Clarification? Repeating units for condensation polymers

• Metanoia

  8 Aug 16, 18:10

  Hi all, 

  seeking some clarification on what consitutites a repeating unit during condensation polymerisation (O-Level Pure Chem)

  The monomer below is able to polymerise on its own

  HO-X-COOH

  The question requires the student to represent the polymer using "two repeating units"

  Will the correct answer be

  1) -O-X-COO-X-CO-

  Or 

  2) -O-X-COO-X-COO-X-COO-X-CO-

  I thought it would be the first answer, but my tutee tells me that her school expects the 2nd option.

  Opinions and inputs welcomed.

• UltimaOnline

  8 Aug 16, 18:25

  Originally posted by Metanoia:

  Hi all, 

  seeking some clarification on what consitutites a repeating unit during condensation polymerisation (O-Level Pure Chem)

  The monomer below is able to polymerise on its own

  HO-X-COOH

  The question requires the student to represent the polymer using "two repeating units"

  Will the correct answer be

  1) -O-X-COO-X-CO-

  Or 

  2) -O-X-COO-X-COO-X-COO-X-CO-

  I thought it would be the first answer, but my tutee tells me that her school expects the 2nd option.

  Opinions and inputs welcomed.

   

  - updated in thread below -
• Metanoia

  10 Aug 16, 07:45

  Thanks so much for your input Ultima, I'm still trying to visualise the "boundaries" of a repeat unit

  1) When you said that there are 2 full repeat units in option 2, are you refering to the parts highighted in blue below?

  Would that make it half repeat units on either side or one repeat unit on either side?

  -O-X-COO-X-COO-X-COO-X-CO-

  2) How many repeat units would you say option 1 consists of?

  -O-X-COO-X-CO-

• UltimaOnline

  10 Aug 16, 14:04

  Originally posted by Metanoia:

  Thanks so much for your input Ultima, I'm still trying to visualise the "boundaries" of a repeat unit

  1) When you said that there are 2 full repeat units in option 2, are you refering to the parts highighted in blue below?

  Would that make it half repeat units on either side or one repeat unit on either side?

  -O-X-COO-X-COO-X-COO-X-CO-

  2) How many repeat units would you say option 1 consists of?

  -O-X-COO-X-CO-

  
  No prob, Metanoia.

  Actually, your student's school teacher is wrong here. Because the monomer (undergoing condensation polymerization) here is a hydroxycarboxylic acid, so for 2 repeat units, you just require 2 R groups (usually illustrated by a shaded-box at O levels) in the section of the polymer. Exactly as you've drawn.

  The reason for your student's school teacher's error, is because in some other instances of condensation polymerization, 2 different monomers are required, eg. a diamine and a dicarboxylic acid in Nylon 66. In such instances, 2 repeat units must include 4 R goups and 0.5+3+0.5 amide linkages, ie. -- diamine -- dicarboxylic acid -- diamine -- dicarboxylic acid --.

  Thus, perhaps by misguided instruction from the HOD, many secondary school teachers just apply this requirement blindly across the board, including for condensation polymers that only involve a single monomer, eg. hydroxycarboxylic acid.

  Long story short : you're correct, Metanoia. Your student's school teacher is wrong.

• Metanoia

  10 Aug 16, 16:54

  Thanks for making it much clearer.

  I agree that there is a chance that the teacher was using the 'blanket rule' which caters to two different condensation mononers.

  It was actually from a past year prelim question, and the teacher insisted that answer key was correct to draw option 2.

  I've asked the student to clarfy again with the teacher.

  Sometimes they can get quite distraught with conflicting expectations.  

• UltimaOnline

  10 Aug 16, 19:07

  Originally posted by Metanoia:

  Thanks for making it much clearer.

  I agree that there is a chance that the teacher was using the 'blanket rule' which caters to two different condensation mononers.

  It was actually from a past year prelim question, and the teacher insisted that answer key was correct to draw option 2.

  I've asked the student to clarfy again with the teacher.

  Sometimes they can get quite distraught with conflicting expectations.  

  
  Oh no worries (about your student being distraught over conflicting answers). just tell your students what I tell my students, "For your school exams, write your answers the way your school teachers taught you. For your O/A level exams, write your answers the way I taught you. Because I personally get A grade every year, so that's Cambridge saying you should trust me over your school teachers."