POLYMERS

Introduction to Polymers

Polymer: A substance composed of macromolecules.
Macromolecule: A molecule with high relative molecular mass chiefly consisting of repetitions of units derived from lower relative molecular mass molecules.
Monomer: A molecule that can undergo polymerization, contributing structural units to a macromolecule.
Repeating Unit: The smallest structural unit repeated to form a regular macromolecule, oligomer, or chain.
Degree of Polymerization: The number of monomeric units in a macromolecule, polymer, or oligomer.

Types of Polymers Based on Source

Natural Polymers: Obtained from natural resources. - Examples: Proteins, cellulose, starch, rubber.
Synthetic Polymers: Produced through chemical reactions. - Examples: Polyethylene, nylon, Teflon.

Types of Polymers Based on Structure

Composition Classification

Homopolymer: Consists of only one type of monomer unit. - Example: Polyethylene is synthesized from ethylene.
Copolymers: Comprises two or more types of monomers. - Example: Synthetic rubber from styrene and 1,3-butadiene; produces a polymer with two types of repeating units.

Monomer Arrangement in Copolymers

Random Copolymers: Repeating units arranged randomly.
Regular Copolymers: Contain a sequence of regularly alternating units.
Block Copolymers: Occurs in blocks of different lengths.
Graft Copolymers: A chain of one repeating unit grafted onto another polymer's backbone.

Classifications Based on Microstructure

Linear Polymers

Definition: One-dimensional polymers; monomer units held in a long chain without lateral branches.
Structure: Can fold back on themselves in random configurations.

Branched Polymers

Definition: Polymers with branches regularly spaced along the chain.
Impact: Reduces regular packing of polymer molecules, leading to less crystalline structures.
Structure: Contains short side chains extending from the main backbone.

Cross-linked Polymers

Formation: Created when linear or branched chains join through covalent bonds during cross-linking.
Effect on Properties: Increases elasticity with fewer cross-links, and enhances rigidity with more cross-links. - Types: - Slightly Cross-linked Polymers: Elastic (elastomers). - Highly Cross-linked Polymers: Rigid (thermosets).

Thermoplastics vs Thermosets

Thermoplastics: Linear and branched polymers that flow when heated and maintain their shapes upon cooling.
Thermoset Plastics: Highly cross-linked polymers with fixed shapes that cannot be reshaped once formed.

Polymerization Processes

Polymerization: A chemical reaction where monomer molecules react to form polymer chains or 3D networks.
Types of Polymerization Reactions: - Addition Polymerization (Chain-growth): Combines molecules with double/triple carbon-carbon bonds. - Condensation Polymerization (Step-growth): Involves the elimination of smaller molecules (e.g., water) during polymer formation.

Addition Polymerization

Definition: Joining small molecules (monomers) to form polymer chains without the elimination of smaller molecules.
Common Monomers: Unsaturated alkenes or derivatives with carbon-carbon double bonds.
Empirical Formula: Addition polymers share the same empirical formula as their monomers.

Classes of Addition Polymerization

Radical Polymerization
Ionic Polymerization: - Cationic mechanism - Anionic mechanism
Coordination Polymerization
Living Polymerization
Ring-opening Polymerization
Reversible-deactivation Polymerization

Mechanism of Free Radical Polymerization

Steps:

Initiation: Initiator (e.g., benzoyl peroxide) decomposes into free radicals, creating active centers by grabbing electrons from monomers' double bonds.
Propagation: Free radicals continuously react with alkene molecules, forming new free radicals and chaining together.
Termination: Occurs when two free radical ends of different chains encounter, forming a bond.

Mechanism of Cationic Polymerization

Initiation: Formation of a positive center (H+) usually catalyzed by acids (e.g., chloric acid, BF3).
Process: H+ attacks an alkene molecule forming a carbocation, which continues to react, creating polymer chains.
Termination: When the carbocation reacts with a negative charge.
Monomers: Preferably those with electron-releasing substituents to stabilize the cation intermediate.

