentioned previously: polyethers, polyesters, polycarbonates, polyamides, and polyurethanes.

27.3.1.3.2.1 Polyethers

Polyethers can be formed by self-condensation reactions of diols; however, in some cases, the competing formation of cyclic ethers complicates the process. For example, treating 1,2-ethanediol with a catalytic amount of acid results in the preferential formation of 1,4-dioxane rather than polyethylene glycol (Figure 27.22). If the monomer structure is chosen carefully so as to avoid the formation of cyclic ethers, then self-condensation reactions can result in polymerization as is the case when 1,4-benzenedimethanol is allowed to react in the presence of an acid catalyst (Figure 27.23).

Figure 27.22. Ethylene glycol does not polymerize.


Figure 27.23. 1,4-Benzenedimethanol does polymerize because a stable ring cannot be formed.

27.3.1.3.2.2 Polyesters

Polyesters can be formed by many of the methods you have already seen for the preparation of small molecule esters including direct esterification, trans-esterification, and the reaction of a diol with a diacid chloride or acid anhydride. The most common polyester (PET) is formed via direct esterification between 1,2-ethanediol and terephthalic acid as seen in Figure 27.24.

Figure 27.24. Polymerization mechanism for the formation of poly(ethylene terephthalate). Note that some steps have been condensed to conserve space; generally, no more than three lone pairs move per elementary step.


27.3.1.3.2.3 Polycarbonates

Polycarbonates are often formed via a reaction between phosgene and a diol. For example, BPA-PC is formed via the nucleophilic addition reaction between bisphenol A and phosgene (Figure 27.25).

Figure 27.25. Polymerization mechanism for the formation of polycarbonate.

27.3.1.3.2.4 Polyamides

Like polyesters and polycarbonates, the formation of polyamides depends on the nucleophilic attack on a carbonyl carbon. For example, nylon 6,6 can be prepared via reaction between 1,6-hexanediamine and adipoyl chloride (Figure 27.26).

Figure 27.26. Polymerization mechanism for nylon 66.

27.3.1.3.2.5 Polyurethanes

Polyurethanes can be formed via the reaction of bischloroformates with diamines (Figure 27.27), but this method has the disadvantage of producing hydrogen chloride as a byproduct. From an industrial, economic, and environmental standpoint, the production of this corrosive byproduct is quite costly. An economic method for producing polyurethanes is via the reaction of diisocyanates with diols (Figure 27.28); in this case, no byproducts are formed.

Figure 27.27. Polyurethanes from bischloroformates​.


Figure 27.28. Polyurethanes from diisocyanates.


Q27.18 - Level 2

What is the byproduct of the following polymerization reaction?

question description
A

H2_2O

B

HCl

C

No byproduct

D

CO2_2

E

O2_2


Q27.19 - Level 2

What is the byproduct of the following polymerization reaction?

question description
A

H2_2O

B

HCl

C

No byproduct

D

OCl2_2

E

Cl2_2CO


Q27.20 - Level 2

What is the byproduct of the following polymerization reaction?

question description
A

H2_2O

B

HCl

C

No byproduct

D

NCO

E

(NH2_2)CO


Q27.21 - Level 2