Functional Macromolecules and Nanoscience, Ian Manners FRS

Introduction to Polymeric Materials

Wikipedia articles of general relevance

History

Association theory (1861) proposed small-molecule monomers were held together by an unknown force, forming so-called "colloids" (which now has a different meaning)
Superseded by Hermann Staudinger's macromolecular hypothesis (1930) that polymers are simply very large molecules held together by conventional covalent bonds
Wallace Carothers' classic review: Carothers, Wallace H. (1931). "Polymerization". Chem. Rev. 8: 353–426. doi:10.1021/cr60031a001.

Classification of Synthetic Routes to Polymers

Chain-growth vs. step-growth

Chain-growth polymerisation versus step-growth polymerization
- Key point: get step-growth when all species (monomer, dimer, trimer etc) can react with each other. Get chain growth when reaction requires at least one of the two reacting molecules to be activated (have a reactive centre, e.g. a radical, anion or cation).

There are many exceptions, but:
- chain-growth is usually associated with radical polymerisation and addition polymers (all monomer atoms incorporated into chain)
- step-growth is usually associated with condensation polymerisation and condensation polymers (not all monomer atoms incorporated into chain: some expelled as small molecules)

Chain-growth polymerisation

Step-growth polymerization

Carothers equation

Carothers equation

Synthetic Control of Polymer Architectures

Living polymerisations

Living polymerization occurs when chain termination is prevented
It's really useful because:
- You get very good molecular weight control
- You get very high polydispersity (PDI approaches 1.00), thus excellent molar mass distribution control
- You can functionalise the end groups, which are still reactive when all the monomer has been used up
- You can synthesise block copolymers, and various other architectures like star polymers and polymer brushes
  - The living polymer is actually a macromonomer
  - PS-PB-PS or PS-PI-PS triblock copolymers are called "Kratons" and are useful synthetic rubber substitutes
  - Kratons are thermoplastic elastomers

Living anionic polymerisation

Living anionic polymerization
Practical issues:
- Need extremely low levels of reactive impurities (eg. O₂, CO₂, H₂O) since the anion concentration is very low
  - Need to carefully purify solvents and reagents
- MMA polymerisation requires an organocaesium initiatior (CsR) and low temperatures (−75 °C)
  - The large counterion and low temp. prevent nucleophilic attack at C=O

Living ring-opening polymerisation

Ring-opening polymerization
Usually driven by enthalpy
- relief of ring strain
- negative enthalpic term must outweigh negative entropic term
  - the entropy of the starting materials (many small cyclic monomer molecules) is usually greater than that of the product (one long polymer chain)

ROMP

Ring-opening metathesis polymerisation is a living polymerisation
Proceeds via an alkene metathesis mechanism (Chauvin mechanism)

Living radical polymerisation

Living free-radical polymerization
Circumvents the major problem with radical polymerisation: chain termination and transfer leading to poor molecular weight control
Key principle: lower concentration of radical-ends on growing polymer chains by reversibly trapping them as dormant species
The cost is slower reactions that require higher temperatures

Nitroxide-mediated

Nitroxide Mediated Radical Polymerization
Georges, 1993
Initiator R–Z is in equilibrium with reactive radical R^• and stable radical Z^•
R^• reacts with monomers M to form a propagating radical-capped polymer chain, R–M_n^•
R–M_n^• is also in equilibrium with a dormant, Z-capped form R–M_n–Z, i.e. reversible chain termination
Z^• is a nitroxide such as TEMPO
Bimolecular termination (radical combination and disproportionation of pairs of chains) is suppressed
Polymerisations are quite slow (1-3 days) and require heat (125–140 °C) but...
Mol. wt. is ok (5500-11000), PDI's (1.15-1.21) nowhere near as good as anionic polymerisation, but functional group tolerance is nice
PDI increases at high conversions (> 80%) because less monomer is present, so RZ formation is suppressed

