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Organic Chemistry
Table of Contents
Unit 31 Overview: Synthetic Polymers
31.1 Chain-Growth Polymers
31.2 Stereochemistry of Polymerization: Ziegler–Natta Catalysts
31.3 Copolymers
31.4 Step-Growth Polymers
31.5 Olefin Metathesis Polymerization
31.6 Intramolecular Olefin Metathesis
31.7 Polymer Structure and Physical

Step-growth polymers are formed by reacting two different monomers with functional groups at each end. This process creates long chains through condensation reactions, building up from dimers and trimers to full polymers.

The resulting materials, like nylon and polyester, have unique properties based on their structure. These polymers are used in everyday items from plastic bottles to car parts, showcasing their versatility and importance in modern life.

Step-Growth Polymers

Formation of step-growth polymers

  • Step-growth polymerization occurs between two different bifunctional or multifunctional monomers (diols and dicarboxylic acids) with reactive functional groups at each end
  • Reaction proceeds in a stepwise manner, forming dimers, trimers, and eventually long-chain polymers through condensation reactions
  • Nylon formation involves a condensation reaction between a diamine (hexamethylenediamine) and a dicarboxylic acid (adipic acid) to form nylon 6,6 or the ring-opening polymerization of caprolactam to form nylon 6
  • Polyester formation involves a condensation reaction between a diol (ethylene glycol) and a dicarboxylic acid (terephthalic acid) to form polyethylene terephthalate (PET) used in plastic bottles, fibers, and food packaging
  • The degree of polymerization, which indicates the number of monomer units in a polymer chain, affects the properties of the final product

Polycarbonates vs polyurethanes

  • Polycarbonates formed from the reaction of bisphenol A and phosgene or diphenyl carbonate have a structure of repeating units of bisphenol A linked by carbonate groups
    • Transparent, impact-resistant, and heat-resistant properties make them suitable for eyewear lenses, electronic components, and automotive parts
  • Polyurethanes formed from the reaction of polyols and diisocyanates have a structure of repeating units of polyol and diisocyanate linked by urethane groups
    • Versatile properties ranging from flexible foams to rigid plastics make them suitable for insulation, cushioning, coatings, and adhesives
  • The functionality of monomers used in these reactions determines the structure and properties of the resulting polymers

Production and uses of polyurethanes

  • Flexible polyurethane foams produced using long-chain polyols and diisocyanates with a blowing agent create a soft and resilient material used in mattresses, upholstery, and automotive seating
  • Rigid polyurethane foams produced using short-chain polyols and diisocyanates with a blowing agent create a dense and insulating material used in building insulation, refrigeration, and packaging
  • Polyurethane elastomers produced using long-chain polyols and diisocyanates without a blowing agent create an elastic and abrasion-resistant material used in shoe soles, conveyor belts, and sports equipment
  • Polyurethane coatings and adhesives produced using a combination of polyols and diisocyanates with various additives provide protective and bonding properties used in wood finishes, automotive paints, and laminating adhesives

Polymer characteristics and properties

  • Molecular weight distribution affects the physical and mechanical properties of step-growth polymers
  • Crosslinking between polymer chains can significantly alter the material's properties, often increasing strength and heat resistance
  • Thermoplastic polymers can be melted and reshaped multiple times, while thermoset polymers form irreversible chemical bonds during curing, resulting in a rigid structure

Key Terms to Review (37)

