Addition Polymerisation and Polymer Disposal

Spec Mapping — OCR H432 Module 4.1.3 — Alkenes, covering addition polymerisation of alkenes (the opening of C=C double bonds in many monomer molecules to give a single long-chain polymer with no other products), the structural drawing of repeat units from monomers and vice versa, the named industrial polymers polyethene, polypropene, polychloroethene (PVC), polyphenylethene (polystyrene) and polytetrafluoroethene (PTFE), the environmental and economic factors associated with the disposal of waste polymers (mechanical recycling, feedstock / chemical recycling, incineration with energy recovery, landfill), the removal of HCl from PVC incineration flue gases by alkaline scrubbing, and the role of biodegradable and photodegradable polymers as alternatives (refer to the official OCR H432 specification document for exact wording).

The last alkene reaction you need to know is addition polymerisation: the joining of thousands of alkene monomer molecules into a single long-chain molecule, a polymer. This reaction underpins the entire modern plastics industry — over 400 million tonnes of polyethene, polypropene, PVC, polystyrene and PTFE are produced globally each year, more than the combined annual production of all metals except iron. The chemistry is conceptually simple: each C=C π bond opens and joins to the next monomer in turn, regenerating a new C=C π bond at the chain end (the propagating radical / cation / carbanion) ready to attack the next monomer. The mechanism is chain polymerisation — radical-initiated for polyethene and polystyrene; Ziegler-Natta organometallic-catalysed for polypropene; ionic for some specialty polymers. Industrially, the polymers themselves are wonderful: cheap, light, strong, mouldable, electrically insulating, chemically inert. But that same inertness creates a colossal disposal problem — most addition polymers are non-biodegradable and persist for centuries. This lesson develops both halves: the polymerisation chemistry (drawing repeat units, naming polymers, the industrial monomers), and the disposal-environment chemistry (landfill, incineration with energy recovery, mechanical recycling, feedstock recycling, biodegradable alternatives). OCR examines this lesson with structural drawings and essay-style environmental questions in roughly equal measure.

Key Definition — Addition polymerisation: a reaction in which many unsaturated monomer molecules (typically alkenes) join to form a single long-chain polymer molecule with no other product — every atom of the monomer is in the polymer. Compare with condensation polymerisation (Year 13), in which each step releases a small molecule (water or HCl). The repeat unit of an addition polymer has the same number of atoms as the monomer, with the C=C double bond replaced by a C–C single bond plus two extension bonds.

1. What is addition polymerisation?

Key features of addition polymerisation:

The C=C π bond in each monomer opens, allowing a new C–C σ bond to form between adjacent monomers.
No atoms are lost — the polymer contains all the atoms of the monomers. The atom-economy is 100 %.
Many monomers (usually thousands to millions) join together in succession.
The reaction proceeds by a chain mechanism: an initiator generates an active species (radical, carbocation, or transition-metal complex), which adds to one monomer; the new active species at the chain end adds to the next monomer; the chain extends until termination.

1.1 The generic scheme

$n\,\text{CH}_2{=}\text{CHR} \rightarrow [\text{-CH}_2\text{-CHR-}]_n$

The C=C double bond becomes a C–C single bond, and the repeat unit gains two extension bonds (one to the previous monomer, one to the next). The polymer chain can extend for thousands of repeat units; molecular weights of 10⁵–10⁶ g mol⁻¹ are typical.

Monomer:       H   R            Repeat unit:    [ H   R ]
                \ /                              [ |   | ]
                 C = C                           [ C - C ]
                / \                              [ |   | ]
               H   H                             [ H   H ]
                                                          n

1.2 The radical chain mechanism (industrial polyethene)

Industrial polyethene (LDPE, low-density poly(ethene)) is made by radical chain polymerisation of ethene at 1000–3000 atm and 100–300 °C with an organic peroxide initiator. The mechanism mirrors the three-stage chain mechanism of free-radical substitution (Lesson 6):

Initiation: R–O–O–R → 2 R–O• (homolysis of the peroxide O–O bond).
Propagation: R–O• + CH₂=CH₂ → R–O–CH₂–CH₂• (adds across the C=C). Then the new radical adds to another ethene: R–O–CH₂–CH₂• + CH₂=CH₂ → R–O–CH₂–CH₂–CH₂–CH₂•. Repeat thousands of times.
Termination: two growing chains meet — chain-chain combination ("R" + "R'" → "R–R'") or disproportionation (one chain abstracts an H from the other, giving one alkane and one alkene-terminated chain). Termination ends that chain.

