Section B

Question 1

Polymers are often produced using heterogeneous production methods. Name major safety issue that can result in the loss of control over this process that is identified with the manufacture of chain polymers using homogeneous manufacturing routes?

Answer 1

Tromsdorff-Norish effect or gel point or auto acceleration

Question 2

Explain the influencing factors which may result in this onset of this loss of control of a homogeneous polymerisation

Answer 2

• the reaction is exothermic
• conversion of monomer to polymer/ molecular weight increases during reaction
• this leads to increased viscosity
• thus mass and heat transfer is severely reduced
• Thus bulk reaction medium temperature increases
• thus increasing rate of reaction so it cannot be controlled

Question 3

Explain how adopting heterogeneous polymerisation methods mitigate this safety risk

Answer 3

• The reaction is conducted by forming droplets called micelles
• the micelles are dispersed in a continuous phase
• the continuous phase does not participate in the reaction
• It acts as a cooling medium only
• conversion of monomer to polymer during the reaction occurs in these droplets
• Thus mass transfer is improved as there is no increased viscosity within the bulk of the reactors
  -Heat transfer is improved because micelles have very high surface to volume ratios

Question 4

Heterogenous polymerisation processes

Answer 4

• Dispersion
  -Suspension
• Emulsion

Question 5

Rank the three heterogeneous polymerisation methods to manufacture polystyrene from least safe to most

Answer 5

• Least: Suspension
• Intermediate: Emulsion
• Most: Disperion

Question 6

Explain the reasoning behind the rank

Answer 6

• All methods rely on successfully breaking up a large monomer into smaller droplets

• If this doesn’t happen, auto-acceleration can occur in the large volume of monomer

• Because emulsion polymerization uses a surfactant, the physical chemistry of the system -forms micelles, thus the level of mixing is not as critical as it is with suspension polymerization.

• however suspension completely relies on shear forces to create the droplets, and the interfacial aid in this system only stabilizes the particles as they form. Thus, emulsion is safer than suspension.

• In dispersion polymerization, the monomer is miscible with the continuous phase, so there is much less chance of monomer domain issues due to poor diffusion of the monomer in the system at the outset. Again, the interfacial aid in this system only stabilizes the particles as they form.

• In emulsion polymerization, the monomer is not miscible with the continuous phase. Therefore, dispersion is safer than emulsion.

Question 7

In dynamic equilibrium, increasing more pressure will…

Answer 7

Move the position of equilibrium to the side with less number of molecules to reduce pressure

Question 8

Does a catalyst influence the position of equilibrium

Answer 8

No- it speeds both forward and reverse reactions

Question 9

Step chain growth

Answer 9

• polymer growth by reaction between monomers
• low- moderate DP
• monomers are consumed quickly but MW increases slowly
• no termination
• no initiation
• polymerisation rate decreases as functional groups are consumed

Question 10

Chain growth polymerisation

Answer 10

• polymer growth by successive additions do monomers to the end of growing chain
• high DP
• monomers are consumed slowly MW increases quickly
• polymerisation rates increases as initiator is generated

Question 11

Gibbs free energy

Answer 11

G= moles * chemical potential ( reactants + products)

Question 12

At equilibrium

Answer 12

Cu + Du = au + bu. ( abcd are coefficient)

Question 13

Ksp solubility constant

Answer 13

A large KSP means forward reaction is favoured eg CaF2 KSP= Ca2+ + 2F-

Question 14

Equilibrium constant

Answer 14

How far a chemical reaction will go

Question 15

Adding inert gas for dynamic equilibrium

Answer 15

• Total pressure of system increases but no Keq change
• but if volume is allowed to increase position of equilibrium will move to the side with more moles of gas

Question 16

Van Hoff equation

Answer 16

DlnKeq/ dT. =. Delta H/ RT^2

Question 17

Van Hoff Plot for graph

Answer 17

LnKeq. =. -Delta H / RT. + delta S/R
Delta S/ R is y intercept
-delta H/R is gradient
Negative slope is endothermic

Question 18

Le chatelier’s principle

Answer 18

If a system is disturbed the position of equilibrium will change to contract the effect of the disturbance

Question 19

Equilibrium constant equation

Answer 19

Kc= [C]^c * [D]^d / [A]^a * [B]^b

Question 20

Tromsdorff effect

Answer 20

• conversion of monomer increases, MW increases, viscosity increases , HMT is reduced, reaction temperature increases, rate of propagation increases

Question 21

How to prevent Tromsdorff effect

Answer 21

• add inhibitor to kill radicals
• add crash volume of solvent/ monomer to reduce viscosity
• drop reaction contents out of bottom of reactor

Question 22

Acrylic sheet casting

Answer 22

• initiator in a polymer resin is poured between the moulding sheets ( resin prevents leaking from cell and increases viscosity )
• resin allowed to pymerizw under controlled heating
• sheet allowed to cool in mould before removing it

Question 23

Cell casting

Answer 23

• two sheets of glass kept apart by rubber polymer and pour resin into it
• affords high surface area to volume ratio
  Batch process

Question 24

Film casting

Answer 24

• a the in sheet of solution or low viscosity resin is applied to a heat roller
• solvent is evaporated way
  Continuous

Question 25

Deborah number

Answer 25

- describes viscoelastic behaviour of a material , tells you if you can process material - bigger De the more solid the material, small De is more fluid - De= deformation time*mean velocity flow / pipe length

Question 26

Emulsion

Answer 26

- surfactant is added to water to form micelles and droplets of water migrate into micelle start swelling them - initiator is put into water and breaks down to form radical and it now wants to be with oil - it goes into the monomer and starts reacting with the monomer in the micelle - reaction continues micelle still swelling and monomer is still transferring - we now have polymer in monomer

Question 27

Emulsion

Answer 27

In emulsion polymerization, a surfactant forms micelles in water, stabilizing droplets of monomer. An initiator breaks down into radicals, which enter the micelles and start polymerizing the monomer. As the reaction continues, the micelles swell with growing polymer particles, and the polymerization progresses within the monomer droplets.

Question 28

Emulsion advantages

Answer 28

- continuous water phase is a good conductor of heat - droplets give large surface area - viscosity remains close to water

Question 29

Emulsion advantages

Answer 29

- continuous water phase is a good conductor of heat - droplets give large surface area - viscosity remains close to water

Question 30

Suspension

Answer 30

- suspension polymerization involves dispersing water-insoluble monomers in water, typically as droplets - initiator is dissolved in the monomer, and when activated, it forms free radicals - stabilisers used to prevent the polymer particles from agglomerating. These stabilizers keep the particles suspended in water. - it avoids issues such as micelle formation, which can occur in emulsion polymerization.

Question 31

Dispersion

Answer 31

- a homogeneous mixture of the monomer and (dispersant) - When polymer reach a certain molecular weight, the polymer becomes insoluble in the solvent and starts to precipitate out of the solution. - dispersant, preventing the polymer particles from aggregating or “crashing out” of the solution. Key to maintain dispersion - the polymer’s precipitation starts forming droplets or micelles, where the growing polymer particles are contained within. The stabilizer ensures the polymer particles remain dispersed. - Without stabilizer, the polymer would precipitate uncontrollably.