Instantaneous composition concentration of the monomer

Define F1 and F2 to be the mole fraction of monomer 1 and 2 in the polymer, at any instant,

If f2 and f2 represent mole fraction of monomer 1 and 2 in the monomer concentration,

The copolymer equation can be written as

This equation can be used to calculate curves of original concentration of monomers versus instantaneous polymer composition for various monomer reactivity ratios. A couple of curves are show in figure 5-1 and figure 5-2.

Evaluation of Monomer reactivity ratios

Recommended method--Plots of r₁ versus r₂. The copolymer equation may be solved for one the reactivity ratios:

Each experiment with a given original concentration gives a straight line; the intersection of several of these allows the evaluation of r1 and r2. If the experimental errors are high, the lines may not be intersect in a single point; the region within which the intersections occur gives information about the precision of the experimental results.

B. Composition of copolymers (omitted)

C. Mechanism of copolymerization

Free radical copolymerization:

The reactivity of monomers and radicals in copolymerization is determined by the nature of the substituents on the double bond of the monomer. They may:

i.                     active the double bond

ii.                   stabilize the resulting radical by resonance

iii.                  provide steric hindrance at the reaction site

Reactivity of monomer:  CH₂=CH-Ph > CH₂=CH-CH=CH₂ > CH₂=CH-COCH₃ > CH₂=CH-CN > CH₂=CH-COOR > CH₂=CH-Cl > CH₂=CH-CH₂Y > CH₂=CH-OCOCH₃ > CH₂=CH-OR

Reactivity of radicals: similar to that of the monomers

Steric effects: Steric hindrance can reduce the reactivity of both monomers and radicals.

Ionic Copolymerization: depends on the stabilities of ionic intermediate and the ________.

Step-reaction copolymerization: depends on the reactivity of the monomer.

Chapter 6. Polymerization Conditions and Polymer Reaction

A. Polymerization in Homogeneous Systems

Bulk Polymerization

The bulk addition polymerization is the simplest of all polymerization processes. It is a homogeneous system with an organic initiator, where the reactions are only mildly exothermic. It is not quite useful for vinyl monomers, since the reaction are highly exothermic and it is seldom used commercially for the manufacturing of vinyl polymers.

If the polymer is insoluble in monomer, then initiation, propagation, and termination might happen in the monomer phase.

If the polymer is soluble in the monomer, then the concentration of monomer decreases continuously and the viscosity changes.

Solution Polymerization

The main advantage of a diluent (either water or an organic solvent) is to take up the heat of polymerization. For solution polymerizations, there are two possibilities:

• monomer is soluble and the polymer is soluble in the diluent:

example: polystyrene in toluene

• monomer is soluble and the polymer is insoluble in the diluent:

example: acrylonitrile in chloroform

The advantage of solution polymerization over bulk polymerization is better heat control; the disadvantage solution polymerization is the removal of the diluent from the polymer. This requires a distillation, and that costs an appreciable amount of money.

B. Polymerization in Heterogenous Systems.

Polymerization from Gaseous Monomers

Polymerization is slow since the most of polymerization form a liquid phase or solid polymer while the monomer are in gaseous phase.

Suspension Polymerization

(Pearl Polymerization) If the monomer is insoluble in water, polymerization can be carried out in suspended droplets, i.e., monomer is mechanically dispersed. The water phase becomes the heat transfer medium. Since it (the water) is a continuous phase, viscosity changes very little as the monomer converts to polymer, so the heat transfer is very good. In this system, the monomer must be either 1) insoluble in water or 2) only slightly soluble in water, so that when it polymerizes it becomes insoluble in water.
The behavior inside the droplets is very much like the behavior of bulk polymerization, but since the droplets are only 10 to 1000 microns in diameter, more rapid reaction rates can be tolerated (than would be the case for bulk polymerization) without boiling the monomer.

The advantages are better heat control of the reaction, and separation is much easier than in solution polymerization. The disadvantage is that few monomers are water soluble.

