Polymerization and Polymer Composition

01 / UNDERSTAND & APPLY

1. A repeat unit is a connectivity pattern, not an isolated monomer

A polymer contains macromolecules built from many linked units. The monomer is a starting molecule; the constitutional repeat unit is the structural segment repeated along the polymer chain. For ethene addition polymerization, CH₂=CH₂ becomes a chain segment –CH₂–CH₂–. The original pi bond is consumed, while the two carbon atoms become part of the saturated backbone.

Brackets around a repeat structure must cut actual backbone bonds, with connections continuing through both bracket ends. A formula such as[–CH₂–CH(Cl)–]ₙ represents poly(vinyl chloride), not a chain retaining an alkene C=C in every repeat. Substituents remain on the correct backbone carbon; chlorine is not removed during ordinary vinyl chloride polymerization.

The degree of polymerization counts repeat units according to the stated convention. A polyester repeat may contain residues from two monomer molecules, so “one repeat” is not always “one original molecule”. When converting chain molar mass to a repeat count, use the repeat mass after any eliminated small molecules and state whether end-group mass is neglected.

Real polymer samples usually contain chains of different lengths. A single value n in a sketch often represents an average or an idealized chain, not proof that every molecule has identical molar mass. Chain ends contribute little to a very long-chain mass but can dominate the chemistry of functional end groups.

02 / UNDERSTAND & APPLY

2. Chain-growth transfers an active centre along a growing chain

In chain-growth polymerization, an active chain end reacts with monomer and regenerates an active end after each propagation step. In a radical example, an initiating radical adds to a vinyl double bond, creating a new carbon-centred radical. That radical adds to another monomer, extending the chain while leaving the radical at the new terminus.

Initiation creates active centres, propagation extends chains, and termination removes their ability to continue by that mechanism. Two radicals can combine to form a covalent bond, or disproportionation can transfer H so that one chain gains a saturated end and another an unsaturated end. Chain transfer moves activity to another molecule and can alter chain length without consuming all monomer.

Use single-headed fishhook arrows for individual-electron motion in radical mechanisms. A propagation step redistributes the two pi electrons and the radical electron to form a new sigma bond and a new radical centre. Ordinary double-headed curved arrows instead represent electron pairs and belong to polar mechanisms.

Chain-growth need not be radical. Cationic polymerization uses a cationic active end, anionic polymerization an anionic one, and coordination polymerization can insert monomer into a metal–carbon bond. Monomer substituents and the catalyst or initiator determine which pathway is viable. Polyisobutylene is associated with 2-methylpropene, not ordinary propene; confusing those names produces a wrong repeat structure.

High-molar-mass chains can appear early in a chain-growth process even while much monomer remains. This differs from ideal step-growth, where the average chain length rises sharply only as functional-group conversion becomes very high. These are mechanistic trends, not a guarantee of identical distributions for all industrial processes.

03 / UNDERSTAND & APPLY

3. Common addition polymers preserve their substituents

Ethene gives polyethylene,[–CH₂–CH₂–]ₙ. Propene gives polypropylene,[–CH₂–CH(CH₃)–]ₙ. Vinyl chloride gives PVC,[–CH₂–CH(Cl)–]ₙ. Styrene gives polystyrene,[–CH₂–CH(C₆H₅)–]ₙ. Tetrafluoroethene gives PTFE,[–CF₂–CF₂–]ₙ. Each formula retains every monomer atom in the ideal addition repeat, apart from chain-end details.

Methyl methacrylate CH₂=C(CH₃)CO₂CH₃ gives PMMA with both CH₃ and CO₂CH₃ attached to the same substituted backbone carbon. Its ester group is pendant, rather than a linkage in the main backbone. That distinction matters in predicting hydrolysis: changing a pendant ester does not automatically cleave every carbon–carbon backbone bond.

A conjugated diene can undergo 1,4 polymerization, leaving one C=C per repeat. For isoprene CH₂=C(CH₃)–CH=CH₂, the common 1,4 repeat is –CH₂–C(CH₃)=CH–CH₂–. Natural rubber is predominantly cis-1,4-polyisoprene; trans-1,4-polyisoprene, associated with gutta-percha, has different packing and properties. Other microstructures such as 1,2 addition are possible with different conditions.

Neoprene is based on chloroprene, CH₂=C(Cl)–CH=CH₂. Styrene–butadiene rubber contains residues of styrene and butadiene; nitrile rubber contains butadiene and acrylonitrile residues. A copolymer name identifies its monomer sources but does not by itself specify their exact mole fractions or sequence distribution.

04 / UNDERSTAND & APPLY

4. Step-growth can join monomers, oligomers and longer chains

In step-growth polymerization, functional groups on molecules of many sizes can react with one another. A dimer can react with a monomer, another dimer or a longer chain, provided compatible groups remain. Many small species coexist early, and long chains require a high extent of functional-group reaction and appropriate stoichiometric balance.

