Polymers

Polymers are macromolecules made up of repeating structural units called monomers. The number of repeating units is the degree of polymerization (n). A polymer with small n is an oligomer.

Classification by type of reaction:

• Addition polymerization (chain growth): monomers add one after another without loss of small molecules. The polymer has the same elemental composition as the monomer. Typical for vinyl monomers (C=C). Example: polyethylene from ethylene.
• Condensation polymerization (step growth): monomers react with elimination of small molecules (H₂O, HCl, CH₃OH). Requires monomers with 2+ functional groups. Example: nylon 6,6 from adipic acid + hexamethylenediamine.

Classification by thermal properties:

• Thermoplastics: soften when heated and harden when cooled (reversible process). Have linear or branched chains, not cross-linked. Examples: PE, PP, PVC, PS, PET.
• Thermosets: undergo irreversible cross-linking during polymerization. Once formed, cannot be remelted. Examples: epoxy resins, phenolic resins (Bakelite), cross-linked polyurethanes.

Classification by chain structure:

• Linear: monomers linked in a continuous chain.
• Branched: side chains attached to the main chain.
• Cross-linked: covalent bonds between different chains, forming a three-dimensional network.

Real-world applications: polymers are ubiquitous: packaging (PE, PET), construction (PVC), textiles (nylon, polyester), electronics (epoxy resins for circuit boards), medicine (PMMA for intraocular lenses, PLA for absorbable sutures).

Radical polymerization is a chain-growth process involving free radicals. It mainly applies to vinyl monomers (CH₂=CHR). The three phases are:

1. Initiation: generation of primary radicals from an initiator (peroxides, azo compounds). Example: benzoyl peroxide (PhCOO)₂ → 2 PhCOO• (heat or UV light). The radical attacks the C=C double bond of the monomer: $$\mathrm{R\!-\!O\!-\!O\!-\!R \xrightarrow{\Delta} 2\,RO\bullet}$$ $$\mathrm{RO\bullet + CH_2=CHR' \to RO\!-\!CH_2\!-\!\!\!\stackrel{\bullet}{C}HR'}$$
2. Propagation: the growing radical attacks a new monomer, adding one unit to the growing chain: $$\mathrm{RO\!-\![CH_2\!-\!CHR']_{n}\!\!\!-\!CH_2\!-\!\!\!\stackrel{\bullet}{C}HR' + CH_2=CHR' \to}$$ $$\mathrm{RO\!-\![CH_2\!-\!CHR']_{n+1}\!-\!CH_2\!-\!\!\!\stackrel{\bullet}{C}HR'}$$ Propagation is very fast (10³–10⁴ L/mol·s) and the addition is typically head-to-tail (the more stable radical forms on the more substituted C atom).
3. Termination (2 main modes):

• Combination: two radicals join forming a covalent bond: R—(CH₂—CHR'){n}—CH₂—•CHR' + •R'CH—CH₂—(CH₂—CHR'){m}—R → R—(sequence)—R. The chain is about twice as long as n.
• Disproportionation: one radical transfers an H• to another, forming a terminal alkene and a saturated chain. Molecular weight is about n.

Polymerization rate: the overall rate is proportional to [initiator]½ [monomer] (steady state). Increasing [initiator] increases the number of chains but reduces molecular weight.

Real-world applications: low-density polyethylene (LDPE, production ≥ 2000 atm, radical), polystyrene (PS, packaging, insulation), polyvinyl chloride (PVC, pipes, windows), polymethyl methacrylate (PMMA, Plexiglas).

