Polymerisation processes and polyethylene production

Chain-growth vs step-growth polymerisation, process types, molecular-weight averages, and LDPE, HDPE and LLDPE production with heat-removal limits.

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Why it matters

Polyethylene is the world's most-produced plastic, used in films, pipes, containers and cable insulation. How it is made — the mechanism, catalyst, pressure and reactor type — fixes its branching, density and molecular weight, and therefore its properties. Polymerisation reactors are also some of the most heat-transfer-limited reactors in the industry, which makes them a favourite for process questions.

Key ideas

Two families of mechanism.

  • Chain-growth (addition) polymerisation: an active centre (free radical, cation, anion or metal–carbon bond on a catalyst) adds monomer units one by one; high molecular weight chains appear early, and no small molecule is released. Examples: polyethylene, polypropylene, PVC, polystyrene.
  • Step-growth (condensation) polymerisation: functional groups react pairwise, often releasing water or methanol; high molecular weight appears only at very high conversion. Examples: polyesters, nylons (next topic).

Free-radical polymerisation has initiation (initiator I → 2R•, R• + M), propagation (R–Mₙ• + M), termination (combination or disproportionation) and chain transfer (to monomer, solvent or a deliberately added agent, which lowers molecular weight). With the steady-state assumption for radicals, the rate is proportional to [M] and to [I]^0.5.

Polymerisation process types. Bulk (pure monomer; viscosity and heat removal are hard), solution (solvent helps heat removal, must be recovered), suspension (monomer droplets in water with initiator inside; beads, e.g. PVC), emulsion (monomer in surfactant micelles with water-soluble initiator; high molecular weight at high rate, e.g. SBR latex), slurry (polymer precipitates in a diluent) and gas phase (fluidised bed).

Molecular weight. A polymer has a distribution of chain lengths. Number-average Mₙ weights each chain equally; weight-average M_w weights by mass. The polydispersity index PDI = M_w / Mₙ ≥ 1 (about 2 for many catalytic polymers, much broader for LDPE). Degree of polymerisation DP = Mₙ / M₀ (M₀ = repeat-unit mass).

Polyethylene grades and processes.

  • LDPE (density about 0.910–0.940 g/cm³): free-radical polymerisation at very high pressure (about 1000–3000 bar) and 150–300 °C, in tubular or stirred autoclave reactors, initiated by oxygen or organic peroxides. Intramolecular "back-biting" gives short branches and transfer to polymer gives long branches, so crystallinity and density are low. Per-pass conversion is only about 15–35 % because of heat removal; unreacted ethylene is recycled through high- and low-pressure separators.
  • HDPE (about 0.941–0.965 g/cm³): coordination polymerisation on Ziegler–Natta (TiCl₄ + aluminium alkyl cocatalyst) or Phillips (chromium oxide on silica) catalysts at low pressure (about 10–40 bar) and 70–110 °C, in slurry (hexane or isobutane diluent, loop or stirred reactors) or gas-phase fluidised beds. Nearly linear chains crystallise well, giving stiffness and strength.
  • LLDPE (about 0.915–0.940 g/cm³): ethylene copolymerised with 1-butene, 1-hexene or 1-octene on Ziegler–Natta or metallocene catalysts in gas-phase or solution processes. The comonomer gives uniform short branches, so the polymer is linear but low density, with better toughness and puncture resistance than LDPE.
  • Hydrogen is the chain-transfer agent used to control molecular weight (melt flow index) with coordination catalysts.
  • Metallocene (single-site) catalysts give narrow molecular weight distribution (PDI about 2) and uniform comonomer placement.

Heat removal. Ethylene polymerisation releases about 95 kJ/mol (about 3.4 MJ/kg). In a gas-phase fluidised bed, the cycle gas (sometimes partly condensed, "condensed mode") carries heat to external coolers; in slurry loops, jacketed walls remove it; in LDPE tubes, the per-pass conversion is limited by the allowed temperature rise. Runaway leads to ethylene decomposition, a serious hazard in LDPE plants.

