Lecture Twelve - Synthetic polymers Flashcards
What is a monomer and a polymer?
Monomer = Simplest non-redundant unit from which a polymer is synthesised. Smallest repeating unit.
Polymer = Long chain molecules synthesised by linking together single parts called monomers.
The most important characteristics:
Size - Meausred by average molecular weight.
Shape - Various architectures affect physical properties.
Where can polymers be found?
Nature polymers:
Proteins, cellulose, lignin, natural rubber.
Synthetic polymers:
Plastics.
Fibre.
Elastomer - rubber.
How does notation and nomenclature work for polymers?
The monomer in a polymer is shown by putting brackets around the smallest repeating unit.
Nomenclature: Poly (repeat unit); poly (monomer name).
n = the average degree of polymerisation.
What is chain growth polymerisation?
Mark (1950) - Step reactions and chain reactions.
Involves growth generally from one side of the chain.
Technique where unsaturated monomer molecules add onto the active site of a growing polymer chain one at a time without the loss of atoms.
From the perspective of the chemical industry, chain growth polymerisation is the single most important reaction of alkenes.
What is step growth polymerisation?
Carothers (1929): Condensation and addition.
Refers to a type of polymerization mechanism in which bi-functional or multifunctional monomers react to form first dimers, then trimers, longer oligomers and eventually long chain polymers.
Can add monomers to the growing chain from both ends.
What is redical polymerization?
Step one - Chain initiation:
A step in a radical chain reaction characterized by the formation of radicals from non-radical compounds.
Step two - Chain propagation:
Reaction of a radical and a molecule (monomer) to give a new radical.
Chain length, n: the number of times the cycle of chain propagation steps repeats in a chain reaction.
Step three - Chain termination:
A step in a radical chain mechanism that involves destruction of radicals one type of chain termination is rafical coupling or ‘combination.’ Termination can also occur by ‘disproportionation.’
What is low density polyethylene?
Low density polyethylene (LDPE):
Soft, tough polymers.
Chains are highly branched due to chain transfer reactions.
Branched chains do not pack efficiently - LDPE is largely amophous and transparent.
Approx. 65% is fabricated into films.
Chain transfer = Reactivity of an end group is transferred from one chain to another, or from one position on a chain to another position on the same chain.
What is ziegler-natta polymerization?
Ziegler-Natta chain-growth polymerization is an alternative method that does not involves radicals.
Ziegler-Natta catalysts are heterogeneous materials composed of:
A MgCl2 support.
A group 4B transition metal halide such as TiCl4.
An alkylaluminium compound such as diethylaluminium chloride - Al(CH2CH3)2Cl.
What is high density polyethylene?
Polyethylene from Ziegler-Natta systems is termed high density polyethylene (HDPE).
Less chain branching than LDPE.
Higher degree of crystallinity (up to 90%, Tm>125 degrees C).
Higher density.
Higher melting points.
Approx. 45% of all HDPE is blow molded into containers.
What is the importance of molecular weight?
Molecular weight of a polymer:
n = Average degree of polymerization.
Molecular weight of ethylene.
When n=1000, molecular weight of ethylene = 28 x 1000 = 28000.
Polymers are almost always of varying lengths, we only can determine average molecular weight of polymers.
Polymer properties by molecular weight:
Two very different substances (e.g. paraffin wax and polyethylene) can also be chemically identicle, but their different properties are due to differences in average chain length (molecular weight).
What is syndiotactic and isotactic
Syndiotactic - (Of a polymer or polymeric structure) in which the repeating units have alternating stereochemical configurations.
Isotactic - Denoting a polymer in which all the repeating units have the samestereochemical configuration.