A diacid and a diol can form a polyester by repeated ester formation, while a diacid and a diamine can form a polyamide. With carboxylic acids as starting groups, water is eliminated per newly formed ester or amide linkage in the ideal condensation balance. Using acid chlorides changes the small-molecule byproduct to HCl; the repeat connectivity can be the same.

Chain-growth/step-growth classify how chains grow. Addition/condensation describe a different aspect of the chemistry, especially whether small molecules are eliminated in building the polymer. Many familiar examples align chain-growth with addition and step-growth with condensation, but the labels are not universally interchangeable.

Polyurethane formation from a diol and a diisocyanate is a useful counterexample: stepwise addition forms urethane links without necessarily eliminating a small molecule in the ideal chain-forming reaction. Ring-opening polymerization provides further cases where the simple school-level addition/condensation division is not a complete mechanism classification.

Monomer functionality controls architecture. A purely difunctional ideal set favours linear chains; monofunctional molecules can cap chains, and functionality greater than two can create branching and eventually a network. Side reactions, unequal reactivity and cyclic oligomer formation can make actual products more complicated than the simplest model.

05 / UNDERSTAND & APPLY

5. Reconstruct monomers by cutting the correct linkage

PET is formed from ethane-1,2-diol and terephthalic acid or suitable derivatives. Its repeat can be written –O–CH₂CH₂–O–C(=O)–C₆H₄–C(=O)–, with the aromatic carbonyl substituents para to one another. Hydrolysis of the main-chain ester links separates glycol-derived and terephthalate-derived fragments.

Nylon 6,6 is based on hexane-1,6-diamine and hexanedioic acid, also called adipic acid. The repeat is –NH–(CH₂)₆–NH–C(=O)–(CH₂)₄–C(=O)–. Each monomer has six carbons, but the diacid’s six include its two carbonyl carbons, leaving only four CH₂ groups between them. Writing six CH₂ groups in the acid residue would give the wrong polymer.

Nylon 6 has repeat –NH–(CH₂)₅–C(=O)– and can be related to caprolactam ring opening or 6-aminohexanoic-acid connectivity. The six-carbon count again includes the carbonyl carbon. The final repeat does not have the same composition as a nylon 6,6 repeat simply because both names contain 6.

Polyamides can form interchain hydrogen bonds involving N–H donors and carbonyl oxygen acceptors, contributing to strength and fibre formation. Polyesters have polar ester groups but lack the corresponding amide N–H donors. Aromatic rings and chain regularity can increase rigidity; the exact material properties also depend on processing, crystallinity and molecular mass.

To infer monomers from a condensation polymer, identify the main-chain ester or amide connections and conceptually reverse their formation. Restore OH on the appropriate acyl fragment and H on the alcohol or amine fragment. Do not break arbitrary C–C bonds simply to make the name fit a remembered monomer pair.

06 / UNDERSTAND & APPLY

6. Sequence, tacticity and branching are different structural questions

A homopolymer derives from one monomer species; a copolymer contains more than one. An alternating copolymer has an ideal ABAB sequence, a block copolymer contains longer A and B runs, and a graft copolymer has branches of one composition attached to a backbone of another. A statistical sequence is governed by incorporation probabilities and can depart from a perfectly random simple model.

Monomer feed fraction need not equal polymer composition. Different monomers can react at different rates with different chain ends, and the remaining feed can change during conversion. Unless the problem supplies equal reactivities or direct incorporation data, do not infer an exact copolymer formula from the initial feed ratio alone.

Tacticity describes the relative configuration of successive stereogenic backbone sites in polymers such as polypropylene. Isotactic, syndiotactic and atactic describe regular same-relative, regular alternating-relative and irregular stereochemical sequences in the conventional extended-chain depiction. These are configurational patterns, not merely arbitrary rotations of a flexible chain drawn on paper.

Branching concerns connectivity: side chains emerge from the main backbone. Cross-linking connects different chains into a network. Neither is the same as placing all methyl substituents on the same side in a tacticity drawing. A polypropylene methyl pendant group is part of its repeat structure, not by itself evidence of the long-chain branching meant in polyethylene architecture comparisons.

Coordination catalysts can control chain insertion and stereochemistry, allowing highly regular polymers. The details depend on the catalyst system, not on the generic phrase metal catalyst. Regularity often aids close packing and crystallinity, which in turn affects stiffness, melting behaviour and permeability.

07 / UNDERSTAND & APPLY

7. Explain uses through packing, mobility and network structure

Thermoplastics soften on heating and can often be reshaped because their chains are not permanently connected into an extensive covalent network. Thermosets form cross-linked networks and do not simply remelt into freely flowing chains; sufficient heating can degrade them instead. These categories describe material behaviour, not a claim that every repeat unit contains a particular functional group.