Mechanism of Anionic Polymerization

Initiation: Formation of carbanions, commonly catalyzed by bases (e.g., sodamide).
Termination: When a carbanion combines with a positive particle.
Monomers: Containing electron-withdrawing groups help stabilize the anion intermediate.

Addition Polymer Examples

Poly(ethene) (PE): Most common; double bond opens to form single bonds, creating long chains of polyethylene. - Structure: Noted for repeating units and long chains.
Low Density Poly(ethene) (LDPE): Discovered in 1933; characteristics include: - Properties: Highly branched, low melting point (150°C), low density (0.92 g/cm³), and easily deformed. - Uses: Plastic bags, wrapping sheets, bottles, electrical insulation. - Production Conditions: 1200 atm and 200°C with O2.
High Density Poly(ethene) (HDPE): Manufactured by Ziegler-Natta process; characteristics: - Properties: 0.96 g/cm³ density, higher melting point (130-140°C). - Uses: Molding rigid articles. - Production Conditions: 1 atm and 60°C using TiCl4 + (C2H5)3Al catalyst.
Poly(propene) (PP): Mixed structures lead to unique properties: - Properties: Lower density due to methyl group presence, higher melting point. - Uses: Ropes, molds, bottles, kitchenware, carpets.
Poly(chloroethene) / PVC: Formed from vinyl chloride: - Properties: Strong due to polar C-Cl bonds. - Uses: Curtains, artificial leather, pipes, cables.
Poly(1,3-butadiene): Dienes undergo addition polymerization: - Uses: Tyre threads, coating resins. - Features: Can further undergo cross-linking like vulcanization.
Natural rubber: An elastomer from isoprene, can be poly-cis- or poly-trans-isoprene: - Properties: Soft and sticky when hot; hard and brittle when cold. - Vulcanization: Overcome limitations by forming sulfur cross-links; enhances durability and oxidation resistance. - Uses: Tires, footwear, gloves.
Synthetic rubber: Example: Neoprene from 2-chloro-1,3-butadiene: - Resistant to chemicals; uses include hoses and containers.
Styrenebutadiene Rubber (SBR): Copolymer of styrene and 1,3-butadiene; ratio 1:3: - Uses: Car tires, footwear, carpet backing.

Key Addition Polymers Summary

Name	Formula	Monomer	Properties	Uses
Polyethylene (LDPE)	$(CH_2-CH_2)_n$	Ethylene	Soft, waxy solid	Film wrap, plastic bags
Polyethylene (HDPE)	$(CH_2-CH_2)_n$	Ethylene	Rigid, translucent solid	Electrical insulation, bottles
Polypropylene (PP)	$[CH_2-CH(CH_3)]_n$	Propylene	Atactic: soft, elastic; isotactic: hard, strong	Carpet, upholstery
Poly(vinyl chloride) (PVC)	$(CH_2-CHCl)_n$	Vinyl chloride	Strong, rigid solid	Pipes, siding, flooring
Poly(vinylidene chloride)	$(CH_2-CCl_2)_n$	Vinylidene chloride	Dense, high-melting solid	Seat covers, films
Polystyrene (PS)	$[CH_2-CH(C_6H_5)]_n$	Styrene	Hard, rigid, clear solid	Toys, cabinets, packaging
Polyacrylonitrile (PAN)	$(CH_2-CHCN)_n$	Acrylonitrile	High-melting solid	Rugs, blankets, clothing
Polytetrafluoroethylene	$(CF_2-CF_2)_n$	Tetrafluoroethylene	Resistant, smooth solid	Non-stick surfaces
Poly(methyl methacrylate)	$[CH_2-C(CH_3)CO_2CH_3]_n$	Methyl methacrylate	Hard, transparent solid	Lighting covers, skylights
Poly(vinyl acetate) (PVAc)	$(CH_2-CHOCOCH_3)_n$	Vinyl acetate	Soft, sticky solid	Latex paints, adhesives
Natural rubber	$[CH_2-CH=C(CH_3)-CH_2]_n$	Isoprene	Soft, sticky solid	Requires vulcanization
Polychloroprene (Neoprene)	$[CH_2-CH=CCl-CH_2]_n$	Chloroprene	Tough, rubbery solid	Synthetic rubber, oil resistant

Condensation Polymerization

Definition: Monomers join to form long-chain polymers with the elimination of smaller molecules (e.g., H2O, methanol).
Empirical Formula: Generally, the empirical formula of condensation polymers differs from that of their monomers.
Functional Groups: Monomers need at least two functional groups to promote polymerization (leading to polyesters or polyamides).
Steps: Dimer formation, trimer formation, and subsequent steps until long polymeric chains are formed.
Also Know As: Step-growth polymerization.
Examples: Proteins, starch, cellulose.

