Tae Lin Pak Choi - 2 Flashcards
Vinyl Polymerization
Polymerization by reactive intermediates
C=C—X
For anionic, what should by the X? For cationic? For radical?
- Anionic: smth that make the monomer electrophile
- Cationic: make the monomer nucleophile
- Radical. less sensitive to the electronic nature of X
Vinyl Polymerization
What about regio-selectivity
Other consideration? (vs condensation?)
Vinyl polymerization results in the addition of monomers without the los of small molecules whic can be advantageous for maintaining a clean reaction environment without by-products.
Regioselectivity: preferemce of one direction of monomer addition over others –> affect the structure of the polymer backbone (important for tacticity)
Commercial products via radical polymerization
Some materials & properties
- LDPE: highly branched, low crystallinity
- PS: more used as a copolymer and blends
- PVC: high mechanical strength
- PMMA: highly transparent
Which is the easiest polymer to recycle, why?
PET
Both with physical (melted and re-used)
and chemical recycling (depolymerization)
Chain transfer to polymer: types and when
- Intramolecular transfer: short-chain branches
- Intermolecular transfer: long-chain branches
Merrifield’s Synthesis (Solid-phase peptide synthesis)
Synthesis of a peptide of significant length
Involves attaching the C-terminus of the peptide chain to a polymeric solid
Separation and purification is accomplished by filtering and washing the beads with solvents,
The filan step, in which the completed peptide is released from the polymerr support, is a simple benzyl ester cleavage
Free energy of polymerization
driving force of polymerization, ceiling temperature
ΔGₚ = ΔHₚ - TΔSₚ
Driving force of polymerization is the change in enthalpy
Ceiling temperature is the temp at which ΔGₚ = 0: T_c = ΔH / ΔS
Cracking Hydrocarbons
Process whereby complex organic molecules are broken down into simpler molecules by the breaking of C-C bonds in the precursors
It is strongly dependent on the temperature and presence of catalyst
Coordination polymerization
Background
Polystyrene obtained from free-radical polymerization has no crystallinity due to absence of stereoregularity
Polyeethylene has low crystallinity due to branching caused by chain transfer
Polypropylene from FRP has oily liquid properties due to low molecular weights and atacitc structure
Coordination polymerization
K. Ziegler
Ziegler catalyst: 70-90% crystallinity
Tacticity
Types
- Isotactic: same enantiomers –> helical structure
- Syndiotactic: alternating enantiomers
- Atactic: random
Chain transfer reactions
Mechanism & Limitations of Z-N catalyst
- heterogeneous nature→ sensitive to surface defects which lead to atactic fractions
- variation in catalytic activity (various terminations), leading to broad PDI> 5
- lack of control of copolymer composition (together with MW and stereo-regularity)
- Metallocene catalysts (homogeneous, single-site catalysts) developed in 1985
Metallocene catalysts
Originally prepared in 1950s but alkyl aluminum was not effective
Zirconocene: most popular for its high activity and selectivity
Polymerization with Metallocene Catalysts
- Homogeneous, single-site catalysts (contrary to Z-N catalysts)
- Polyethylene with narrow MW distribution (PDI: 2~5)
- More active than Z-N (about 100 times)
- Linear Low Density Polyethylene (ethylene-octene (hexene) copolymers)
- Syndiotactic PS and PP possible
- Precious control of ligand structure affects polymer structure
- Harder to make, more expensive. Only recently commercialized. More expensive, more environmentally safer.
Mechanism of metallocene catalyst
Accidental discovery: MAO (trimethyl aluminum + H 2 O)
Mixture of linear, cyclic and other 3-D structure (cage-like) with n=5~20
Alkylating agent: Added as excess (100~10^4 fold) also removes impurity
Post-metallocene catalyst
- Bercaw @ Caltech (more open-environment more reactive: can incorporate longer bulkier olefins:PE copolymer with 1-alkene, norbornene etc Also long-chain branching from in-situ generated macromonomer via b-H elimination) Also known as Dow catalyst
- Brookhart @ U of North Carolina (incorporation of polar monomers)
- Grubbs @ Caltech (tolerate polar monomers and solvents)
Commercial products
LLDPE (Linear low-den. PE): copolymer of ethylene and 1-hexene (or higher)
controlled branching, better mechanical property than LDPE
Mechanical properties: tensile test
Elastomer: rubber-like, low modulus ( 2,000 N cm -2 = 20 MPa)
Plastic: medium modulus (15,000 N cm -2 = 150 MPa )
Fiber: high modulus (> 30,000 N cm -2 = 300 MPa )
Crystallinity
Amorphous vs. Semi-crystalline
Am is transparent, crystalline is opaque
Olefin methatesis
Typically a thermodynamic equilibrium process
The reaction produces a new carbon-carbon double bond
Diene methatesis reactions
driving force
Driving force is entropy because you go from liquid to gas
Cross methatesis reaction
early catalyst
In-situ generation of metal carbene (ill-defined catalysts)
never know the active catalyst. hard to do mechanistic studies
Well defined catalyst
Titanium, tungsten, molybdenum, ruthenium
N-Heterocyclic carbene ligands
- Second generation ruthenium benzylidene
- Enhanced activity and stability relative to 1
- Retains functional group tolerance of 1
Mechanism dependence
Depending on the catalyst design we have different properties and different uses:
1. new drugs
2. total synthesis of natural products
3. engineering plastics
4. new polymers
Ring-Opening Metathesis Polymerization
driving force
Driving force: High ring-strain energy (most efficient method)
what material can be used as a ballistic protection?
Poly DCPD
(diocane porcodio)
Monomers for ROMP
Which with low size and high strain, which with high size and low strain
- Nbn
- CDT & COT
Less-strained: Concentration, and temperature may affect polymerization
Well defined catalyst for ROMP
All
- Highly tolerant to many functional groups, but less active than 3. Promotes living ROMP of norbornene derivatives
- Highly active than 3. Retains high functional group tolerance of 1. Initiation is slow with high kp (poor mw control). Efficient chain transferring
- Highly active but sensitive to moisture and air. Promotes living polymerization of norbornene derivatives
Catalyst for Living ROMP
3-bromopyridine
Very labile ligand.
Ligand dissociation is million times faster than 2 nd -Gen catalyst.
Catalyst 1 promotes living polymerization with high activity.
