Table of Contents
Basic Ideas: From Small Molecules to Giant Chains
Synthetic organic polymers are very large molecules built from many small, repeating organic units. Two closely related ideas are useful:
- Monomer – a small organic molecule that can be linked to others (e.g. ethene, styrene, vinyl chloride).
- Polymer – a macromolecule formed by connecting many monomers.
- Degree of polymerization $n$ – the number of repeating units per polymer chain (often in the hundreds or thousands).
We write a polymer structure using a repeating unit in brackets:
$$
[-\text{CH}_2-\text{CH}_2-]_n \quad \text{poly(ethene), usually written as polyethylene (PE)}
$$
The chemical identity of the repeating unit (atoms and functional groups) is the main determinant of the polymer’s properties; how these units are connected and arranged (chain architecture and packing) then fine‑tunes those properties.
Chain Architecture: How the Repeating Units Are Connected
Linear, Branched, and Crosslinked Chains
- Linear polymers
- Chains consist of one long backbone with no (or very few) side chains.
- Example structures (schematic):
- Polyethylene (idealized):
$[-\text{CH}_2-\text{CH}_2-]_n$ - Poly(vinyl chloride), PVC:
$[-\text{CH}_2-\text{CHCl}-]_n$ - Chains can pack relatively closely, allowing crystalline regions and often giving higher density and strength (e.g. high‑density polyethylene, HDPE).
- Branched polymers
- Main chain with side chains attached.
- In polyethylene, branches are additional $-\text{CH}_2-\text{CH}_3$ or longer alkyl chains.
- Branching reduces the ability of chains to pack closely:
- Lower crystallinity and density.
- Typically softer and more flexible (e.g. low‑density polyethylene, LDPE).
- Crosslinked polymers (network polymers)
- Chains are covalently connected at various points, forming a 3D network:
- Slight crosslinking → materials that still deform somewhat (e.g. vulcanized rubber).
- Extensive crosslinking → rigid, infusible networks (e.g. epoxy resins, phenol–formaldehyde resins).
- Crosslinks reduce chain mobility, giving higher modulus (stiffness), heat resistance, and solvent resistance but lower ability to melt and be reshaped.
Homopolymers, Copolymers, and Architectures
- Homopolymer
- Only one type of repeating unit:
- Example: polyethylene from ethene monomer only.
- Copolymer
- Two (or more) different monomers are incorporated into the same chain. For two monomers A and B:
- Random copolymer: units A and B appear in random order along the chain.
Example: styrene–butadiene rubber (SBR) often approximates a random arrangement. - Alternating copolymer: strict alternation $(-A-B-A-B-)_n$.
Example (idealized): poly(styrene-alt-maleic anhydride). - Block copolymer: long sequences (“blocks”) of one monomer followed by blocks of another:
$A\_mB\_nA\_mB\_n \dots$
Example: styrene–butadiene–styrene (SBS) triblock copolymer. - Graft copolymer: a main chain of one monomer with side chains (“grafts”) of another.
Copolymer composition and sequence strongly influence glass transition temperature, elasticity, transparency, and compatibility with other materials.
- Chain shape: linear vs star vs dendritic
- Linear: one backbone.
- Star polymer: multiple linear arms radiating from a central core.
- Dendritic and dendrimers: highly branched, tree-like structures.
These architectures mainly affect viscosity, solution behavior, and how chains pack.
Tacticity: Spatial Arrangement Along the Chain
When a monomer has a substituent (e.g. a $-\text{CH}_3$ group in propene), the positions of these groups along the chain matter.
For a polymer with repeating unit $[-\text{CH}_2-\text{CH(R)}-]_n$:
- Isotactic: all substituents R on the same side of the chain.
- Syndiotactic: substituents alternate sides.
- Atactic: random arrangement of substituents.
Example: poly(propene)
- Isotactic polypropylene can pack more regularly → higher crystallinity, hardness, and melting point.
- Atactic polypropylene is more amorphous, softer, and often sticky or rubbery.
Tacticity is controlled mainly by the polymerization mechanism and catalysts used.
Molecular Weight and Distributions
Polymer samples contain molecules of many lengths:
- Number-average molar mass $M\_n$ and weight-average molar mass $M\_w$ describe the distribution.
- The ratio $M\_w / M\_n$ (polydispersity index) indicates how broad that distribution is.
Higher average molar mass generally increases tensile strength, toughness, and viscosity (chains are longer and more entangled), but also makes processing more difficult.
Types of Polymerization Reactions
Synthetic organic polymers are formed from organic monomers through characteristic reaction types. For beginners, it is useful to distinguish what changes in bonding during polymerization, without going into detailed mechanisms.
Chain-Growth (Addition) Polymerization
Key idea:
Unsaturated monomers (often with C=C double bonds) add to a growing chain one by one. The polymer’s repeating unit has (almost) the same atoms as the monomer; no small molecules are eliminated.
Typical Features
- Monomers: vinyl-type (e.g. ethene, propene, styrene, vinyl chloride, acrylates).
