Organic Mechanism

By: Duyen Vuong 12D

Content • Organic Mechanism • Vinyl polymers• Low density poly(ethene), LDPE

–Free radical formation• High density poly(ethene), HDPE

–Ziegler- Natta catalyst

• Bibliography

Organic mechanisms

• To manufacture of poly(ethene)

• used to make polymers from vinyl monomers

• Mostly covalent bonds

• 2 types: – LDPE– HDPE

Vinyl polymers• from vinyl monomers

(contain double carbon- carbon bonding)

• largest family of polymers

• made from the monomer ethylene/ ethene

• form a long chain of many thousands of carbon atoms containing only single bonds between atoms

Examples of Vinyl Polymers

• polypropylene

• Polystyrene

• poly(vinyl chloride)

• Polyisobutylene/ type of rubber

• poly(methyl methacrylate)

• polytetrafluoroethylene

Objective C.9.1

• Describe the free- radical mechanism involved in the manufacture of low- density polyethene

Low density poly(ethene)

• Carried out at very high pressures (1000-3000 atm)

• At temperature about 500K (227ºC)• Average polymer molecule contains between 4 x

10³ to 4 x 10^4 carbon atoms • With short branches • Has a density of about 0.92g cm^-3• Used mostly for packaging • Formed under free radical formation

Free radical formation

• Definition- molecules with unpaired electrons • Highly reactive due to unpaired electrons• Biology example- PEROXIDATION (producing

ATP inside the body)• Elements example- oxygen (ROS- reactive

oxygen species) , transition metals (Cu- copper, can lose and gain electrons easily)

• Progress= initiate propagation termination

Initiate • First step• Homolytic fission

– when a bond is broken, each of the bonded atoms or groups takes one of the bonding pair of electrons.

–

• Not heterolytic fission – when a bond is broken and two electrons go to one of

the element/compound

• Examples:– Cl2 (UV light) 2Cl·– Br2 2 Br·

Propagation • Each step must have odd number of unshared electrons • Two compounds containing one unshared electron each

never, ever, ever combine • Unpaired electrons wanted will swipe one of themselves

to pair with • Leading us back to the beginning • adding of more and more monomer molecules to the

growing chains • Examples:

– Cl · + CH4 · CH3 + HCL– · CH3 + Cl2 CH3Cl + Cl ·– Cl · + CH4 -> · CH3 + HCL– · CH3 + Cl2 -> CH3Cl + Cl·

Termination • Each of two unpaired compounds form

radical–radical combination (coupling) or a disproportionation

• Examples: – · Cl+ · Cl -> Cl2– · CH3+ · CL-> CH3Cl– · CH3+ · CH3->C2H6

Examples of the free radical formation

OR

Objectives C.9.2

• Outline the use of Ziegler- Natta catalyst in the manufacture of high- density polyethene

High density poly(ethene)

• Carried out at pressures (1-50 atm)• At low temperature about 350K (77ºC)• Using Zieler- Natta catalyst • Maily of linear chains with very little

branching• Higher density 0.96 g cm^-3• More rigid structure• Used to make containers and pipes

Ziegler- Natta catalyst

method of vinyl polymerization• discovered by two scientists: Ziegler and Natta

in the 1950s • Can’t make some other kinds of monomers,

such as poly(vinyl chloride)• Suspension of titanium (III) or titanium (IV)

chloride together with an Alkyl- aluminum compound

• Catalyst and co catalyst:– TiCl3 and Al(C2H5)2Cl – TiCl4 with Al(C2H5)3

Industry process

Chemical process

• TiCl3 can arrange itself into a number of crystal structures

• each titanium atom is coordinated to six chlorine atoms= octahedral geometry

• Ti-

– transition metal

– Have six empty orbitals (4s and five 3d-orbitals )

• Needs 2 electrons to fill up the orbital

• Filled 1 up by Al(C2H5)2Cl

• Combine with vinyl monomer like propylene nth(C3H6)

• two electrons in the π-system of a carbon-carbon double bond fill the left over empty orbital

• Form a complex

• Several pair of electrons shift position

• The propylene monomer migrates

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