HOME PAGE * KS3 SCIENCES * GCSE BIOLOGY  CHEMISTRY  PHYSICS * ADVANCED LEVEL CHEMISTRY

A Level Organic Chemistry: Reaction mechanisms - free radical polymerisation of alkenes

Doc Brown's Chemistry Advanced Level Pre-University Chemistry Revision Study Notes for UK KS5 A/AS GCE advanced level organic chemistry students US K12 grade 11 grade 12 organic chemistry

A Level Revision Notes PART 10 Summary of organic reaction mechanisms

A mechanistic introduction to organic chemistry and explanations of different types of organic reactions

10.3 Reaction mechanisms of ALKENES

Index of all the ALKENE revision notes

[SEARCH BOX]

Free radical polymerisation

doc b oil notes



 

10.3.7 The free radical addition polymerisation of an alkene

The organic synthesis of poly(alkenes) like poly(ethene) from alkenes

  • What is the mechanism of polymerising ethene to form poly(ethene)?

  • e.g. nCH2=CH2 ==> -(-CH2-CH2-)-n      [see mechanism 32 below]

  • Polyalkenes such as low density poly(ethene) can be made by polymerising the alkene monomer with oxygen or peroxide catalysts.

organic reaction mechanisms

mechanism 32 - the peroxide catalysed free radical polymerisation of ethene

  • Step (1) For peroxide catalysed polymerization, the initial free radicals are formed by the homolytic bond fission of the O-O bond in a peroxide molecule.

    • The O-O bond (bond enthalpy 146 kJmol-1) is the weakest and will break before any others e.g. the C-C bond enthalpy is 348 kJmol-1 and the C-H bond enthalpy is even stronger at 412 kJmol-1. (see similar arguments in alkane chlorination)

      • Step (1) illustrates how to use half-arrows to show a homolytic bond fission step.

  • Step (2) is a chain propagation step, in which the 'alkoxy radical' adds onto an alkene molecule.

    • If you split open half the alkene double bond (i.e. one bond pair), one electron pairs up with the unpaired electron of the 'alkoxy radical' to form a C-O bond. The other electron remains unpaired at the end of the radical, so that one active free radical is replaced by another.

      • Step (2) illustrates how to use half-arrows in a polymerisation chain propagation step, where the radical adds onto the polymer molecule to make an even longer radical to continue the chain reaction.

  • Step (3) illustrates all the further subsequent chain propagation steps in which the polymer free radical adds onto another monomer alkene molecule to make the chain longer and longer.

  • Step (4) represents one possible chain termination, were the electrons of two free radicals pair up to form a covalent C-C bond. 

  • FURTHER COMMENTS

    • At GCSE level the presence of a very tiny % of oxygen atom in poly(ethene) is unlikely to have been mentioned, but has no influence on the properties of the polymer, since the extremely long chain of -CH2-CH2-'s predominates!

    • The use of Ziegler-Natta catalysts to make more stereo-specific high density poly(ethene) and high density poly(propene) involves an ionic mechanism.

  • -


keywords phrases: reaction conditions formula intermediates organic chemistry reaction mechanisms steps free radical mechanism of the polymerisation of alkenes, ethene polymerised to poly(ethene)

Index of all the ALKENE revision notes

TOP OF PAGE

[SEARCH BOX]


Doc Brown's Advanced Level Chemistry Revision Notes

APPENDIX -  COMPLETE MECHANISM and Organic Synthesis INDEX (so far!)

Doc Brown's Advanced Level Chemistry Revision Notes

[SEARCH BOX]

See also INDEX of ALL advanced revision notes on ALKENES

TOP OF PAGE

KS3 BIOLOGY QUIZZES ~US grades 6-8 KS3 CHEMISTRY QUIZZES ~US grades 6-8 KS3 PHYSICS QUIZZES ~US grades 6-8 HOMEPAGE of Doc Brown's Science Website EMAIL Doc Brown's Science Website
GCSE 9-1 BIOLOGY NOTES GCSE 9-1 CHEMISTRY NOTES and QUIZZES GCSE 9-1 PHYSICS NOTES GCSE 9-1 SCIENCES syllabus-specification help links for biology chemistry physics courses IGCSE & O Level SCIENCES syllabus-specification help links for biology chemistry physics courses
Advanced A/AS Level ORGANIC Chemistry Revision Notes US K12 ~grades 11-12 Advanced A/AS Level INORGANIC Chemistry Revision Notes US K12 ~grades 11-12 Advanced A/AS Level PHYSICAL-THEORETICAL Chemistry Revision Notes US K12 ~grades 11-12 Advanced A/AS Level CHEMISTRY syllabus-specificatio HELP LINKS of my site Doc Brown's Travel Pictures
Website content © Dr Phil Brown 2000+. All copyrights reserved on revision notes, images, quizzes, worksheets etc. Copying of website material is NOT permitted. Exam revision summaries & references to science course specifications are unofficial.

 Doc Brown's Chemistry 

*

TOP OF PAGE

best Christmas gift deals latest video game release, best Christmas gift deals best bargains in new year January shop sales latest pop music releases, best New Year sales deals download free music, latest film releases, best Christmas gifts for teenagers latest high street fashion in clothes, fashionable trending in clothing, best Christmas gift deals best bargains in new year January shop salesgirls buy clothes, spend a lot of money on clothes, best New Year sales deals shoes, sweets and chocolates, magazines and make-up best Christmas gifts for teenagers  Boys buy food and drink, computer games best Christmas gift deals best bargains in new year January shop sales DVDs and CDs, girls and boys spend a lot of money on credit for mobile phones best New Year sales deals best bargains in new year January shop sales buses and trains to transport them going out best Christmas gifts for teenagers best bargains in new year January shop sales Girls spend a lot of money on clothes best Christmas gift deals  color colour preferences in clothes, cool sunglasses best New Year sales deals boys buy expensive things like best Christmas gifts for teenagers designer sports clothes and trainers, teenagers save for holidays, best New Year sales deals clothes, mobile phones, birthday presents and electronic goods, teenage accessories, Favourite style of jeans. best Christmas gifts for teenagers A typical girl’s shopping list includes mobile phone credit deals best Christmas gift deals fashionable quality cool airpods, air pods, fashionable clothes best New Year sales deals the most popular favourite sneakers best Christmas gifts for teenagers fancy shoes, sweets, chocolates, magazines, best Christmas gift deals best bargains in new year January shop sales lip moisturizer best bargains in new year January shop sale slots on make-up, well being, teenage decor decorating their room best New Year sales deals teenagers like LED string lantern lights, best Christmas gifts for teenagers beauty products for teenagers, denim jackets, scrunchies coolness, fashionable back packs, typical boy’s shopping list includes mobile credit deals, eating out takeaway food and drinks, what teenagers like to buy in clothes best Christmas gifts for teenagers computer games, DVDs, CDs, what teenagers talk about best Christmas gift deals what teenagers worry about, what teenagers like to do for fun sports best New Year sales deals  what cool things do teenagers buy, resale websites like eBay Teenager