- Driving force: conversion of many relatively reactive double bonds into more stable single bonds within a long chain.
- Growth: occurs mainly at active chain ends (radical, cationic, or anionic).
- Rate: many chains start and grow in parallel, often very rapidly once initiated.
Simple Structural Example
From ethene to polyethylene:
$$
n\,\text{CH}_2{=} \text{CH}_2 \;\longrightarrow\; [-\text{CH}_2-\text{CH}_2-]_n
$$
The double bond opens, and carbon atoms now form single bonds in the backbone.
Initiation, Propagation, Termination (Conceptual)
While mechanistic details belong elsewhere, three basic stages help understand chain formation:
- Initiation – formation of an active species (e.g. radical) that can attack a monomer’s double bond.
- Propagation – the active chain end adds many monomer molecules; the chain grows.
- Termination – the active site is destroyed by combination of two chains or other reactions; chain growth stops.
Different initiation methods (radical, cationic, anionic, coordination catalysts) largely determine:
- Tacticity (iso-/syndiotactic vs atactic).
- Molecular weight and branching.
- Ability to form block or graft copolymers.
Step-Growth (Condensation and Related) Polymerization
Key idea:
Any two suitable molecules (often containing two or more functional groups) can react and link together, step by step. Chains grow slowly; small molecules (often water, HCl, or methanol) are frequently eliminated.
Typical Features
- Monomers: difunctional or polyfunctional organic molecules:
- Diacids + diols,
- Diamines + diacids,
- Diisocyanates + diols, etc.
- Driving force: formation of strong covalent bonds (e.g. ester, amide, urethane linkages) between functional groups.
- Growth: starts with dimers, trimers, oligomers; high molar mass appears only at very high conversion of monomer.
- Often involves condensation: elimination of small molecules, though there are also “step-growth” processes without elimination.
Structural Examples
- Polyesters (e.g. PET)
Reaction of a diacid (or diacid chloride) with a diol:
$$\text{HO-}\text{R-OH} + \text{HOOC-}\text{R'}\text{-COOH} \rightarrow \text{-O-R-OOC-R'-CO-}\_n + \text{(water)}$$
Repeating unit: ester linkages $(-\text{CO-O}-)$ connecting the pieces R and R′.
- Polyamides (e.g. nylon)
Reaction of a diamine with a diacid:
$$\text{H}_2\text{N-R-NH}_2 + \text{HOOC-R'-COOH} \rightarrow \text{-NH-R-NHCO-R'-CO-}\_n + \text{(water)}$$
Repeating unit: amide linkages $(-\text{CO-NH}-)$.
Because any chain can react with any other as long as functional groups are present, very high functional group conversion is required to obtain high molar mass material.
Common Polymerization Methods (Process View)
Without going into reactor engineering, it is useful to see how polymers are physically formed from monomers in practice, because this is connected with structure and purity.
Bulk (Mass) Polymerization
- Only monomer(s) plus initiator/catalyst.
- High polymer purity, no solvent to remove.
- Heat removal is difficult (reaction is often strongly exothermic), and viscosity increases as polymer forms.
- Often used for polymers like polystyrene or PMMA under controlled conditions.
Solution Polymerization
- Monomer and growing polymer are dissolved in a solvent.
- Solvent helps remove heat and control viscosity.
- Polymer must later be recovered by precipitation or evaporation of solvent.
- Useful for controlled reactions and for more uniform products.
Suspension and Emulsion Polymerization
- Monomer is dispersed in another liquid (often water) as droplets (suspension) or stabilized much more finely (emulsion).
- Polymerization takes place within droplets or micelles.
- Good heat removal and control; results in polymer beads or latex particles.
- Widely used for PVC (suspension PVC), styrene–butadiene latex, and many coatings.
These methods do not change the fundamental chemical structures, but they affect:
- Particle size and morphology of the polymer,
- Impurities and residual monomer content,
- How easily the polymer can be processed (e.g. as powder, beads, latex).
Linking Structure and Formation to Properties (Overview)
For synthetic organic polymers, specific structural features formed during polymerization are key to performance:
- Backbone type and functional groups:
Hydrocarbon backbones (e.g. polyethylene) are chemically inert and hydrophobic; polar backbones (polyesters, polyamides) can hydrogen-bond and interact with water. - Chain architecture:
Linear vs branched vs crosslinked → meltability, elasticity, and solvent resistance. - Tacticity and regularity:
More regular structures often crystallize better → higher stiffness and defined melting behavior. - Molecular weight and distribution:
Longer chains and narrower distributions generally improve mechanical strength and toughness (up to practical limits). - Composition in copolymers:
Mixing different monomer types in one chain allows tuning glass transition temperatures, flexibility, and compatibility with other materials.
Understanding how a polymer’s chemical structure and chain architecture arise from the chosen monomer and polymerization method is the foundation for designing tailor-made synthetic polymers, which are discussed in a later chapter.