The same mechanism, with different conditions and catalysts, gives all the major industrial addition polymers. Modern polyethene is largely made by Ziegler–Natta catalysis (TiCl₄/AlEt₃ on a support; Karl Ziegler and Giulio Natta, 1953-1955, Nobel 1963) which gives HDPE (high-density poly(ethene), much more linear and crystalline) at far lower pressure (~10 atm) and temperature (~80 °C). Even more modern is metallocene catalysis (post-1990) which gives extremely uniform chain lengths and stereoregularity.

2. Common addition polymers

Monomer	Polymer (IUPAC / common)	Repeat unit	Industrial uses
Ethene CH₂=CH₂	Poly(ethene) / polythene / PE	–(CH₂–CH₂)–	Bags, bottles, film, packaging — most-produced polymer
Propene CH₂=CHCH₃	Poly(propene) / PP	–(CH₂–CH(CH₃))–	Food containers, rope, carpet, automotive trim
Chloroethene CH₂=CHCl (vinyl chloride)	Poly(chloroethene) / PVC	–(CH₂–CHCl)–	Window frames, pipes, cable insulation, flooring
Phenylethene CH₂=CHC₆H₅ (styrene)	Poly(phenylethene) / polystyrene / PS	–(CH₂–CH(C₆H₅))–	Packaging, disposable cups, insulation (expanded PS / EPS)
Tetrafluoroethene CF₂=CF₂	Poly(tetrafluoroethene) / PTFE / Teflon	–(CF₂–CF₂)–	Non-stick coatings, bearings, gaskets, chemically-inert linings
Methyl 2-methylpropenoate CH₂=C(CH₃)CO₂CH₃	Poly(methyl methacrylate) / PMMA / Perspex	–(CH₂–C(CH₃)(CO₂CH₃))–	Transparent panels, acrylic paint, dental composites

The world produces ~400 million tonnes of these six polymers per year combined.

3. Writing polymer equations

3.1 Drawing the repeat unit — OCR rules

The repeat unit is the smallest unit that, when repeated, generates the polymer.

For an alkene CH₂=CHR, the repeat unit is:

  [  H   R  ]
  [  |   |  ]
  [  C - C  ]
  [  |   |  ]
  [  H   H  ]
              n

OCR drawing rules (you will lose marks if any are missing):

Open the C=C: draw a single bond between the two carbons; the double bond is broken in polymerisation.
Square brackets (or clear curly brackets) around the repeat unit.
Extension bonds (short lines) coming out of the left and right ends, showing where the polymer continues.
Subscript n outside the right-hand bracket.

3.2 Worked Example — Poly(propene)

Propene: CH₂=CHCH₃. Reaction:

$n\,\text{CH}_2{=}\text{CHCH}_3 \rightarrow [\text{CH}_2\text{-CH(CH}_3\text{)}]_n$

Repeat unit:

   [ H   H  ]
   [ |   |  ]
 — [ C - C  ] —
   [ |   |  ]
   [ H  CH₃ ]
              n

The methyl branch dangles off every other carbon. Tacticity (whether the methyls all point the same way or alternate) is determined by the catalyst — Ziegler-Natta gives isotactic poly(propene) (all methyls same side, crystalline, high m.p.) while older radical processes give atactic PP (methyls random, amorphous, lower-grade material).

3.3 Worked Example — PVC

Chloroethene: CH₂=CHCl. Reaction:

$n\,\text{CH}_2{=}\text{CHCl} \rightarrow [\text{CH}_2\text{-CHCl}]_n$

The repeat unit has one Cl on every other carbon. The C–Cl bond is the source of PVC's environmental problem on incineration (Section 5.2).

