Designer Polymers Flashcards
Describe the conductance of polymers.
Most organic polymers are insulators, but conjugated polymers such as trans-polyacetylene can be made to be conducts with the strength of metals when treated with dopants.
Describe and use a diagram to explain the basis of band theory for conjugated polymers.
What cannot an undoped polymer conduct?
When you increase the conjugation of a molecule, the pi orbitals change in energy where some will increase and some will decrease in energy. This reduces the gap between the antibonding and bonding orbitals.
The electrons must move into a space. Before doping the only spaces are in the LUMO which requires crossing the band gap which is unfavourable and will just end with recombination into the valance band.
Describe the methods of doping.
Doping is where electrons are either added to the conduction band or removed from the the valance band.
Reduction - putting electrons into the conduction band, produces n-type (negative) polymers. This is done with a reduction agent such as Na or NaBH4.
Oxidation - removing an electron from the valance band creating a ‘hole’, produces a p-type (positive) polymer. This done with an oxidising agent such as I2.
Give the two ways of synthesising polyacetylene.
Reaction of ethyne in the presence of Ti and Al complexes will form the cis polymer which will convert to the trans polymer with heat. Problems with this method is that the product is insoluble and cannot be melted so it is very hard to use in a useful way.
The Durham precursor route involves the reaction of a tri-ring system. ROMP occurs with a four membered ring to form the precursor polymer. Then reverse diels-alder reaction removes a benzene. The polymer can then be converted to the trans-polymer. This is driven by the aromaticity and can be manipulated by changing the groups on the ring.
Describe the alternate bond model of polymer conductivity and simplifcations that it makes.
Neutral polymers don’t conduct as imperfections in the chain lead to spare radicals in the chain. These stop the bonds alternating to that carbon, interrupting the HOMO and LUMO band gaps. The radical is called a neutral soliton, when doped it becomes either a positive or negative soliton. The charges are free to move along the chain, just not the radical.
This model is simplified as for bulk conductivity the charge will have to jump across polymer chains.
Describe the conduction and synthesis of polyphenylene.
What happens to the band gaps when the molecule is doped?
The conduction is from para-dibromobenzene and Mg, forming the Grignard. It can also be formed from a diester diene to form a precursor polymer.
The ground state has set aromaticity which has to be broken to alternate its double bonds. After doping the polymer conducts via two fragments, an unstable radical with broken aromaticity, the quinonoid, and the stable benzenoid.
Either reducing or oxidising forms intermediate bands in the band gap. The valance and conduction bands become lower and higher in energy respecitvely.
Describe the structure and properties of polypyrrole.
Pyrrole rings joined at the C2 postition. They form p-type conductors via electrochemical oxidation. They are stable in air and conduct up to 570 K. The pyrrole solution is polymerised onto the anode forming a film.
The counter ion of the polymer and the group attached to the nitrogen can vary its conductivity. Poor packing is due to large R groups and decrease conductivity.
Describe the synthesis of polythiophene, what adaptations are made and the resulting regiochemistry. How can this regiochemistry be controlled?
Thiophene with Br or I at C2 and C5 is reacted with Mg or Zn with a nickel (0) catalyst. This has increased conjugation than polypyrrole as it has less aromatic stability.
To increase solubility and processability, alkyl groups are introduced on C3 and C4. This creates 3 different types of links between thiophene links; 2,5’ Head to Tail, 2,2’ Head to Head and 5,5’ Tail to Tail. The lowest energy form is 2,5’ Head to Tail as it has the best packing, this is seen by a red shift.
Regioregular polymers have a higher conductivity and can be introduced by introducing bromine specifically at one side (next to the R group) and using a very hindered catalyst. The regioregularity can be quantified by NMR analysis.
Describe the superstructure of regioregular polypyrrole and how it depends on temperature.
The chains align with the R groups overlapping with the groups from the other chains. This can be observed with atomic force microscopy. The polymer backbones are normally planar but when the temperature increases a blue shift can be seen as the chain twists. This is associated with a decrease in conjugation.
What are the conjugating polymers used for?
Plastic batteries in very weight dependant situations (space exploration, sailing). Stable rechargable batteries. Coating to remove staic charge on surgical instruments.
How can conducting polymers sense metal ions?
Metal ions drive self assembly. Small ions will form linear chains around regular ions by facilitating the entropy with ionic interatctions. This can be identified by increased conduction and a redshift in absorbtion.
If the metal ions are too large the structure will be unzipped until there is no conduction or colour (huge blueshift).
How can conjugating polymers be used in the detection of chemcial weapons?
Chemical weapons commonly have F- ions which can be detected by a red-shift by removing Si groups from a polymer which enhances conjugation.
Define and describe electroluminescence as well as electroluminescent devices. What are the requirements of OLED cells for commercial uses.
Electroluminescence is the emission of light due to injection of charges of opposite sign into a material. When an electric field is induced, electrons are injected into the LUMO and removed from the HOMO. The exciton then decay by radiative emission.