Formulas

Mₙ = Σ Nᵢ Mᵢ / Σ Nᵢ = 1 / Σ (wᵢ / Mᵢ) M_w = Σ Nᵢ Mᵢ² / Σ Nᵢ Mᵢ = Σ wᵢ Mᵢ

  • Nᵢ: number of moles of chains of mass Mᵢ (g/mol), wᵢ: mass fraction.

PDI = M_w / Mₙ, DP = Mₙ / M₀

R_p = k_p [M] (f k_d [I] / k_t)^0.5 (free radical, steady state)

  • k_p, k_d, k_t: propagation, initiator decomposition and termination rate constants; f: initiator efficiency; concentrations in mol/L.

ΔT_ad = X · (−ΔH_p) / (M_m · c_p)

  • X: fractional conversion, ΔH_p: heat of polymerisation (J/mol), M_m: monomer molar mass (kg/mol), c_p: specific heat of reacting mixture (J/kg·K). Estimates the adiabatic temperature rise.

Worked examples

Example 1 (standard): molecular weight averages. A polyethylene sample contains 20 % by mass of chains of 20 000 g/mol, 50 % of 50 000 g/mol and 30 % of 100 000 g/mol. Find Mₙ, M_w, PDI and DP.

  1. Mₙ = 1 / (0.2/20 000 + 0.5/50 000 + 0.3/100 000) = 1 / (1.0 + 1.0 + 0.3) × 10⁻⁵ = 43 478 g/mol.
  2. M_w = 0.2 × 20 000 + 0.5 × 50 000 + 0.3 × 100 000 = 59 000 g/mol.
  3. PDI = 59 000 / 43 478 = 1.357.
  4. DP = 43 478 / 28.05 = 1550.

Answer: Mₙ ≈ 43 500 g/mol, M_w = 59 000 g/mol, PDI ≈ 1.36, DP ≈ 1550.

Example 2 (GATE level): why LDPE conversion is low. A tubular LDPE reactor receives 50 t/h ethylene. Take ΔH_p = −95 kJ/mol and c_p = 2.5 kJ/kg·K. Find (a) the heat released at 30 % conversion and (b) the adiabatic temperature rise per 1 % conversion and at 30 %.

  1. Heat per kg polymer = 95 000 J/mol / 0.028 05 kg/mol = 3.387 MJ/kg.
  2. Polymer = 0.30 × 50 000 = 15 000 kg/h; heat = 15 000 × 3.387 MJ / 3600 s = 14.1 MW.
  3. ΔT_ad per 1 % = 0.01 × 3387 kJ/kg / 2.5 = 13.5 K.
  4. At 30 %: ΔT_ad = 30 × 13.5 = 406 K.

Answer: (a) about 14.1 MW; (b) about 13.5 K per % conversion, about 406 K at 30 % — impossible without heavy cooling and multiple initiator injections, which is why per-pass conversion is held to roughly 15–35 %.

Common mistakes

  • Calling M_w / Mₙ the degree of polymerisation; it is the polydispersity index.
  • Saying LDPE uses a Ziegler–Natta catalyst; LDPE is free-radical at high pressure.
  • Thinking LLDPE is LDPE made differently; it is a linear copolymer with short comonomer branches.
  • Swapping density ranges: more branching means lower crystallinity and lower density.
  • Ignoring heat removal when judging per-pass conversion.

For GATE CH

Expect questions comparing LDPE, HDPE and LLDPE (mechanism, catalyst, pressure, branching, density), polymerisation techniques (bulk, solution, suspension, emulsion), the role of hydrogen and comonomers, and chain-growth vs step-growth. Numericals: Mₙ, M_w, PDI and DP from distributions; heat release and adiabatic temperature rise; simple free-radical rate ratios (rate ∝ [I]^0.5).

Quick check

  1. What catalyst system is used for HDPE?
  2. What is the role of hydrogen in coordination polymerisation?
  3. If initiator concentration is quadrupled, how does the free-radical polymerisation rate change?
  4. Which PE grade has long-chain branching?

Answers: 1. Ziegler–Natta (TiCl₄ + aluminium alkyl) or Phillips chromium catalysts; 2. chain-transfer agent to control molecular weight; 3. it doubles (rate ∝ [I]^0.5); 4. LDPE.

