Organic-Inorganic Hybrids and Structural Colour

Question 1

TiCl₄/AlEt₃ can polymerise polypropene of high moleculear weight. Define and describe the tacticity of the product as well as the polydispersity.

Answer 1

Tacticity describes the stereochemistry of the product from the reaction which in this case is low, meaning it is not controlled well. The chains produced also have a high polydispersity.

Question 2

Draw the mechanism of the synthesis of polypropylene using a transition metal catalyst and a co-catalyst. Give the three roles of the co-catalyst.

Answer 2

The co-catalyst, methyl aluminoxane or MAO, is formed when AlMe₃ is partly hydrolysed with water. MAO:catalyst is about 1000:1 The three roles it serves is:

1. Removes Cl, adds Me to the metal complex
2. Removes one Me^- to create a vacant site on the complex
3. Removes water from the reaction which can poison the catalyst

The product M_w depends on the temperature, catalyst concetration and reagent concentration.

Question 3

How can the metal catalysed reaction to form polypropylene be made stereospecific?

What are the benefits of the stereospecific polymers?

How can a mix of the stereospecifities be obtained?

Answer 3

Linking and increasing the size of the cyclopentyl rings will form the isotactic polymer. The link is a SiMe₂R₂ group where the R is the rings. A benzene ring is linked to the Cp ring on opposite sides laterelly.

Syndiotactic polymers can be formed by having both benzene rings on the same Cp ring. The polymers have a higher melting point and are easier to work with.

By changing reaction temperature the catalyst can be changed to favour a certain stereochemistry of product.

Question 4

Why are early transition metals good polymerisation catalysts? What about late transition metals?

Answer 4

Early TMs:

1. Are electrophilic, rapid bonding of alkene increases rate of polymerisation, k_p.
2. Group 4 are d⁰ so back bonding is weak and M-alkene bond is labile.
3. For metallocenes, k_p>>>k_t.
4. They are however, very sensitive to water so require lots of expensive MAO and cannot be used for some functional groups.

Late TMs would need less MAO, tolerant to polar functional groups and be more stable to hydrolysis, reducing k_t. But:

1. Strong alkene back bonding, smaller k_p.
2. Rapid beta-H elimination, large k_t.

All factors lead to k_p ≈ k_t which means they are only useful in dimerisation and oligomerisation.

Question 5

How have recent developments given increased functionality to late TM polymerisation catalysts?

Answer 5

Nickel with very large imine ligands and an ether ligand. Improved catalyst because:

1. Imine ligand is a hard donor and complex has a positive charge. Increased electophilicity
2. Bulky N-substituents prevent chain transfer and beta-H elimination.
3. Chelating di-imine ligand forces polymer chains to be cis to vacant site.
4. Ether is a weak ligand and easily substituted.

Question 6

How do the products of the late transition metal polymersiation catalytsts compare to those of the early transition metals?

Answer 6

Ni and Pd, late TMs, give highly branched products whereas early TM catalysts give linear products. New late TM complexes have also produced linear polymers however.

Question 7

Describe a general ROMP reaction including the type of catalyst and the driving force of the reaction.

Answer 7

A ROMP (ring opening metathesis polymerisation) reaction involves the release of a sterically hindered cyclic alkene. They produce polymers with reproducible gaps between alkene groups. Metal carbene complexes are the catalysts. Schrock or Grubbs catalysts can be used but Grubbs is more tolerent of functional groups and conditions so that is the typical choice.

Question 8

Draw the ROMP mechanism of cyclopentene with the Grubbs catalyst.

Answer 8

Question 9

Describe the polymers produced from ROMP in terms of structure, dispersity and stereochemistry.

Answer 9

Living polymerisation so a very small polydispersity, chains will extend as the reagent is contiunally added. Adding different reagents will form co-block polymers and even tri-block polymers can be formed.

The end groups depends on the aldehyde or ketone added for termination and the starting carbene.

Bicyclic reagents are often used as they form hard polymers and are cheap and abundant in crude oil.

It is hard to regulate stereochemistry of ROMP as 4 outcomes of each polymerisation can occur but it is possible (not covered in course).

Question 10

Give basic structural properties of polysiloxanes and their synthesis.

Answer 10

They are flexible, have low transition temperatures and gas permeability. They can also be made to be hydrophobic. PDMS (polydimethylsiloxanes) are the most widely used.

Question 11

Describe and give the reasons for the thermal and oxidative stability and the flexibility of polysiloxanes.

Answer 11

They are very strong to both as the Si-O bonds are 100 kJ mol^-1 stronger than a C-C bond and there are no multiple bonds that will react with radicals or other highly reactive species.

The backbone is very flexible with a low T_g. The Si-O-Si bond angle is wider than the C-C-C bond angle and the bonds are longer. Every oxygen has no side groups all meaning that rotation of the backbone is easy with low steric hinderence.

Question 12

How are polysiloxanes easy to modify?

Answer 12

Leaving some reactive Si-H bonds in ths structure can be reacted after the polymer has formed.

Cross-linking is easy which allows for the creation of silicone rubbers which hold their shape.

Silica can be used in cross-linking to make reinforced fillers.

Co-hydrolysis with a cross-linking agent can be done to strengthan the material.

