free radical styrene

7/28/2019 Free Radical Styrene

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Dr. D. A. Savin Techniques PSC341L/720

Electronic Handout2008 D. A. Savin

Experiment 5: Free-Radical Polymerization of Styrene

Key differences between step growth and chain growth polymerization reactions:

Step Growth

Reaction occurs in slow, stepwise manner

Any two species can reactmonomer + monomer dimerdimer + monomer trimerdimer + dimer tetramer

Slow inc rease in MW

Distribution of molecular species

Rapid disappearance of monomer

Generally long reaction timesnecessary to obtain HMW polymer

Chain Growth

Rapid addition of monomer to growing chainend

Few initiated chains,monomer adds to growingchain end

High MW polymer formedat once

Only high polymer and

monomer present

Monomer concentrationdecreases steadily

Long reaction times givegreater yield, but not higher MW

Polystyrene

Styrene monomer, a petrochemical typically produced from benzene and ethylene, ispolymerized to form polystyrene.

Polystyrene


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Tacticity in Polystyrene

Tacticity relates to the stereochemistry of the side-chain phenyl groups:

H R

H R

H R

H R

H R

H R

H R

H R

R H

R H

H R

H R

H R

R H

R H

H R

H R H R

R H R H

R H

isotactic syndiotactic atactic

R R RR

R RR R

R R R R


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Atactic polystyrene, the common commercial form of polystyrene, is a commodity plastic withan estimated global consumption of 10.5 million metric tons in 2000.1 It is amorphous and

transparent with a Tg of approximately 100C. Typical applications for polystyrene include

food packaging, toys, appliances and CD cases. Commercial production of polystyrene began inthe 1930s in Germany (BASF) and was introduced in the US by Dow Chemical in 1937.

Styrofoam, trademark of Dow Chemical for foamed polystyrene, was introduced in 1957.Dow (Styron) is the leading global producer of polystyrene.2 Other major producers includeBASF (Polystyrol), Chevron Phillips, Nova, Atofina, and Hunstman. Atactic polystyrenehomopolymer is often referred to as crystal polystyrene to differentiate it from the manycommon polystyrene copolymers and blends in commercial use.

Syndiotactic polystyrene is produced using metallocene catalysts, and is substantially moreexpensive than atactic commodity polystyrene. Dow recently introduced syndiotactic polystyrene

products under the tradename QUESTRA. Syndiotactic polystyrene, in contrast to atacticpolystyrene, is semi-crystalline, and exhibits enhanced chemical resistance and thermal stability.

Free-Radical Polymerization

Vinyl monomers may be polymerized via free-radical polymerization, a type of chain growthreaction. Typical polymers manufactured via free-radical polymerization include low densitypolyethylene, poly (methyl methacrylate), polyacrylonitrile, poly (vinyl chloride),polyacrylamide, poly (acrylic acid), and polystyrene. Due to the reactivity of vinyl monomers,it is often necessary to add an inhibitor to stabilize the monomer and prevent premature radicalformation and polymerization. Inhibitors scavenge any radicals formed and produce low-

reactivity radicals which will not initiate polymerization of the vinyl monomer. Commoninhibitors include substituted phenols and quinones, with 4-tert buylcatechol commonly used toinhibit styrene polymerization.

Free-radical polymerization proceeds via multiple steps: chain initiation, chain propagation,chain transfer, and chain termination. Chain initiation involves formation of reactive radicals.Initiating radicals may be provided via thermal decomposition of added initiators. Typicalinitiators of this type include dialkyl peroxides, diacylperoxides, and azo compounds.

Initiator Formula Useful polymerization

Temperature (C) 3

O O

Benzoyl Peroxide Ph C O O C Ph 40 90


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t-Butyl Peroxide (CH3)3 C O O C (CH3)3 80 150

CN CN

Azobisisobutyronitrile CH3 C N = N C CH3 25 80

CN CN

Initiation:

Initiator 2 R R =radical

R + M R1 M =monomer

In chain propagationsteps, monomer molecules add to the active chain end while forming a newactive chain end. Monomer will continue to add to the chain end until chain transfer ortermination occurs. Monomers may add to the growing chain viahead-to-head or head-to-tailreactions.

Propagation:

R1 +M R2R2 +M R3. . .

Rn +M Rn+1

~CH2 CH + CH2= CH ~CH2 CH CH2 CH Head-to-tail Ph Ph Ph Ph

~CH2 CH + CH2= CH ~CH2 CH CH CH2 Head-to-head Ph Ph Ph Ph


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Chain transfer involves transfer of the active radical to another molecule, which stops chainpropagation at the growing polymer chain. The molecule to which the active site was transferredcan initiate another chain. In some cases an external chain transfer agent is added to thepolymerization reaction. Butyl mercapatan is commonly added to polystyrene reactions.

Additionally, chain transfer to solvent, monomer, initiator, and/or polymer may also occur.

Chain Transfer:

Rn +ZY RnY +Z ZY =chain transfer agent

Inchain termination the active chain center is destroyed. Dead polymer may be formed throughcombinationor disproportionation reactions. Polystyrene has been shown to terminate primarily

by combination reaction.

Termination:

Rn +Rm Pn +m combination P =polymerRn +Rm Pn +Pm disproportionation

Study Questions

1. Which is more likely, head-to-head or head-to-tail addition in the propagation step?2. What are some reasons for addition of a chain transfer agent to a polymerization

reaction?

General References4,5,6,7

1http://pardos.marketing.free.fr/37.htm2http://www.dow.com/homepage/index.html3 Bovey, F. A., Winslow, F. W. Macromolecules, Academic: Orlando, 1979, p. 314http://www.cheresources.com/polystymonzz.shtml 5http://www.huntsman.com/petrochemicals/ShowPage.cfm?PageID=11296http://www.devicelink.com/mpb/archive/98/07/001.html7 Allcock, H. R.; Lampe, R. W.; Mark, J . E. Contemporary Polymer Chemistry, Ed. 3, Pearson Education: UpperSaddle River, NJ, 2003.

free radical styrene

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