3.4 Worked Example — Polystyrene (poly(phenylethene))

Styrene: CH₂=CHC₆H₅. Reaction:

$n\,\text{CH}_2{=}\text{CHC}_6\text{H}_5 \rightarrow [\text{CH}_2\text{-CH(C}_6\text{H}_5\text{)}]_n$

The benzene ring (phenyl group, –C₆H₅) dangles off every other carbon. The bulky phenyl rings interlock to give rigid, glassy polystyrene — the standard packaging material in disposable cups and expanded polystyrene (EPS) insulation.

3.5 Finding the monomer from the polymer

If you are given the polymer repeat unit and asked for the monomer:

Draw the repeat unit (in brackets, with extension bonds).
Remove the extension bonds and the brackets.
Re-draw the two carbons in the repeat unit's backbone as a C=C double bond.
Name the resulting alkene.

Example: the repeat unit –(CH₂–CF₂)– gives the monomer CH₂=CF₂ (1,1-difluoroethene, the monomer for poly(vinylidene fluoride), PVDF).

4. Properties that make polymers so useful

Chemically inert. Do not react with acids, bases, air, water, or most chemicals at room temperature. The backbone consists only of strong, non-polar C–C and C–H σ bonds with no functional groups to react.
Mechanically strong yet flexible. Long entangled chains and the chain entropy of polymer melts give materials that resist tearing and breakage but can be moulded into any shape.
Lightweight. Densities ~0.9-1.5 g cm⁻³, far less dense than metals (Fe = 7.9, Al = 2.7). A plastic petrol bottle weighs ~5 % of an equivalent steel can.
Cheap. Produced from oil-derived alkene feedstocks at ~£1-2/kg for commodity polymers, vs ~£10/kg for aluminium.
Easily moulded. Thermoplastics (PE, PP, PVC, PS, PMMA) can be melted, re-shaped and re-melted indefinitely — this is the key to mechanical recycling. Thermosets (Bakelite, epoxy) cross-link on curing and cannot be re-melted; they must be ground and used as filler.
Electrically insulating. Non-polar backbones with no free electrons make excellent cable sheathing.
Easy to colour, print on, foam, fibre-spin. Polymer processing is extraordinarily versatile.

5. The disposal problem

The very inertness that makes polymers useful also makes them an environmental problem. The C–C and C–H bonds of the backbone are biologically un-degradable — no microorganism produces enzymes that hydrolyse non-polar C–C bonds (enzymes preferentially attack C–O, C–N or C–halogen bonds, which are essentially absent in PE, PP, PVC). An unprotected polyethene bag persists for 500+ years in landfill. Production worldwide is ~400 million tonnes per year; recycling rates are typically <15 %. The remainder is landfilled (still the majority in most countries), incinerated (with or without energy recovery), or mismanaged — ending up as rivers-and-oceans waste, the "plastic soup" and microplastics crisis.

5.1 Landfill

Advantages: simple, technologically mature, cheap, contains the waste in one geographically-managed location. Modern lined landfill cells use HDPE liners and leachate collection.

Disadvantages: occupies land; polymers take centuries to break down; landfill leachate (acidic, biologically-rich solution percolating through the buried waste) can contaminate groundwater if liners fail; landfilled polymers represent embodied-energy losses (the crude oil could have been used as fuel or feedstock); UK landfill tax (~£100/tonne in 2026) makes this option increasingly uneconomic.

5.2 Incineration with energy recovery

Burning waste polymers releases heat energy, which can be used to generate electricity (energy-from-waste, EfW) and reduces waste volume by ~90 %.

Combustion equations (example: PVC):

$[\text{CH}_2\text{-CHCl}]_n + \tfrac{5n}{2}\text{O}_2 \rightarrow 2n\,\text{CO}_2 + n\,\text{H}_2\text{O} + n\,\text{HCl}$

Advantages:

Recovers useful heat / electrical energy.
Dramatically reduces volume of waste sent to landfill (90 % volume reduction).
Destroys pathogens and decomposes most toxic organic species.

