Polymers: structure and production of poly(ethene)
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Basic polymer formation from ethene
Addition polymerisation causes ethene monomers (CH2=CH2) to open their double bonds and link together into long chains. Each ethene unit becomes a repeating -CH2-CH2- group in the polymer backbone. Polymerisation requires an initiator or catalyst to start chain growth; the reaction type is chain-growth addition, not condensation, so no small molecules are lost during linking. Limiting factors include monomer purity and reaction control. Temperature, pressure and catalyst type influence chain length and branching, which determine the polymer’s density and mechanical properties.
Low density poly(ethene) (LDPE) production
LDPE forms when ethene polymerises under high pressure (typically several thousand atmospheres) and high temperature (about 200–300 °C) with free-radical initiators. Free radicals produce random chain branching because radicals react at many positions along growing chains. Branching prevents chains from packing closely, reducing intermolecular van der Waals forces. Reduced packing lowers density and crystallinity, producing a soft, flexible polymer with a relatively low melting point. Limiting factors include degree of branching and control over reaction conditions. Higher branching increases amorphous regions and further lowers density.
High density poly(ethene) (HDPE) production
HDPE forms when ethene polymerises using specific catalysts (such as Ziegler–Natta or chromium-based catalysts) at lower pressures and temperatures compared with LDPE. Catalyst-controlled polymerisation produces long, linear chains with minimal branching. Linear chains pack closely and form more crystalline regions, increasing density, tensile strength and melting point. HDPE therefore appears harder, more rigid and more chemically resistant than LDPE. Limiting factors include catalyst choice and reaction conditions. Catalyst poisons or incorrect temperature reduce control over branching and lower crystallinity.
Thermosoftening (thermoplastic) polymers: structure and properties
Thermosoftening polymers consist of linear or lightly branched chains with weak intermolecular forces between chains. Heating increases chain mobility; chains slide past each other and the material softens and melts. Cooling reverses the effect and the polymer hardens again. Repeated heating and reshaping is possible because no strong covalent bonds link chains together. Limiting factors include the strength of intermolecular forces, chain length and amount of branching; stronger intermolecular attractions or crosslinking reduce thermosoftening behaviour.
Thermosetting polymers: structure and properties
Thermosetting polymers contain polymer chains held together by many strong covalent crosslinks forming a three-dimensional network. Crosslinking prevents chains from sliding when heated. Heating initially does not melt the material; excessive heating causes decomposition rather than remelting. The extensive covalent bonding makes thermosets hard, rigid and unable to be reshaped after curing. Limiting factors include crosslink density and the nature of covalent bonds. Higher crosslink density increases rigidity and thermal stability but prevents recycling by melting.
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