Basic Condensation Reactions

Carboxylic Acid + Alcohol → Ester
Carboxylic Acid + Amine → Amide
Acyl Chloride + Alcohol → Ester

Polyester Synthesis

Example Reaction: Carboxylic acids or acyl chlorides react with alcohols, losing water. - E.g.: Ethanoic acid + ethanol forms ethyl ethanoate + H2O.
When dicarboxylic acids react with diols, they can form polymers through linking -OH and -COOH groups at polymer ends.

Nylon Synthesis

Nylon 6

Example: Formed from 6-aminohexanoic acid (a single monomer with two functional groups).
Industrial Process: From caprolactam through nucleophilic addition leading to polymerization.
Uses: Fibers, large molded articles.

Nylon 6,6

Type: Polyamide formed by the condensation of dicarboxylic acids with diamines, eliminating water.
Definition of Numbers: Each '6' indicates the respective carbon atoms in the diamine and dicarboxylic acid.
Properties: Higher melting/softening points due to strong hydrogen bond formations (polar –NH and –CO groups).

Kevlar Synthesis

Raw Materials: 1,4-benzenedicarboxylic acid (terephthalic acid) and 1,4-diaminobenzene.
Characteristics: Strong and flexible, applied in bulletproof vests.

Terylene (PET, Dacron) Synthesis

Components: Dicarboxylic acid (benzene-1,4-dicarboxylic acid) and dihydroxy compound (1,2-ethanediol).
Properties: High tensile strength, resistance to stretching and chemical degradation.
Uses: Clothing, tire cords, carpets.

Polymer Properties and Terminologies

Crystallites

Definition: Highly ordered regions within a polymer.
Importance: More crystalline polymers are denser, harder, and more heat-resistant due to stronger van der Waals forces.

Elastomers

Definition: Polymer that stretches and returns to original shape; requires some cross-linking.
Example: Rubber; behaves as randomly oriented amorphous polymers.

Fibers

Definition: Thin threads produced by extruding molten polymer through small holes.
Formation Process: Cooling and drawing the fibers align crystalline regions for tensile strength.
Examples: Nylon, Dacron, PE.

Plasticizers

Function: Added to polymers for flexibility; lowers attractions between polymer chains.
Example: Dibutyl phthalate.

Modification of Polymer Properties

Change polymer chain length.
Vary chemical composition of monomers.
Alter polymer chain branching.
Crosslink polymer chains (add strength).
Change solid chain arrangement.
Modify orientation of monomer units within chains.

Polymer Research Fields

Applications in building materials, vehicle parts, etc.
Natural vs synthetic fiber usage.
Focus on sustainability, reducing manufacturing costs, and environmental friendliness.

Challenges & Solutions

Drawbacks of Natural Fibers: Hydrophilicity leads to weak adhesion with hydrophobic HDPE, affecting mechanical properties and moisture resistance.
Research Solutions: Use of compatibilizers to improve fiber-Polymer matrix adhesion leading to better physical properties.

Applications and Innovations

Sustained Drug Delivery Materials

Function: Capsules designed for slow/release drug delivery.
Advantage: Lower strain on kidneys compared to immediate-release capsules.

References

Lecture notes credited to En Nasrulzamani B. Mohd. Rodzi; Prof. Madya Dr. Norbani Abdullah; Dr. Hazar Bebe Mohd Ismail.
Relevant literature exploring mechanical performance in composite materials, solar cell applications, and advanced hydrogels with self-healing properties (as per listed articles).