Tae Lin Pak Choi - 2

Question 1

Vinyl Polymerization

Polymerization by reactive intermediates
C=C—X

For anionic, what should by the X? For cationic? For radical?

Answer 1

• Anionic: smth that make the monomer electrophile
• Cationic: make the monomer nucleophile
• Radical. less sensitive to the electronic nature of X

Question 2

Vinyl Polymerization

What about regio-selectivity
Other consideration? (vs condensation?)

Answer 2

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)

Question 3

Commercial products via radical polymerization

Some materials & properties

Answer 3

• LDPE: highly branched, low crystallinity
• PS: more used as a copolymer and blends
• PVC: high mechanical strength
• PMMA: highly transparent

Question 4

Which is the easiest polymer to recycle, why?

Answer 4

PET

Both with physical (melted and re-used)
and chemical recycling (depolymerization)

Question 5

Chain transfer to polymer: types and when

Answer 5

• Intramolecular transfer: short-chain branches
• Intermolecular transfer: long-chain branches

Question 6

Merrifield’s Synthesis (Solid-phase peptide synthesis)

Answer 6

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

Question 7

Free energy of polymerization

driving force of polymerization, ceiling temperature

Answer 7

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

Question 8

Cracking Hydrocarbons

Answer 8

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

Question 9

Coordination polymerization

Background

Answer 9

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

Question 10

Coordination polymerization

K. Ziegler

Answer 10

Ziegler catalyst: 70-90% crystallinity

Question 11

Tacticity

Types

Answer 11

• Isotactic: same enantiomers –> helical structure
• Syndiotactic: alternating enantiomers
• Atactic: random

Question 12

Chain transfer reactions

Mechanism & Limitations of Z-N catalyst

Answer 12

• 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

Question 13

Metallocene catalysts

Answer 13

Originally prepared in 1950s but alkyl aluminum was not effective

Zirconocene: most popular for its high activity and selectivity

Question 14

Polymerization with Metallocene Catalysts

Answer 14

• 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.

Question 15

Mechanism of metallocene catalyst

Answer 15

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

Question 16

Post-metallocene catalyst

Answer 16

1. 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
2. Brookhart @ U of North Carolina (incorporation of polar monomers)
3. Grubbs @ Caltech (tolerate polar monomers and solvents)

Question 17

Commercial products

Answer 17

LLDPE (Linear low-den. PE): copolymer of ethylene and 1-hexene (or higher)
controlled branching, better mechanical property than LDPE

Question 18

Mechanical properties: tensile test

Answer 18

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 )

Question 19

Crystallinity

Answer 19

Amorphous vs. Semi-crystalline
Am is transparent, crystalline is opaque

Question 20

Olefin methatesis

Answer 20

Typically a thermodynamic equilibrium process
The reaction produces a new carbon-carbon double bond

Question 21

Diene methatesis reactions

driving force

Answer 21

Driving force is entropy because you go from liquid to gas

Question 22

Cross methatesis reaction

early catalyst

Answer 22

In-situ generation of metal carbene (ill-defined catalysts)
never know the active catalyst. hard to do mechanistic studies

Question 23

Well defined catalyst

Answer 23

Titanium, tungsten, molybdenum, ruthenium

Question 24

N-Heterocyclic carbene ligands

Answer 24

• Second generation ruthenium benzylidene
• Enhanced activity and stability relative to 1
• Retains functional group tolerance of 1

Question 25

Mechanism dependence

Answer 25

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

Question 26

Ring-Opening Metathesis Polymerization

driving force

Answer 26

Driving force: High ring-strain energy (most efficient method)

Question 27

what material can be used as a ballistic protection?

Answer 27

Poly DCPD
(diocane porcodio)

Question 28

Monomers for ROMP

Which with low size and high strain, which with high size and low strain

Answer 28

1. Nbn
2. CDT & COT

Less-strained: Concentration, and temperature may affect polymerization

Question 29

Well defined catalyst for ROMP

All

Answer 29

1. Highly tolerant to many functional groups, but less active than 3. Promotes living ROMP of norbornene derivatives
2. Highly active than 3. Retains high functional group tolerance of 1. Initiation is slow with high kp (poor mw control). Efficient chain transferring
3. Highly active but sensitive to moisture and air. Promotes living polymerization of norbornene derivatives

Question 30

Catalyst for Living ROMP

3-bromopyridine

Answer 30

Very labile ligand.
Ligand dissociation is million times faster than 2 nd -Gen catalyst.
Catalyst 1 promotes living polymerization with high activity.