Less-branched polyethylene can pack more efficiently and generally has higher crystallinity and density than highly branched polyethylene. HDPE is commonly associated with more rigid containers and piping applications; LDPE with flexible films and related products. Exact grades and processing histories matter, so a trade name or density label is not a complete performance specification.

PVC can be rigid or flexible depending on formulation, especially plasticizers that increase segmental mobility. It is used in pipes, coatings and insulation in suitable formulations. Polystyrene is commonly rigid and can be foamed for lightweight packaging. PMMA is transparent and used where optical clarity and stiffness are useful; PTFE has low surface adhesion and strong chemical resistance for suitable sealing and coating applications.

Fibres such as nylon and polyester benefit from chain alignment, intermolecular attraction and sufficient crystallinity. Drawing can orient chains and increase tensile strength along a direction, so a polymer’s properties are not determined by repeat formula alone. Directional mechanical behaviour is particularly important in oriented fibres.

The glass-transition temperature Tg marks a change in segmental mobility of amorphous regions; it is not the same as the melting temperature Tm of crystalline regions. A semicrystalline polymer can show both. Below Tg, an amorphous region is typically glassy; above Tg it becomes more mobile, though cross-links or crystalline domains can still prevent free flow.

Biodegradability depends on chemical linkages, morphology, environment and available organisms or enzymes. The presence of ester groups can support hydrolytic degradation but does not guarantee rapid disappearance in every environment. Natural origin, recyclability and biodegradability are separate claims. PHBV-type polyesters and biodegradable copolyamides are standard examples, but their actual breakdown conditions must be specified.

08 / UNDERSTAND & APPLY

8. Elastic networks and rigid networks use different cross-link densities

An elastomer can stretch substantially and recover when force is released. Flexible chains between sparse cross-links change conformation during extension; the cross-links prevent unrestricted chain sliding. Entropic preference for many coiled conformations contributes to the restoring force. This is different from saying every covalent bond stretches dramatically and then shrinks back.

Vulcanization introduces sulfur-containing cross-links in suitable rubbers, improving dimensional stability and resistance to permanent flow. Increasing cross-link density changes stiffness and extensibility; very extensive linking can give hard materials rather than highly extensible rubber. The elementary sulfur-bridge diagram is a schematic because actual cross-link structures and formulations vary.

Natural rubber’s predominantly cis-1,4-polyisoprene microstructure differs from more readily packed trans material. Synthetic elastomers allow different chemical resistance and mechanical behaviour: styrene–butadiene rubber is widely associated with tyre applications, nitrile rubber with oil-resistant seals, and chloroprene-based rubber with useful weathering resistance. These are typical applications, not interchangeable certifications.

Phenol–formaldehyde resins illustrate network formation through methylene and related links between activated aromatic units. Urea–formaldehyde and melamine–formaldehyde resins are other condensation-based network families. Early-stage structures can be soluble or fusible while further linking produces a cured network. Describing the final resin simply as one neat linear repeating chain can conceal the chemistry responsible for thermoset behaviour.

09 / UNDERSTAND & APPLY

9. Count molecules for Mn and weight molecules for Mw

If Ni molecules have molar mass Mi, the number-average molar mass is Mn=ΣNiMi/ΣNi. It weights each molecule equally. The mass-average, commonly called weight-average, molar mass is Mw=ΣNiMi²/ΣNiMi. It gives greater weight to heavier molecules. Their ratio Đ=Mw/Mn is the molar-mass dispersity and is at least one for a positive distribution.

A sample can therefore have a small number of very long chains that raise Mw substantially more than Mn. End-group analysis and ideal colligative measurements are linked to molecule number and hence Mn, while other techniques weight the distribution differently. A quoted average molecular mass is incomplete unless the average type is identified.

For a sufficiently long homopolymer with negligible end-group mass, number-average degree of polymerization is approximately Mn divided by repeat molar mass. In a copolymer, the average repeat mass must be calculated from incorporated monomer fractions under the specified model. Mole fractions and mass fractions are not interchangeable when the repeat masses differ.

For ideal linear AB step-growth with equal group reactivity, no rings and no branching, start with N₀ monomer molecules. If fraction p of the initial A groups reacts intermolecularly, pN₀ bonds form and the number of molecules falls to N₀(1−p). Thus Xn=N₀/N=1/(1−p). The derivation explains why 99% conversion gives only 100 average units. Stoichiometric imbalance, capping and intramolecular cyclization invalidate that simple version.

10 / UNDERSTAND & APPLY

10. Composition examples and solved practice

Keep three counts separate: monomer molecules supplied, residues incorporated and polymer molecules produced. In a condensation problem, also count eliminated small molecules and terminal groups. In a copolymer problem, use the incorporated composition supplied rather than silently substituting the feed composition.