Ionic polymerization uses ionic initiators (Lewis acids/bases, alkali metals) instead of radicals. It is divided into:

Anionic polymerization: the initiator is a strong nucleophile (n-BuLi, NH₂⁻, NaNH₂). The monomer must have electron-withdrawing groups (styrene, butadiene, methyl methacrylate). Advantage: living polymers — without termination, the chain remains active until monomer exhaustion. Adding a second monomer produces block copolymers. $$\mathrm{BuLi + CH_2=CHPh \to Bu-CH_2\!-\!\!\stackrel{\ominus}{C}HPh\,Li^+}$$
Cationic polymerization: the initiator is a Lewis acid (BF₃, AlCl₃) + a co-initiator (H₂O). Requires monomers with electron-donating groups (isobutylene, vinyl ethers). Temperature-sensitive: low T favors long chains. $$\mathrm{BF_3 + H_2O \to H^+ BF_3OH^-}$$ $$\mathrm{H^+ + CH_2=C(CH_3)_2 \to (CH_3)_3C^+}$$
Polymer stereochemistry: the arrangement of substituents along the chain determines tacticity:

• Isotactic: all substituents on the same side (same R/S configuration). Crystalline, high Tm, strong.
• Syndiotactic: alternating substituents (R,S,R,S...). Crystalline.
• Atactic: random arrangement. Amorphous, low or no Tm.

Real-world applications: living anionic polymerization produces polybutadiene and polystyrene with narrow molecular weight distributions (PDI ≈ 1.0). Block copolymers (SBS, styrene-butadiene-styrene) are thermoplastic elastomers used in shoe soles and adhesives.

Ziegler-Natta catalysts (Nobel Prize 1963) revolutionized polyolefin production. They are heterogeneous catalysts based on transition metal compounds (TiCl₄, TiCl₃) + alkyl-aluminum co-catalysts (AlEt₃, AlEt₂Cl). Mechanism:

1. Coordination of the monomer (ethylene, propylene) to the metal center (Ti) via the C=C double bond.
2. Insertion of the monomer into the Ti—C bond (1,2-migration).
3. Repetition: the active site is always the Ti—C bond of the last inserted unit.

Advantages: polymerization at moderate pressure and temperature (1–50 atm, 50–100°C), high molecular weight, control of tacticity (isotactic PP crystalline, Tm ≈ 165°C, vs atactic PP amorphous), no byproducts.

Metallocene catalysts (Kaminsky, 1980s): homogeneous single-site catalysts (e.g., Cp₂ZrCl₂ + MAO). Produce polymers with very narrow molecular weight distribution (PDI ≈ 2) and exceptional steric control (syndiotactic PP, linear low-density polyethylene).

Copolymers: polymers formed from 2+ different monomers:

• Random (statistical): random distribution — AABABBAB... Properties intermediate between the two homopolymers.
• Alternating: regular sequence — ABABAB... Maximum interaction between different units.
• Block: long sequences of AAAAA-BBBBB-AAAAA... Combined properties (e.g., SBS elastomeric).
• Graft: side chains of one polymer attached to the main chain of another.

Real-world applications: isotactic PP ("Moplen") is used for containers, fibers, film. High-density polyethylene (HDPE, Ziegler-Natta) for bottles, pipes. Metallocene catalysts produce linear low-density polyethylene (LLDPE) superior for stretch film.

The main commodity polymers and their properties:

Polyethylene (PE):

• HDPE (high density, 0.94–0.97 g/cm³): linear, Ziegler-Natta or metallocene catalysts. Tm ≈ 130°C. Uses: bottles, pipes, containers.
• LDPE (low density, 0.91–0.94 g/cm³): branched, high-pressure radical polymerization (1000–3000 atm). Tm ≈ 105°C. Uses: film, bags, coatings.
• LLDPE (linear low density): ethylene/1-butene or 1-hexene copolymer, metallocenes. Stretch film.

Polypropylene (PP): isotactic (Tm ≈ 165°C). Light, rigid, fatigue and chemical resistant. Uses: microwave containers, fibers (geotextiles), car bumpers.

Polyvinyl chloride (PVC): formula (—CH₂—CHCl—)ₙ. Strong, durable, can be rigid (pipes, windows) or flexible with plasticizers (cables, flooring). Environmental issue: releases HCl when burned. Tm ≈ 100–260°C (variable).

Polystyrene (PS): formula (—CH₂—CHPh—)ₙ. Transparent, rigid, brittle. Expanded PS (EPS, Styrofoam): thermal insulation. Tm ≈ 240°C.