Interview questions

All Chemical Technology interview questions

Try answering each one aloud before you open it.

  1. 1.What is polymerisation and how does it relate to polyethylene production?Concept

    Polymerisation is a chemical process where small molecules called monomers join together to form a large chain-like molecule called a polymer. In the case of polyethylene production, the monomer ethylene (C2H4) undergoes polymerisation to form polyethylene, a widely used plastic.

  2. 2.Explain the difference between addition polymerisation and condensation polymerisation.Concept

    Addition polymerisation involves the joining of monomers without the loss of any small molecules, typically involving unsaturated monomers like ethylene. Condensation polymerisation, on the other hand, involves the joining of monomers with the simultaneous elimination of small molecules such as water or methanol. Polyethylene is produced through addition polymerisation.

  3. 3.What are the main types of polyethylene and how do they differ?Concept

    The main types of polyethylene are Low-Density Polyethylene (LDPE), High-Density Polyethylene (HDPE), and Linear Low-Density Polyethylene (LLDPE). LDPE has a highly branched structure, making it less dense and more flexible. HDPE has a linear structure with minimal branching, resulting in higher density and strength. LLDPE is similar to LDPE but with a linear structure and short branches, offering a balance of strength and flexibility.

  4. 4.Why is Ziegler-Natta catalyst used in the production of polyethylene?Application

    Ziegler-Natta catalysts are used in the production of polyethylene because they allow for the control of polymer structure, enabling the production of high-density polyethylene (HDPE) with a linear structure. These catalysts facilitate the polymerisation of ethylene at relatively low pressures and temperatures, making the process more efficient and cost-effective.

  5. 5.What happens if the temperature is too high during the polymerisation of ethylene?Application

    If the temperature is too high during the polymerisation of ethylene, it can lead to uncontrolled reactions, resulting in a lower molecular weight polymer and potentially causing degradation of the polymer. This can affect the mechanical properties of the polyethylene, making it less suitable for certain applications.

  6. 6.How does the branching of polyethylene affect its properties?Application

    The branching of polyethylene affects its density, crystallinity, and mechanical properties. More branching, as seen in LDPE, results in lower density and crystallinity, making the material more flexible and less rigid. Less branching, as in HDPE, leads to higher density and crystallinity, resulting in a stronger and more rigid material.

  7. 7.Calculate the degree of polymerisation if the molecular weight of polyethylene is 280,000 g/mol and the molecular weight of ethylene is 28 g/mol.Numerical

    The degree of polymerisation (n) is calculated by dividing the molecular weight of the polymer by the molecular weight of the monomer. n = 280,000 g/mol / 28 g/mol = 10,000. Therefore, the degree of polymerisation is 10,000.

  8. 8.What are the environmental concerns associated with polyethylene production and disposal?Application

    Environmental concerns associated with polyethylene production include the release of greenhouse gases and other pollutants during manufacturing. Disposal issues arise because polyethylene is not biodegradable, leading to accumulation in landfills and oceans. Recycling and developing biodegradable alternatives are important strategies to mitigate these concerns.

  9. 9.Explain the role of pressure in the polymerisation process of ethylene.Application

    For LDPE, very high pressure (about 1000-3000 bar) is essential: it makes supercritical ethylene dense enough for fast free-radical propagation and keeps the polymer dissolved, but it brings large compression costs and the hazard of ethylene decomposition. Coordination catalysts (Ziegler-Natta, Phillips, metallocene) are active enough to polymerise ethylene at about 10-40 bar in slurry or gas-phase reactors, giving HDPE and LLDPE. In those processes ethylene partial pressure mainly sets reaction rate and productivity.

  10. 10.If a polyethylene sample has a density of 0.95 g/cm³, what type of polyethylene is it likely to be?Application

    A polyethylene sample with a density of 0.95 g/cm³ is likely to be High-Density Polyethylene (HDPE). HDPE typically has a density range of 0.94 to 0.97 g/cm³, indicating a linear structure with minimal branching, resulting in higher density and strength.

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