Alkene Polymerisation

Question 1

Aluminium Oligomer synthesis

Answer 1

At high pressure and temps migration reactions occur on aluminium complexes

1. coordination
2. migration
3. repeat
4. beta H elimination

Question 2

Why Aluminium poor catalyst

Answer 2

Vacant d orbitals mean no π interactions to activate alkene

Question 3

Ziegler- Natta Catalytic cycle

Answer 3

1. activation of Ti complex
2. alkene coordination
3. alkyl migration
4. repeat
5. beta H elimination
6. alkene coordination
7. hydride migration and regeneration

Question 4

How know Ti is active centre

Answer 4

starter unit of Ti is 2C

starter unit on Al is 1C

ethene polymeristaion adds 2C per step

products have even carbon

thus must be Ti

Question 5

MAO function

Answer 5

Zr metallocene co-catalyst

1. exchanges halide for alkyl group
2. creates vacant coordination site

Question 6

BARF

Answer 6

weakly coordinating ligands that are easily displace to activate catalyst

no added co-catalyst

Question 7

Agostic Interactions

Answer 7

Electron Deficient metals with vacant coordination site have non-bonding electrostatic interactions with hydrides that stabilize the metal

1. b agostic

lead to beta H elimination

2. a/c agostic

stabilise chain growth transition

Question 8

Polymer stereochemistry

Answer 8

isotactic - side groups on same side

syndiotactic- on alternating sides

atactic - random

Question 9

Factors Determining Polymerisation

Answer 9

1. orientation of polymer chain

reduce sterics with ligands

2. orientation of alkene

orientate for beta migration

orientate away from polymer chain

3. migration mechanism

agostic mean same position before/after

same mechanism/face each time - isotactic

Question 10

Regular Polymerisation

Answer 10

• sterics must be constant e.g. ansa-metallocene
• sterics must be asymmetric so preferred orientation
• migration must be same each time

Question 11

why few head to tail errors for metallocenes

Answer 11

Steric congenstion at alpha carbon makes migration to substitued carbon kinetically favourable

Question 12

Transition metal polymerisation catalysts

Answer 12

naturally favour elimination

however higher steric bulk makes chain growth more favourable

elimination has 5-coordinate TS

chain growth has 4-coordinate TS

Question 13

Alkene Isomerisation why

Answer 13

Catalysts that don’t polymerise isomerise

due to energy differences in alkenes

generally not selective, get mixture

Question 14

Alkene Isomerisation Cycle

Answer 14

1. activation
2. alkene coordination
3. hydride migration
4. beta H elimination
5. decoordination