Types of Vinyl Polymerization MethodAdvantagesDisadvantages Bulk (Neat)Simple equipment Rapid reaction Pure polymer isolated Heat buildup Gel effect Branched or crosslinked product SolutionGood mixing Ready for application Lower mol. Wt. Low R poly Solvent Recovery Suspension (Pearl) Low viscosity Direct bead formation Removal of additives EmulsionHigh R poly Low Temperatures High Mol. Wt. High surface area latex Removal of additives Coagulation needed Latex stability Inverse EmulsionWater in oil latex formed Inversion promotes dissolution in water

Fate of Initiator Radicals Radical reactions Recombination in solvent cage Chain initiation, R i = 2 f kd [I] Efficiency factor, f = Reaction with polymer radicals (k t )--primary termination Reaction with initiator (MIH) Hydrogen abstraction from polymer chains (chain transfer to polymer) Reaction with solvent or inhibitor

Radical Initiators Azo Initiators

Decomposition of Azo Initiators 2- bond cleavage to liberate nitrogen Cage Recombination ---Side reaction- irreversible coupling of succinonitrile radicals, efficiency decreases at high conversion

Peroxy Initiators High temperature initiators Moderate temperature initiators

Peroxy Initiators Low temperature initiators, C 3-bond cleavage process?  -cleavage to carbon centered radical

Redox Initiation 0-5 C in water 0-5 C in organic/aqueous phase

Decomposition of Peroxy Initiators 1-bond cleavage process If R = aryl, acyl radical initiates = alkyl, CO2 lost before initiation occurs

Reaction of benzoyloxy radicals with styrene

Chain Transfer Hydrogen transfer to growing polymer chain Reinitiation of growing chain using transferred radical

Effect of Chain Transfer on R p and DP Relative rate constants Type of effectEffect on R p Effect on DP k p.>> k tr k a ~ k p NormalNoneDecrease k p << k tr k a ~ k p TelomerizationNoneLarge decrease k p >> k tr k a < k p RetardationDecrease k p << k tr k a << k p InhibitionLarge decrease

Control by Chain Transfer Chain transfer depends upon nature and concentration of chain transfer agent. Where Ctr is the chain transfer constant that includes the rate constants for hydrogen abstraction and re-initiation of a new chain Ctr is specific for a given monomer at a given temperature

Common Chain Transfer Agents Transfer agentStyrene, Ctr x 10 4 Vinyl Acetate Ctr x 10 4 Toluene Di-n- butyldisulfide 2410,000 Carbon tetrabromide 22,000390,000 n-butyl mercaptan 210,000480,000

Additional Chain Transfer Processes Chain transfer to monomer, Ctr x 10 4 –Ethylene, ; Styrene, Chain transfer to polymer--branching Polyethylene Vinyl acetate Vinyl chloride Vinyl acetate, Vinyl chloride, Allyl systems,

Transfer to Polymer Polyethylene branching Long branches Short branches

Inhibition of Radical Polymerization Must stop oxygen- and carbon centered radicals Carbon centered radicals stopped by addition Oxygen centered radicals stopped by hydrogen transfer Radicals generated by auto oxidation May be metal catalyzed

Critical Inhibitor Properties An inhibitor should not add to, abstract from or otherwise reach with monomer or solvent Inhibitors should not undergo self reaction or unimolecular decomposition Inhibitors must react rapidly with the propagating and/or initiator derived radicals to terminate polymer chains

Trapping Oxygen Centered Radicals

Trapping carbon centered radicals Carbon centered radicals stopped by addition to oxygen or carbon

Typical Inhibitors

Stable Radical Inhibitors