ATRP

Atom-transfer radical-polymerization (ATRP)
Matyjaszewski, Sawamoto, 1995
Also reversible termination, but this time with transition metal complexes that reversible accept halogen atoms X
- Complex is often a copper(I) halide complex (LCuCl or LCuBr), also L₃RuCl₂, L₂FeCl₂, etc.
Temperatures of 60–120 °C required, unwanted colouring from metal complex can occur

RAFT

Reversible addition−fragmentation chain-transfer polymerization (RAFT)
Rizzardo, 1998
Reversible termination again, this time with a dithioester ZCS₂R
- The radical-capped growing polymer chain P^• adds to the neutral, closed-shell dithioester ZCS₂R to give a sulfur-stabilised carbon radical ZC(SR)(SP)^•
- The R group can depart as a reactive radical R^•, leaving a dithioester-capped polymer chain ZCS₂P
- Two different polymer chains can be linked by the dithioester radical, ZC(SP_m)(SP_n)^•
The majority of the actively propagating polymer chains are trapped in dormant states
- This limits chain termination
Functional group tolerant (styrenes, acrylates, acrylamides, many other vinyl monomers)
PMMA and PAA can be made well using RAFT

Living chain-growth polycondensations

Even in ideal cases, you get PDI = 2
Most polycondensations are step-growth not chain-growth processes
But protein and nucleic acid biosyntheses give perfectly monodisperse polymers
- They can be thought of as living chain-growth polymerisations (LCGP)
Synthetic examples of LCGP now exist
- The Manners and Allcock cationic route to polyphosphazenes
- Yokozawa's anionic routes to polyethers and polyamides
- Yokozawa and McCullough's π-conjugated polymers by GRIM

Protein and nucleic acid biosynthesis

Natural protein biosynthesis and DNA biosynthesis
The ribosome controls peptide synthesis via mRNA and tRNA
Natural structural and functional materials (proteins) are far more sophisticated than current synthetic materials
- Intramolecular hydrogen bonding in polypeptides gives rise to alpha-helices
- Intermolecular hydrogen bonding gives rise to beta-sheets
- Immensely complex and functional tertiary structures occur spontaneously
Can harness nature's ability with recombinant DNA technology
- Clone and modify genes that encode proteins to make protein structures of your choice
  - Use site-directed mutagenesis to tailor your sequence and the polymerase chain reaction to amplify your DNA blueprints
- Get plasmids or viruses to insert your tailored DNA blueprint into a bacterial cell
- Bacteria multiply, offspring contain the blueprints too, your protein gets made in large quantities
- Good route to certain types of protein and other controlled structures in useful quantities
- Much work still required

Polyphosphazenes

Room temperature condensation route to polyphosphazenes
- An example of living cationic polymerization
- Cl₃P=N–SiMe₃ monomer is initiated with PCl₅
- PCl₅ abstracts chloride and somehow causes Me₃SiCl to be eliminated
- [Cl₃P=N=PCl₃]⁺ cation is the reactive intermediate
- further Cl₃P=N–SiMe₃ monomers add to the cation, eliminating Me₃SiCl each time
- generates the (–N=PCl₂–)_n polymer

Polyethers and polyamides

π-Conjugated polymers

Yokozawa, McCullough: π-conjugated polymers by Grignard metathesis (GRIM)
- Basically nickel-catalysed coupling of aryl dibromides with RMgX or RLi
- Chain-transfer polycondensation to poly(3-hexylthiophene) by this route
- Proceeds via R–NiL₂–Br and R₂NiL₂ species
- Reductive elimination allows thiophene monomers to insert between polythiophene chain and nickel catalyst end-group
- Oxidative insertion into a thiophene-bromine bond puts the catalytic nickel centre at the end of the chain again

