Polymerisation techniques Flashcards
What is chain growth polymerisation?
Addition of monomer units to an activated or initated form of the monomer, three-step mechanism: initiation, propagation, termination.
What is free radical chain growth polymerisation?
Chain growth polymerisation with radical initiated form. Mechanism:
Initiation via free radical initiators by homolytic scission followed by addition of initiating radical to monomer unit
Propagation involves growth of polymer chain by sequential addition of monomer units to active radical
Termination via chain combination (dead polymer), disproportionation (alkene formation), intramolecular H abstraction (branched polymer formation - chain transfer)
What is an ionic polymerisation and what type of monomers is it useful for?
Polymerisation between monomers of the general type:
alkene - X, alkene - XY where X/Y stabilise active centre by induction or resonance effects
where the active chain is made active by formation of anion or cation
What monomers can undergo ionic polymerisations?
Cationic polymerisations: alkene with electron donating substituents, conjugated species or hyperconjugation possible
Anionic polymerisations: X must be EWG or able to delocalise charge
Essentially, monomer units which can stabilise a charge.
Give a brief description of the mechanism for cationic polymerisation.
Initation - electrophiles, generally Lewis acids with proton donor to react with monomer unit and generate cation
Propagation - sequential reaction with monomer units
Termination - H elimination to leave the end as an alkene.
How is the speed of the propagation reaction in cationic polymerisation controlled?
Propagation reaction very fast. Done at -78 degrees to try and control. Temperature control important as very exothermic reaction.
How do solvent and counterion effects impact the rate of cationic polymerisation?
Polar solvents with high dielectric constants will stabilise ionic species, increase charge separation and increase rate of polymerisation.
Large counterions are less strongly associated, increasing rate of polymerisation.
What are some of the problems with or considerations that need to be made with cationic polymerisation?
Low temperatures required to suppress side reactions (e.g. water leading to chain transfer) and to reduce heat transfer issues caused by rapid propagation
How is anionic polymerisation initiated and what type of initiators are used?
Electron transfer initiation with sodium naphthalide, initially form radical anions of styrene which couple to form dianion which is the starting chain. Alkyl lithium initiators via H-abstraction with alkane formation to produce anion.
What are the practical issues with anionic polymerisation?
Very sensitive to minute traces of impurities and most functional groups: inert conditions required, rigorously dried aprotic reagents and inert atmosphere required, limited functional group tolerance
What considerations need to be made with respect to solubility in anionic polymerisations?
Hydrocarbon solubility key as allowed production of man-made alternatives to natural rubber. BuLi initiator aggregates in hydrocarbon solvents leading to rapid initation resulting in low dispersity polymers (level of control)
What is polymer microstructure and what effect can it have on polymer properties?
Microstructure = stereochemistry
Key physical properties - glass transition temperature: above this temperature, chains flow over each other and below, rigid.
How can polymer microstructure be influenced?
Solvent and initiator. Polar solvents promote high 1,2-diene microstructure whereas non-polar hydrocarbon solvents promote high 1,4-diene microstructure. Alkyl lithium initiators also promote 1,4.
What are some effects that can impact the rate of anionic polymerisation?
Identity of alkyl group on alkyl lithium initiator. Aggregation can reduce rate of initiation.
What is a living polymerisation?
Class of chain growth polymerisations with no termination events, display first order kinetic behaviour.