PET (polyethylene terephthalate): polyester from terephthalic acid + ethylene glycol (condensation). Tm ≈ 260°C. Uses: beverage bottles, textile fibers (polyester). Recyclable.

Nylon 6,6: polyamide from hexamethylenediamine + adipic acid. Strong, elastic, abrasion-resistant fibers. Tm ≈ 265°C. Uses: textiles, ropes, gears.

Polycarbonate (PC): from bisphenol A + phosgene (or diphenyl carbonate). Transparent as glass, impact resistant. Tm ≈ 150°C (amorphous, Tg ≈ 147°C). Uses: CD/DVD, lenses, medical devices, screens.

Thermal properties:

• Tg (glass transition temperature): transition from hard-brittle (glassy) to soft-rubbery (elastomeric). Below Tg: chains frozen. Above Tg: segmental motion.
• Tm (melting temperature): transition from crystalline solid to liquid. Only in semi-crystalline polymers.

Real-world applications: polymer recovery and recycling is crucial for sustainability (recycled PET for new containers and fibers). Biopolymers (PLA, PHA) are biodegradable and compostable.

Condensation (or step-growth) polymerization occurs between two bifunctional monomers (or one monomer with two different functional groups) with elimination of a small molecule (H₂O, HCl, CH₃OH). Unlike addition polymerization, the molecular weight grows gradually and any pair of chains can react.

Polyesters:

• From a diacid (HOOC—R—COOH) and a diol (HO—R'—OH): repeated esterification.
• PET (polyethylene terephthalate): terephthalic acid + ethylene glycol, for bottles and textile fibers (Dacron, polyester).
• PLA (polylactic acid): biodegradable polymer from lactic acid (corn starch), for compostable packaging and medical implants.

Polyamides:

• From a diacid and a diamine (or from an ω-amino acid).
• Nylon-6,6: adipic acid (6C) + hexamethylenediamine (6C). Strong, elastic, abrasion-resistant fibers.
• Nylon-6: ring-opening polymerization of caprolactam (a cyclic amide).

Polycarbonates: from bisphenol A + phosgene (COCl₂). Polycarbonate (Lexan) is extremely impact-resistant (bulletproof glass, CD/DVD, helmets).

Polyurethanes: from a diisocyanate (OCN—R—NCO) + a diol: no small molecule is eliminated, but it is classed as condensation. Used for foams (mattresses, insulation), elastomers, coatings, adhesives.

Real-world applications: biodegradable plastics (PLA, PHA) are gaining importance against pollution. Nylon revolutionized textiles. Kevlar (an aromatic polyamide) is used in bulletproof vests (5× the strength of steel by weight).

World plastic production exceeds 400 million tonnes per year (2023). Less than 10% is recycled: plastic pollution is a global environmental crisis.

Recycling codes and polymer types:

• PET (code 1): recyclable (bottles → fibers, new bottles). Mechanical recycling: grind, wash, remelt.
• HDPE (code 2): recyclable (milk jugs, detergents).
• PVC (code 3): hard to recycle (contains chlorine and additives).
• LDPE (code 4): recyclable (bags, films).
• PP (code 5): recyclable (caps, containers).
• PS (code 6): rarely recycled (expanded polystyrene).
• Other (code 7: polycarbonate, acrylic, nylon): seldom recycled.

Biodegradable polymers:

• PLA (polylactic acid): biodegradable in industrial composting, from corn starch.
• PHA (polyhydroxyalkanoates): produced by bacteria, biodegradable even in the marine environment.
• PBAT: biodegradable, flexible polyester for compostable films.

Challenges:

• Chemical recycling (depolymerization into monomers) is costly but can yield virgin-grade polymers.
• Microplastics (< 5 mm) are everywhere: environment, drinking water, living organisms.
• Oxo-degradable plastics fragment into microplastics without biodegrading.
• The circular economy needs better collection, sorting and recycling technologies.

Real-world applications: compostable bags (PLA/PBAT) are increasingly common. Chemical recycling of PET (hydrolysis, methanolysis) recovers high-purity monomers. Chitin/chitosan biopolymers (crustacean shells) are studied for water treatment and wound dressings.