In devices these are OLEDs and must have one transparent electrode. The polymer is between two metal electrodes. The devices must have good emission colour purity, long term stability and energy efficient. They must also have good response time and a wide viewing angle.
Poly(p-pheylene vinylene) or PPV can be used in OLED cells, describe the effects of R groups that can be added to it.
+M groups, such as -OMe cause repulsion of electrons in the HOMO as there is greater electron density. This creates a smaller band gap and increases the wavelength of the emission, causing a red shift.
Describe the basic principles of an organovoltaic cell.
Energy absorbed is used to seperate charge. This is done by a conjugated polymer (with the correct band gap) being excited by vis/near UV light. Upon excitation, many conjugated polymers are good electron donors wich is coupled with a molecular electron acceptor. These charges are then seperated to nearby electrodes which are connected with a load to connect the circuit.
Draw a diagram of a photovoltaic cell and describe the roles of each layer.
P3HT is poly(3-hexylthiophene) and will be excited when irradiated by light. Recombination is prevented by PCBM, fullerene-derived polymer, as it is very easily reduced, accepting up to six electrons as it has low energy LUMOs to accept electrons to. The charge is then transferred to the Al and then is carried back to the ITO. This current drives the electrical circuit.
Describe the challenges and recent advances of photovoltaic technology.
Challenges are to optimise:
- Metal electrodes to maximise capture of charges
- The donor-acceptor pairs with appropriate max absorption wavelength
- Mobility of charge carriers to promote seperatation
Advances include polymers with a high absorption coefficient and a broad absorption wavelength to improve efficiency, alternating accepting and donating polymer units for a lower band gap and changing polymer mixtures to maximise absorption across a broad wavelength.
Define an oligomer and their properties. How they compare to polymers in terms of conjugation?
An oligomer is a short chain of monomers with a clearly defined number of repeating units. They have better solubility and processability than polymers. They can act as an molecular wire.
Oligomers have an effective conjugation length (ECL) at which their properties become ‘polymer-like’. The oligomer conjugation only changes the properties up to a certain length.
ECL affects the colour of the material, the max absorption wavelength will change as conjugation changes, up to a certain point. It also affects redox potential and HOMO-LUMO band gap.
Define ECL and describe how it is measured. Describe the challenges with measuring some oligomers.
ECL (effective conjugation length) is the number of repeat units in a π conjugated polymer that are required to have size independant optical and redox properties.
To measure it, plot a graph of a chain property vs chain length and find the number of repeat units where the property becomes constant. Note that as the chain length increases, imperfections become likely which can decrease conjugation.
When only short oligomers can be made and the properties don’t saturate, we must interpolate to find the ECL. Use the polymer to measure the longest wavelength absorption energy, then using the values of this for the short chains, predict the n number when the short chains match the polymer.
For 7 monomers with 3 termini (AB2) where A can only form bonds with B, draw the most extreme branching and linear morphologies, labelling each unit as branched, terminal or linear.
Define the degree of branching and give the value for each of the structures.
What drives the degree of branching?
Degree of branching (DoB) = (B + T)/(B + T + L)
Where B is a branched unit (2 B units have reacted with an A, A can be reacted or unreacted), T is a terminal unit (No B units are reacted) and L is a linear unit (one B unit has reacted). The amounts of each type of unit can be measured by NMR analysis.
For linear molecules, as chain length increases, DoB decreases.
If B is very bulky, linear morphologies may be favoured. When A and B react, the electronic structure of the unit may change, affecting how the second B reacts.
Why is branching in polymers useful to mix with linear polymers? What type of molecule can be used for this function?
Linear polymers can tangle easily and therefore have a high. Branched polymers are used as viscosity lowering additives. The additives are often dendrimers, highly branched polymers that require novel synthesis. They are often monodisperse.
Dendrimers are a molecules which are perfectly branched from a core molecule, with each new generation layer of units also perfectly branched. Each new set of units represents a new generation with the core as G0. The number of units in each generation is given by B x 2n where B is the branch points of each unit and n is the generation number.
Describe the divergent approach to synthesising dendrimers. Give the requirements and problems.
React a core molecule (X3) with a AB2 unit where A reacts. Then convert B to a reactive unit, X, and react that with a new AB2 unit. The two steps can be described as the branching and reactivation reactions.
Number of reactions per growth step increases with each generation as does the number of monomers, the net mass increases per generation. The reactivation requires much more forcing conditions with each generation. The steric crowding in later generations makes the full conversion increasingly difficult and purification is required at each step.
Describe the convergent approach to synthesising dendrimers. Give the requirements and problems.
Build the branching units from the outside in. Once the desired size of branching units has been reached, attach three of them to a core unit.
The number of reactions per growth step is constant but a very high conversion is harder as the generation increases due to steric crowding. Detecting impurities and purification is required at each step and net mass will decrease with each generation.
Give the key comparisons between divergent and convergent synthesises of dendrimers.
Divergent:
- Number of reactions per growth step increases
- Structural defects form at higher generations
- Net mass increases with each growth step
Convergent:
- Number of reactions per growth step is constant
- Structural defects form at higher generations
- Net mass decreases with each growth step