Polymers in nanotechnology

Block copolymers in nanotechnology

Vary the relative lengths of the blocks in a block copolymer to get different morphologies (different phases in the phase diagram)
Can get self-assembly into lamellar, cylindrical, spherical and gyroid phases, depending on the volume fraction of each block
- Triblock copolymers have even more possible phases, which are even more complex!
Applications in semiconductor device patterning, beyond the minimum size limits of photolithography
- PS-b-PB block copolymer on silicon nitride
  - Positive resist: untreated PB blocks removed by ozonation
  - Negative resist: PB blocks stained by RuO₄, survive reactive ion etching better than PS
Block copolymers self-assemble into different morphologies in different solvents, but more difficult to predict than solid state
- Appplications based on micellar shapes and properties
  - Nanolines (cylinders)
  - Controlled drug delivery
  - Catalysis

Metal-containing polymers in nanotechnology

Metallopolymers: G. R. Whittell, I. Manners, Adv. Mater. (2007) 19, 3439–3468
All sorts of scope
- Metal atoms in the main chain
- Metal atoms in the side chains
- Linear and dendritic metallopolymers
- Covalent and supramolecular metallopolymers possible
Mostly undeveloped until the late twentieth century
- Things like poly(vinyl ferrocene), polymetallaynes, polyferrocenylsilane, coordination polymers, polystannanes, π-conjugated metallopolymers

Nitric oxide sensor

B. Holliday, T. Swager, et al. Chem. Mater. (2006) 18 5649–5651
Main-chain metallopolymer containing cobalt
Coordination of NO causes change in conductivity of the metallopolymer, allowing ppm sensitive, selective and reversible NO detection

Air/oxygen pressure sensor

Polymer side-chain contains a phosphorescent ruthenium complex
The phosphorescence is quenched by triplet dioxygen in air
Gives a visual indication (also measurable) of relative air pressure by phosphorescent light intensity
- Higher air pressure, less phosphorescence

Polyferrocenylsilanes

Polyferrocenylsilanes (PFSs) are polymers with alternating ferrocene and SiR₂ backbone units
- Like PDMS with ferrocene in place of oxygen
Heat strained cyclic monomers (silicon-bridged ferrocenophanes) to 130 °C and they undergo ring-opening polymerisation
Easily form high molecular weight polymers, although chain-growth mechanism leads to PDI = 2.3 (broad m.w. distribution)

Monomer synthesis

Dilithiate ferrocene with BuLi + tmeda in hexanes
Add R₂SiCl₂ such as dimethyldichlorosilane, get LiCl ppt and red-orange crystals of the monomer
The monomer is a silicon-bridged ferrocenophane, containing an FeCp₂Si ring
- Ferrocenophanes are named on the pattern of cyclophanes
Strained, ring tilted structures (Jmol models of monomer crystal structures)
- Tilt angles of about 21°
- Strain energies of 70-80 kJ/mol

Polymer forms

PFSs can be amorphous, glassy, semicrystalline or liquid crystalline
Can be soluble in polar or non-polar organic solvents, sometimes even water: I. Manners, Science (2001) 294 1664–1666
Can crosslink PFSs with spirocyclic ferrocenophanes (e.g. Fc₂Si) to get PFS gels
Lightly-crosslinked PFS gels in a highly polar solvent + electrolyte solution swell when neutral Fe(II) is oxidised to cationic Fe(III)
- Non-polar PFS becomes polar upon oxidation
- Osmotic pressure causes solvent to flow into oxidised PFS
- When monodisperse microspheres are embedded in the PFS gel, it acts as photonic crystals
- The PFS oxidation state determines the separation of the spheres, i.e. their Bragg diffraction d-spacing
- This tunable lattice spacing leads to tunable colour when d is on the order of visible light wavelengths (500-700 nm)

PFS block copolymers

Block copolymers of PFS and PI (polyisoprene) adopt unusual structures in hexane solvent
- Assembles into cylindrical micelles in hexane
- PDMS is quite etch resistance (due to Si content) whereas PI isn't
- Etching away PI corona with O₂ plasma yields 8 nm cylinders
- With PDMS, mostly untouched by etching, left with 30 nm cylinders

Wikipedia articles to be incorporated