【Basic Knowledge of Reaction Mechanisms for the “Polymer” Major - 002】Basic Knowledge of the Reaction Mechanism of Polybutadiene Styrene Rubber

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Production principle of styrene-butadiene rubber: 1 Main raw materials: ①1,3-Butadiene. The structural formula of 1,3-butadiene is: CH2=CH–CH=CH2. 1,3-Butadiene is the simplest conjugated diene. It is a colorless gas at room temperature and pressure, with a relative molecular mass of 54.09. Relative density 0.6211, melting point -108.9°C, boiling point -4.5°C. It has a distinctive odor, is narcotic, and is particularly irritating to the mucous membranes. It liquefies easily and is soluble in organic solvents. It is highly reactive and prone to self-polymerization; therefore, tert-butylhydroquinone is added as a polymerization inhibitor during storage and transportation. 1,3-Butadiene is primarily obtained by the dehydrogenation of butane or butylene, or from the separation of C4 fractions. ②Styrene is a colorless or slightly yellow oily liquid with a distinctive odor. Its melting point is -306°C, its density is 900 kg/m3, and its boiling point is 145–146°C. Styrene is insoluble in water, but soluble in organic solvents such as ethanol, ether, propane, and carbon disulfide. The purity of polymer-grade styrene should be above 99.5%. 2. Synthesis principle ① Polymerization principle of emulsion-polymerized styrene-butadiene rubber: Butadiene and styrene undergo polymerization in an emulsion via a free-radical copolymerization mechanism. In typical low-temperature emulsion polymerized copolymers, cis constitutes about 9.5%, trans about 55%, and vinyl about 12% of the macromolecular chains. If high-temperature emulsion polymerization is used, in the resulting polymer, approximately 16.6% of the molecules have a cis configuration, about 46.3% have a trans configuration, and about 13.7% contain vinyl groups. ②Detailed production mechanism — The role of various components in the formula and related chemical reactions I: Chain initiation reaction. If RO. represents the primary radical, and M1 represents the monomer butadiene while M2 represents the monomer styrene, then the reactions that form the monomer radicals can be expressed as follows: RO. + M1 → ROM1.; RO. + M2 → ROM2. It can be seen from the chain initiation reaction that as the initiator decomposes, the concentration of OH- ions in the system increases, thereby raising the pH value of the system. The reaction between OH- and Fe2+ in the system results in the formation of Fe(OH)2 precipitate: Fe2+ + 2 OH- == Fe(OH)2 ↓. (a) Use of the chelating agent EDTA: To prevent the formation of this Fe(OH)2 precipitate, EDTA disodium salt is used industrially as a chelating agent; it forms water-soluble chelates with Fe2+. The water-soluble chelates formed between EDTA disodium salt and Fe2+ have low dissociation degrees and remain stable under both alkaline and acidic conditions, allowing Fe2+ to be retained for an extended period without the formation of Fe(OH)2 precipitate. (b) Use of pigment: As can be seen from the chain-initiated reaction, Fe2+ is oxidized to Fe3+, which is brown in color; if its concentration is high, it will affect the color of styrene-butadiene rubber. To reduce the concentration of Fe3+, calcium hypochlorite (dihydrate of formaldehyde and sodium bisulfite, CH2O.NaHSO3.2H2O) is used industrially as a secondary reducing agent to reduce Fe3+ to Fe2+. 4 Fe3+ + 2CH2O.NaHSO3.2H2O → 4 Fe2+ + 2HCOOH + Na2SO4 + H2SO4 + 8H+ Since the secondary reducing agent, chalk, is consumed, the amount of ferrous sulfate required is significantly reduced. (c) Deoxidizer – The use of sodium dithionite (sodium dithiosulfate dihydrate, Na2S2O4.2H2O) as a deoxidizer; it can react with dissolved oxygen in water: 2Na2S2O4.2H2O + O2 + 2H2O → 2 Na2SO4 + 2 H2SO4 + 8 H+. Dissolved oxygen in water acts as an inhibitor of polymerization at low temperatures, and the addition of sodium dithionite ensures that the polymerization reaction proceeds properly. II: The simplified formula for the chain-growth polymerization reaction is as follows: ROM1 + M1 → ROM1M1; ROM1 + M2 → ROM1M2; ROM2 + M1 → ROM2M1; ROM2 + M2 → ROM2M2. III: Chain termination reaction – When the conversion rate (or Mooney viscosity) reaches the desired level, a terminator such as sodium dimethyldithiocarbamate is added. This terminator reacts with the chain radicals, thereby eliminating their reactivity. Role of the terminator: Sodium methyl dithiocarbamate is an effective terminator, but polymerization still occurs during the monomer recovery process; therefore, polysulfides, sodium hypochlorite, and polyethylene polyamine are added. Polysulfides have a reducing effect; they can react with residual peroxides to eliminate the initiating effect of these peroxides left over from the recycling process, thereby preventing the formation of explosive polymers.