Polyurethane: Polyurethane is a versatile type of polymer formed by reacting a polyol (an alcohol containing multiple hydroxyl groups) with a diisocyanate, in the presence of a catalyst and other additives. It can be tailored to be either flexible or rigid, making it widely used in various applications such as foams, elastomers, and coatings.
Bisphenol A: Bisphenol A (BPA) is a synthetic organic compound that is widely used in the production of various consumer products, particularly in the manufacturing of polycarbonate plastics and epoxy resins. It has become a topic of interest due to its potential health and environmental impacts, especially in the context of its use in everyday items and its ability to mimic the structure and function of natural hormones.
Ethylene Glycol: Ethylene glycol is a colorless, odorless, and sweet-tasting liquid that is widely used as an antifreeze, coolant, and solvent. It is a dihydric alcohol, meaning it contains two hydroxyl groups, and its chemical formula is C₂H₆O₂. Ethylene glycol is a versatile compound that is relevant in the context of various topics in organic chemistry, including spectroscopy of alcohols and phenols, cyclic ethers, reactions of epoxides, and the synthesis of step-growth polymers.
Step-Growth Polymerization: Step-growth polymerization is a type of polymerization process in which monomers or reactive groups on the ends of polymer chains react with each other in a stepwise manner to form larger molecules. This gradual, step-by-step growth of the polymer chain distinguishes it from other polymerization methods, such as chain-growth polymerization.
Adipic Acid: Adipic acid is a dicarboxylic acid with the chemical formula (CH2)4(COOH)2. It is a widely used industrial chemical that serves as a key building block in the synthesis of various polymers, particularly polyamides and polyesters, which are the focus of the topics 21.9 Polyamides and Polyesters: Step-Growth Polymers and 31.4 Step-Growth Polymers.
Terephthalic Acid: Terephthalic acid is a dicarboxylic acid compound that is widely used in the production of polyesters and polyamides, which are important classes of step-growth polymers. It serves as a key building block in the synthesis of various polymeric materials with diverse applications.
Nylon 6,6: Nylon 6,6 is a type of polyamide polymer that is widely used in various applications due to its exceptional mechanical and thermal properties. It is formed through a step-growth polymerization reaction between a diamine and a dicarboxylic acid, specifically hexamethylenediamine and adipic acid.
Hexamethylenediamine: Hexamethylenediamine is a diamine compound with the chemical formula H2N(CH2)6NH2. It is an important monomer used in the synthesis of polyamides, a class of step-growth polymers, and is particularly relevant in the context of topics 21.9 Polyamides and Polyesters: Step-Growth Polymers and 31.4 Step-Growth Polymers.
Molecular Weight Distribution: Molecular weight distribution is a measure of the range of molecular weights present in a polymer sample. It is a critical characteristic that influences the physical and mechanical properties of polymers, including their strength, flexibility, and processing behavior.
Degree of Polymerization: The degree of polymerization (DP) refers to the average number of monomer units that make up a polymer chain. It is a crucial parameter that determines the physical and chemical properties of polymeric materials, particularly in the context of step-growth polymers like polyamides, polyesters, and other macromolecular structures.
Polyurethanes: Polyurethanes are a class of polymers formed through the reaction of diisocyanates and polyols. They are versatile materials used in a wide range of applications, from foams and coatings to adhesives and elastomers.
Nylon 6: Nylon 6 is a type of synthetic polymer classified as a step-growth polymer. It is a polyamide material widely used in various applications due to its exceptional mechanical properties, chemical resistance, and versatility.
Diols: Diols, also known as glycols, are organic compounds that contain two hydroxyl (-OH) functional groups. They are a class of polyhydric alcohols that play a crucial role in the formation of step-growth polymers.
Polyester: Polyester is a type of synthetic polymer that is widely used in various applications, including clothing, textiles, and plastics. It is formed through a step-growth polymerization process, where monomers are linked together to create long polymer chains. Polyester's unique physical and chemical properties make it a versatile material in the context of polymer structure and physical characteristics.
Diphenyl Carbonate: Diphenyl carbonate is an organic compound with the chemical formula (C6H5)2CO3. It is a colorless, crystalline solid that is commonly used as a monomer in the production of polycarbonate polymers, which are known for their high impact resistance, optical clarity, and thermal stability.
Caprolactam: Caprolactam is a cyclic amide compound that is the key monomer used in the production of the synthetic polymer nylon-6. It is an important industrial chemical with widespread applications, particularly in the manufacturing of textiles, plastics, and other engineered materials.
Carbonate Groups: Carbonate groups are functional groups consisting of a central carbon atom bonded to two oxygen atoms, forming a negatively charged $\text{CO}_3^{2-}$ unit. These groups are commonly found in various organic compounds and play a crucial role in the chemistry of step-growth polymers.
Crosslinking: Crosslinking is a process in which polymer chains are chemically bonded together, creating a three-dimensional network structure. This process is particularly important in the context of step-growth polymers, as it can significantly influence the properties and applications of these materials.
