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.
What are the benefits of living polymerisations?
Pre-determinable degree of polymerisation, narrow molecular weight distributions, preservation of end group functionalities.
What are the limits of living polymerisation and what technique can we use to overcome them?
Limited scope due to functional group tolerances, rigorous purification of reagents and solvents and inert conditions without labile protons required. Use pseudo-living polymerisations instead.
Why do we want to use pseudo-living polymerisations?
Desire to emulate fine control found in living methods, ensuring broader functional group tolerance.
What are the main two types of polymer synthesis?
Chain growth and step growth
How does step growth polymerisation work?
Successive reactions between mutually reactive multifunctional monomers, can be condensation or addition polymerisation.
Single monomer approach: monomer contains both reactive functional groups
Double monomer approach: Bifunctional monomers combined.
What are the problems with step-growth polymerisation?
Slower than chain growth, reaction proceeds for very long time for high degrees of polymerisation, reliance on trimers and tetramers coming together
What are the benefits of step-growth methods?
No initiator required and no termination reactions
What is the mechanism for classically-catalysed ROP?
Form a classical initation system with tin complex and alcohol to give the initiation complex. Formation of propagating chain by condensation of next monomer unit to tin. Driven by relief of ring strain.
Exact mechanism: L-Sn attacks carbonyl, oxygen complexes to tin. Reformation of carbonyl bond and ring opening by recomplexation to tin. Repeat.
What is the mechanism for organocatalysed ROP?
Mechanistic debate, but likely via monomer-activated mechanism. Form activated intermediate by covalent linkage to catalyst, ring open this species, alcohol attacks to form polymer chain.