Question 13

List the useful properties of polysiloxanes.

Answer 13

• Higher thermal and oxidative stability
• Flexible at low temperatures
• Wide temperature range of operation
• Very stable to radiation
• Hydrophobic - low surface energy
• Biocompatible
• High permeability to oxygen
• Transparent

Question 14

Give some current and future used of PDMS.

Answer 14

• Stress relieving interlayer for windscreens which also have hydrophobic effects.
• Soft contact lenses.
• Gas seperation membranes.
• Medical imaging tubes combined with silver elastomer for antimicrobe functions.
• Electrical systems for functions such as smart contact lenses.

Question 15

What is aerogel and how is it formed?

Answer 15

Reacting Si(OR)₄ with water will form Si with 4 Si-O bonds and alcohol. The silicon forms a fully 3D polymer network and the alcohol can be evapourated from that to leave the solid. Reaction in a supercritial fluid leaves many nm sized channels with air trapped, forming a very good insulator.

Question 16

What are polyhedral silsesquioxanes?

How are they synthesised?

Answer 16

Mixing polymers and silica particles forms regualrly shaped structures which can take the shape of a triangular, pentagonal or hexagonal prism. They can often form links between other structures with the silicon pointing outwards.

8 RSiCl₃ + 12 H₂O → R₈Si₈O₁₂ + 24 HCl

Question 17

Describe the basic strutural information about polyphosphazenes and draw the synthesis mechanism and how they can be modified.

Answer 17

Alternate P and N atoms with two side groups on the P. The side groups can be organic, inorganic or organometallic. These can be modified during synthesis.

Modification of the starting material is also possible to introduce different groups and modify reactivity.

Question 18

Describe the observations and subsequent understanding about the bonding in polyphosphazenes.

Draw the possible chain conformations.

Answer 18

Cis and trans conformers of the P-N bond exist but are quite small suggesting some π bonding character. The P-N bond is longer than a normal P-N bond but the lengths do not alternate in the chain however they are not coloured and do not conduct electricity which is characteristic of extensive conjugation.

Reactivity suggests that ionic bonding may be dominant.

Question 19

What properties of polyphosphazenes depends on the side groups present?

Answer 19

Hydrophobicity/hydrophilicity - polar groups will cause hydrophilicity, aromatic and aliphatic groups, especially with fluoro groups will cause hydrophobicity.

Flexibility and elasticity at low temperatures can be enhanced by fluoro groups and can even be fire resistant.

Short side chains can encourage gas permeability and polar groups can atract ions and act as crown ether type stabilisers. These can even be cross-linked to form a hydrogel and open and close solvent channels depending on temperature.

Question 20

Describe the biomedical applications of polyphosphazenes.

Answer 20

Stable polymers can be formed with both hydrophobic chains that can be used to replace heart valves and other surgical structures, or with hydrophilic chains which degrade to harmless products (such as amino acids). These will be hydrolysed over time and can release durgs either at a specific site, or just over time. This can either be in a polymer capsule or the drug can be attached to the backbone.

Question 21

What is the origin of structural colour?

Answer 21

Structural colour arises from the interaction of light with a lattice of particles in a regular lattice with spacings close to, or shorter than, the wavelength of visible light. The Bragg equation outlines how the colours constructively and destructively interfere making some colours enhance and some deplete.

The colour can change depending on viewing angle.

Question 22

Describe the structure of opal and how it causes structural colour.

Answer 22

Photonic structures which are normally nano-sized spheres forming a fcc (or similar) lattice, reflect in the 111 planes. In the gemstone opal, a fcc crystal is composed of SiO₂ spheres. The repeat distance is about 250 nm.

The ‘photonic crystals’ are nanostructures that cause high contrast of dielectric constants periodically spaced that allow or forbid certain wavelengths of light. They can be described to have a band gap, similar to electronic band gaps allowing only some wavelengths.

The periodic difference in dielectric properties must have a length similar to a half-wavelength of light (200 - 350 nm)

Question 23

How are lattices that cause structural colour formed?

Answer 23

SiO₂ microspheres are made using controlled hydrolysis, by condensation of Si(OEt)₄ in water, catalysed by NH₃ and EtOH. The colloidal particles formed have an average size in the nanoscale. These crystalise to form the lattices.

Question 24

How can the colour arising from structural colour be changed?

Answer 24

The sphere size will affect the gaps between the reflecting planes. Additionally changing the refractive index of the layer will have the same effect. The Bragg-Snell law describes these changes:

nλ = 2d_hkl(n_eff² - sin²θ)^½

Dependant variables are the distance between planes (d_hkl = (2/3)^½D for the 111 plane) and n_eff, the mean effective refractive index of the crystal which mainly depends on the gaps with good distance between the planes.

Question 25

How can structural colour colour be made responsive?

Answer 25

A responsoive photonic ink, P-ink, is a planar array of silica microspheres embedded in a partially cross-linked network of polyferrocenylsilane. The polymer infiltrates the silica particles and coats them with a layer of the polymer network. This increases the size of the particles which changes the colour. The colour can be further changed by adding solvent, which increases the spacing (red-shifting the colour) or applying a electric field which changes the oxidation state of the iron and affects the solvation of the particles.