Functionality: Functionality refers to the specific capabilities, features, or purposes that a material, compound, or structure is designed to possess or perform. It is a crucial concept in the context of step-growth polymers, as the functionality of the monomers and reactants determines the type of polymer that can be formed and its subsequent properties.
Polycarbonates: Polycarbonates are a type of thermoplastic polymer known for their high impact resistance, transparency, and dimensional stability. They are widely used in a variety of applications due to their unique properties and versatility.
Bifunctional Monomers: Bifunctional monomers are organic compounds that possess two functional groups, allowing them to participate in the formation of step-growth polymers. These monomers serve as the building blocks for the synthesis of linear and cross-linked polymeric materials.
Thermoplastic: A thermoplastic is a type of polymer that becomes soft and pliable when heated and hardens when cooled. This reversible change in physical state allows thermoplastics to be easily molded, extruded, or otherwise shaped into a variety of products and applications.
Multifunctional Monomers: Multifunctional monomers are chemical compounds that possess more than one reactive functional group, allowing them to participate in the formation of multiple covalent bonds during step-growth polymerization. These monomers are essential building blocks in the synthesis of complex polymeric materials.
Diisocyanates: Diisocyanates are a class of organic compounds that contain two isocyanate functional groups (-N=C=O). These highly reactive molecules are widely used in the production of polyurethane polymers, which have a wide range of applications in various industries.
Urethane Groups: Urethane groups are functional groups that consist of a carbonyl (C=O) group bonded to an amino (N-H) group, forming a carbamate ester. They are an important structural component in step-growth polymers, contributing to the formation of polyurethanes and other related polymeric materials.
Nylon: Nylon is a synthetic polymer that belongs to the class of step-growth polymers. It is a versatile material widely used in various applications due to its exceptional mechanical properties and chemical resistance. Nylon's unique structure and physical characteristics make it a valuable material in the field of polymer science.
Polyurethane Elastomers: Polyurethane elastomers are a class of step-growth polymers that exhibit high elasticity and flexibility. They are composed of alternating soft and hard segments, allowing them to undergo reversible deformation under stress and recover their original shape when the stress is removed.
Thermoset: A thermoset is a type of polymer that undergoes an irreversible chemical reaction when heated, resulting in a rigid, cross-linked structure that cannot be remelted or reshaped. This unique property sets thermosets apart from thermoplastics, which can be repeatedly softened and reshaped by heating.
Dicarboxylic Acids: Dicarboxylic acids are organic compounds that contain two carboxylic acid functional groups (-COOH) within their molecular structure. These compounds play a crucial role in the context of step-growth polymers, as they can undergo polycondensation reactions to form long-chain polymers.
Condensation Reactions: Condensation reactions are a type of chemical reaction in which two molecules combine to form a single molecule, typically with the elimination of a small molecule such as water or methanol. These reactions are a fundamental aspect of step-growth polymerization, a process used to synthesize many important polymeric materials.
Polyethylene terephthalate (PET): Polyethylene terephthalate (PET) is a thermoplastic polymer resin of the polyester family, commonly used in the manufacture of plastic bottles, food containers, and other packaging materials. It is known for its strength, durability, and resistance to heat and chemicals, making it a widely used material in the context of step-growth polymers.
Phosgene: Phosgene is a highly toxic gas that was used as a chemical weapon during World War I. It is also an important industrial chemical with applications in the production of various organic compounds and polymers.
Polyols: Polyols, also known as sugar alcohols, are a class of organic compounds that contain multiple hydroxyl (-OH) groups. They are commonly found in nature and are often used as sweeteners and bulking agents in various food and pharmaceutical products.
Rigid Polyurethane Foams: Rigid polyurethane foams are a type of polymer material that are characterized by their high rigidity, low density, and excellent thermal insulation properties. They are produced through a step-growth polymerization process involving the reaction of polyols and isocyanates.
Flexible Polyurethane Foams: Flexible polyurethane foams are a type of polymer material that are lightweight, soft, and cushiony. They are commonly used in a variety of applications, including furniture, bedding, and automotive interiors, due to their unique properties and versatility.
Step-Growth Polymers: Step-growth polymers, also known as condensation polymers, are a class of polymers formed through a step-by-step process where monomers react with each other to form dimers, trimers, and larger oligomers, eventually leading to the formation of high molecular weight polymers. This process is characterized by the release of small molecules, such as water or hydrogen halides, as byproducts.
Blowing Agent: A blowing agent is a substance used in the production of foamed or expanded plastic materials, such as polystyrene foam and polyurethane foam. These agents create the cellular structure within the plastic, resulting in a lightweight, insulating material.