Disadvantages:

Complete combustion of polymers releases CO₂, contributing to climate change.
Incomplete combustion releases CO, soot and unburned hydrocarbons.
PVC combustion releases HCl gas, which contributes to acid rain and corrodes incinerator stacks. Modern incinerators use alkaline scrubbers (NaOH or Ca(OH)₂ slurry) to neutralise HCl:

$\text{HCl(g)} + \text{NaOH(aq)} \rightarrow \text{NaCl(aq)} + \text{H}_2\text{O}$

$2\,\text{HCl(g)} + \text{Ca(OH)}_2\text{(aq)} \rightarrow \text{CaCl}_2\text{(aq)} + 2\,\text{H}_2\text{O}$

PVC combustion can also produce trace dioxins (chlorinated polycyclic aromatic compounds, persistent organic pollutants) under uncontrolled combustion conditions. Modern incinerators operate at >850 °C with 2 s residence time and rapid flue-gas quench to minimise dioxin formation.

5.3 Mechanical recycling

The standard "recycle" route: collect waste polymer, sort by type, clean, shred, melt and re-form into new products.

Difficulties:

Polymers must be sorted by type — different plastics are incompatible in the melt and form weak, brittle blends. Sorting is labour-intensive and error-prone. The resin-identification code (recycling triangle with 1–7 inside) helps but is voluntary.
Contaminants (food residues, inks, labels, additives) must be cleaned out; this is costly and not always complete.
Each melt-reprocess cycle causes some chain scission (heat / shear degradation), shortening the chains and degrading the mechanical properties. Recycled PE typically goes into lower-grade applications (refuse sacks rather than food packaging).
Mixed plastics that cannot be sorted are often downcycled into low-value products: park benches, traffic cones, building blocks.

Despite these difficulties, mechanical recycling avoids landfill, saves the crude oil needed for virgin polymer, and reduces greenhouse-gas emissions. PET (drinks bottles) has the highest UK recycling rate (~70 %) because the bottles are highly homogeneous and the colour-coded; PVC and mixed plastics are <30 %.

5.4 Feedstock (chemical) recycling

In feedstock recycling, waste polymers are chemically cracked back to their monomers or small-molecule feedstocks, which can be re-polymerised to virgin-quality polymer or used as fuels. Two main routes:

Pyrolysis: heating polymer waste in absence of oxygen to crack it into oils and gases.
Solvolysis / glycolysis: specific chemical depolymerisation (mostly used for PET, breaking ester bonds back to terephthalic acid and ethylene glycol).

Feedstock recycling avoids the quality-degradation of mechanical recycling and can handle mixed waste streams, but requires significant energy input and is currently more expensive than virgin-polymer production. Modern catalytic-pyrolysis plants are scaling up rapidly post-2020.

5.5 Biodegradable and photodegradable polymers

An alternative is to design polymers that can be broken down by environmental processes:

Biodegradable polymers are broken down by microbial enzymes. Examples: poly(lactic acid) (PLA) — made from corn starch via fermentation to lactic acid then condensation polymerisation; poly(hydroxyalkanoates) (PHA) — made by bacterial fermentation of glucose. These are condensation polymers, not addition polymers — they contain ester bonds in the backbone that enzymes can hydrolyse.
Compostable polymers are biodegradable under controlled composting conditions (warm, moist, aerobic) — they break down into CO₂, water and biomass within 12 weeks (EN 13432 standard).
Photodegradable polymers ("oxo-degradable plastics") have additives (e.g., transition-metal stearates) that catalyse C–C bond cleavage under UV light. The plastic fragments rather than truly disappearing — controversial because microplastic pollution worsens.
Starch-blended polyethene: starch granules are mixed into PE film; soil microbes digest the starch, the PE fragments. Same caveat as photodegradable PE — micro-fragmentation rather than mineralisation.

graph TD
  A[Waste plastic] --> B{Disposal option}
  B --> C[Landfill<br/>cheap but non-biodegradable<br/>persists centuries<br/>UK tax ~£100/t]
  B --> D[Incineration with energy recovery<br/>~90 percent volume reduction<br/>releases CO2; HCl from PVC<br/>alkaline scrubbing needed]
  B --> E[Mechanical recycling<br/>sort by type clean remelt<br/>quality degrades each cycle]
  B --> F[Feedstock recycling<br/>pyrolysis or solvolysis<br/>back to monomers<br/>energy-intensive]
  B --> G[Biodegradable alternative<br/>PLA PHA<br/>condensation polymers<br/>compostable]

Lesson 10: Addition Polymerisation and Polymer Disposal