Polymerization mechanism of EVA apparatus
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Polymerization mechanism of EVA devices: Ethylene is polymerized via free-radical polymerization under high temperature and pressure to produce LDPE products, while ethylene and vinyl acetate are copolymerized via free-radical polymerization under high temperature and pressure to produce EVA products. The polymeric monomers (ethylene, vinyl acetate) undergo free-radical polymerization under ultra-high pressure (260–280 MPa) and high temperature (210–305 °C), in the presence of peroxide initiators. The initiation temperature for this reaction is 140–160 °C. The entire reaction process involves the following 4 steps: The homopolymerization reaction is: n(CH2=CH2) → (-CH2-CH2-)n. The copolymerization reaction is: n(CH2=CH2) + m(VA) → (-CH2-CH2-)n-(VA)m. Chain initiation: The initiator decomposes to form primary radicals; this is an endothermic reaction: ROOR → RO• + •OR (primary radicals). Primary radicals react with the monomers to form monomer radicals; this is an exothermic reaction: RO• + CH2=CH2 → RO-CH2-CH2• (monomer radicals). Chain growth: The chain-growth reaction is exothermic; the heat of synthesis for polyethylene is approximately 3260 kJ/kg, while the polymerization heat for VA is about 1034 kJ/kg. R•+CH2=CH2→R-CH2-CH2• R-CH2-CH2•+ CH2=CH2→R-CH2-CH2-CH2-CH2• (polymerization). Since the copolymerization rates of ethylene and vinyl acetate under high temperature and pressure are approximately equal, the composition of the gas fed into the reactor is roughly the same as that of the polymer. Chain termination: When two polymer radicals react to form one or two polymer molecules, the activity of the polymer radicals is terminated. Coupling termination: Two free radicals form a macromolecule. R1-CH2-CH2•+•CH2-CH2-R2→R1-CH2-CH2-CH2-CH2-R2 (coupling). Termination by disproportionation: the two free radicals form two polymer molecules, one of which is saturated and the other is vinyl-unsaturated. R1-CH2-CH2•+•CH2-CH2-R2→R1-CH=CH2+CH3-CH2-R2 (disproportionation). Radical loss (β-scission): The radical decomposes to form a polymer molecule and a low-molecular-weight radical; the polymer contains unsaturated structures. Mm* → Mn* + Mm – The impurities (or oxygen) act as agents that deactivate free radicals: by reacting with these chain-like free radicals, they produce non-free radical products, thereby inactivating the free radicals. R1-CH2-CH2•+I (inhibitor) → R1-CH2-CH2I (inert substance). Note: The reaction with impurities is also a chain-transfer reaction, in which the free radical is transferred from the polymer chain to the impurity molecule, thereby terminating the growth of the polymer chain.
Chain transfer reactions interrupt the linear growth of polymer chains by transferring active radical sites from one end of the chain to another site. Two main types of chain transfer: intramolecular chain transfer and intermolecular chain transfer. • Intermolecular transfer occurs to the solvent, comonomer, or modifier (adjusting agent/polymerization regulator). • Intermolecular transfer can also occur to the polymer macromolecules, resulting in long-chain branching. • Intramolecular transfer takes place to a certain carbon atom within the active chain, leading to short-chain branching. Chain transfer reactions to the solvent, monomer, or modifier include transfer to saturated (such as butane) and unsaturated (such as isobutylene or propylene) modifiers, as well as to ethylene, impurities, and depolymerized molecules. Examples: (1) Chain transfer to ethylene – between the growing chain and ethylene monomer molecules: Mn-CH2-CH2* + CH2=CH2 → Mn-CH=CH2 + CH3-CH2*. (2) Chain transfer to modifiers – such as propylene and propanal, which are used in production to control the melt index. The chain transfer of these modifiers occurs through a hydrogen loss mechanism involving the modifier (chain transfer agent); propylene loses hydrogen atoms from the methyl carbon, while propanal primarily loses hydrogen atoms from the aldehyde carbon. Additionally, the methylene carbon in the propanal molecule can also lose hydrogen atoms. The active center can also be transferred to the solvent isododecane, which mainly loses hydrogen atoms from its tertiary carbon atoms. (3) Transfer to macromolecules – Long-chain branching determines a wide molecular weight distribution as well as their rheological behavior (i.e., solution viscosity, viscoelasticity). ① The tertiary or secondary carbon atoms of the macromolecules lose hydrogen and become activated: Mn-CH2-CH2-Mm + Mp-CH2-CH2* → Mn- *CH-CH2-Mm + Mp-CH2-CH3. ② Once the macromolecules are activated, chain growth continues, resulting in the formation of long branches. Due to mixing in the stirrer tank, long-chain branching reactions occur more frequently in autoclave reactors than in tubular reactors ； Long-chain branching reactions occur more frequently at high reaction temperatures ； Extruded coated polymers benefit from a high degree of LCB (long-chain branching reaction) achieved in autoclave reactors. In other words, products produced by the kettle method contain more long side chains than those produced by the tubular method. (4) Intramolecular chain transfer – short-chain branching: The density of homopolymer LDPE resins is primarily determined by the number of short-chain branches, a phenomenon known as the “backbiting reaction”. ①First backbiting reaction – formation of a butyl branch: Mn-(CH2)4–CH2* → Mn-*CH-(CH2)3-CH3. ② Further growth occurs. ③ Transfer to macromolecules – long-chain branching leads to a wide molecular weight distribution as well as specific rheological properties (i.e., solution viscosity, viscoelasticity). ① The tertiary or secondary carbon atoms of the macromolecules lose hydrogen atoms and become activated: Mn-CH2-CH2-Mm + Mp-CH2-CH2* → Mn- *CH-CH2-Mm + Mp-CH2-CH3. ② Once the macromolecules are activated, chain growth continues, resulting in the formation of long branches. Due to mixing in the stirred-tank reactor, long-chain branching reactions occur more frequently in high-pressure reactors than in tubular reactors ； Long-chain branching reactions occur more frequently at high reaction temperatures ； Extruded coated polymers benefit from a high degree of LCB (long-chain branching reaction) achieved in autoclave reactors. In other words, products produced by the kettle method contain more long side chains than those produced by the tubular method.
(4) Intramolecular chain transfer — The density of short-chain branched homopolymer LDPE resins is primarily determined by the number of short-chain branches, a phenomenon known as the “backbiting reaction”. ①First backbiting reaction – formation of butyl side chains: Mn-(CH2)4–CH2* → Mn-*CH-(CH2)3-CH3. ② Further growth occurs. ③ Second backbiting reaction – formation of two ethyl side chains. ④ Growth can continue on the radicals of the polymer containing those two ethyl side chains ； ⑤Growth can also continue on the radicals of the butyl-containing polymer formed during the first backbiting reaction. (5) The unsaturated structure of LDPE is formed by chain transfer and chain termination reactions during the polymerization process: disproportionation termination chain transfer to the monomer β-scission of secondary radicals. In LDPE, the total degree of unsaturation per 1000 carbon atoms is usually less than 0.5. There are three types of unsaturated structures in LDPE: Vinyl group: -CH=CH2- Vinylidene group: -C=CH2 Trans-2-vinylidene group: -CH=CH- (less common)
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