Polymers quiz 2

Peroxide traits as initiators

• reversible

• very efficient as an initiator (0.9-1)

• large solvent effect

• reactive with metals

• high probability of hydrogen abstraction

• can be explosive

Azos traits as initiators

• irreversible

• less efficient as an initiator (0.5-0.7)

• small solvent effect

• not reactive

• small probability of hydrogen abstraction

• can be toxic

benzoyl peroxide

• very common initiator

alkyl peroxides

Asymmetric peroxides

Azobisisobutyronitrile (AIBN)

N-Nitrosoacylanilides

benzenediazohydroxide

Propagation

• The radical on the chain-end react to form a new sigma bond and a new radical

• # of radicals does not change (unlike termination or initiation)

standard pathways for propagation

• rxns between monomer + the polymer chain-end

• results in chain growth(generally)

Chain-transfer pathways

• rxn between the chain end and structures other than the monomer alkene

• does not result in chain growth

Transfer to a monomer (chain-transfer pathways)

• Chain-end abstracts an atom instead of reacting w the alkene

• terminates the polymer and generates a new initiator

• occurs more readily at the beginning of a rxn

  enter mech

Transfer to solvent (chain-transfer pathway)

• chain-end abstracts an atom from the solvent, same outcome as trans fer to a monomer

• equally likely to occur at any point during the rxn

enter mech

transfer to polymer (chain-transfer pathway)

• chain-end abstracts an atom from a separate polymer backbone

• forms branched polymers

  enter ex mech

Ring formation

• intramolecular rxn between the polymer chain-end and it’s backbone

• does not result in chain growth

• usually requires non-conjugated dienes

Alkanes and Vinyl: ethers or amines(ring formation)

• does not add to chain length

Viynyl: Esters or carboxylic acids (ring formation)

Cyclic Dienes (ring formation)

Termination

• Elimination of the chain-end radical

• either combination of radicals

• or formation of a less reactive radical

Combination with chain-end (termination)

• two chain ends react, occurs towards the end of the rxn

Radical Disproportionation (termination)

• hydrogen abstraction terminated with the two chains, can happen at anytime during the rxn

combination with initiator (termination)

• occurs when homolytic cleavage is slow

Combination with impurities (termination)

• oxygen example

addition of terminators (termination)

• molecules that sequester radicals

• often used as stabilizers for monomer storage

addition of terminators, Diphenylpicrylhydrazyl (DPPH)

mech

addition of terminators, Butylated hydroxytoluene (BHT)

mech

addition of terminators, Quinones

mech

Monomer reactivity

Primarily effected by two properties

Electronics

Combination of inductive effects and resonance

Stabilizing effects

Higher stability = lower chain end reactivity

• rate of prop. Is lower

• Decreases alternative rxn pathways

regiochemistry

• dictates regularity

• only occurs in asymmetric alkenes

stereochemistry

• determines tacticity

• impacts physical properties

• results from monomer approach and confirmation of chain-end

polymer sterics impact approach

• monomer prefers to approach from less inhibited side

polymer sterics impact monomer direction

• large groups generally approach on opposite sides(syndio)

• if no preference, then atactic polymers form

conformation of chain end

• free roatation can alter stereochemical outcome, large groups hinder free rotation

• thermodynamics impact free rotation, smaller temp= less energy=less free roatation