Emulsion Polymerization

The "ingredients" for an emulsion polymerization include 1) a water soluble initiator, 2) a chemical emulsifier, and 3) a monomer that is only slightly soluble in water, or completely immiscible.

The two differences between emulsion and suspension polymerization are: 1) that a suspension polymerization is a mechanical process, and must have a stabilizing agent until the droplets are far apart, and 2) the emulsion polymerization is a chemical process which requires a surfactant to make the monomer "emulsify."

Disadvantage- the surfactant is a soap and it contaminates the polymer.
Advantage- better heat control; the size of the emulsion polymer is usually 0.05 to 5 microns, and the size of the droplets is usually in the 10- 1000 micron diameter range.

Water-soluble initiators are used rather than monomer-soluble initiators. The end product is usually a stable latex--an emulsion of polymer in water rather than a filterable suspension.

 

Precipitation Polymerization

The polymers are insoluble in its monomer or the solvent.

Solid-phase Polyemrization

Some monomers can be polymerized from the crystalline solid state, such as styrene, acrylonitrile, methacrylonitrile, formaldehyde, trioxane, etc.

We are going to discuss more details about the four types of polymerization:

1. Bulk
2. Solution
3. Suspension
4. Emulsion

and the following six concerns:

1. Initiation
2. Propagation
3. Termination
4. Molecular Weight
5. Rate (kinetics)
6. Heat effects (thermodynamics)

Addition polymerizations are usually carried out bulk and solution polymerizations.

Condensation polymerizations are carried out mostly without solvents. The polyerization of polyethylene terephthalate (PET), the plastic used for 2 liter soda bottles is an example of a bulk condensation reaction.

Bulk addition polymerization is a homogeneous process which uses an organic initiator. Two possibilities:

• polymer soluble in monomer (example: polystyrene)
• polymer insoluble in monomer (example: vinylidine chloride)

The above probably implies that polyvinylidine chloride precipitates out of the solution of vinylidine chloride monomer when it reaches a certain molecular weight, but that polystyrene is soluble in styrene to infinite molecular weight.

If the polymer is soluble in the monomer, then some physical changes occur with increasing molecular weight (e.g., viscosity, etc.)

If the polymer is insoluble in the monomer, the rate of initiation is proportional to the monomer concentration, the initiator concentration.

The higher the temperature, the lower the molecular weight of the polymer produced. It is thought that the notes here allude to the idea that at higher temperatures, the initiator decomposes to form radicals at a faster rate, then

• for a given amount of monomer
• with more radicals present
• more polymer chains will be started (initiated), and
• the resulting polymers will have a lower molecular weight.
  • Example: If you have 2000 monomer molecules, and 5 chains are initiated, then the average molecular weight will be 400, but if 10 chains are initiated, then the average molecular weight will be 200.
  • All of this ignores the question "wouldn't a higher temperature speed up the polymerization reaction?"
  • If we are to assume the concluse "higher temperature lowers molecular weight", then we must conclude that the effect on the initiation step is more dramatic than on the polymeriation step.

We can have continuous polymerization at very low temperatures if we use light to convert the initiator molecules to radicals (which will start the polymerization.)

Bulk polymerization has a built in hazard. The thermal conductivity of monomers and polymers is low, and as the viscosity builds up, the ability for heat transfer via convection is substantially diminished. If the heat energy cannot be dissipated, temperature rises, and at higher temperatures the reaction is going to go faster, so this is a positive feedback loop with disastrous consequences.

For bulk polymerization, removal of unreacted monomer can be a problem. This is a large concern if your safe polymer was prepared from monomers which are toxic. The Federal Drug Administration puts limits on how much monomer can be present in a polymer system used for food containment.

For solution polymerization where the polymer is insoluble in the solution above a certain molecular weight (i.e., the polymer precipitates out at that molecule weight) then the viscosity is more likely to remain fairly constant. Dimerization termination is more likely, and the rate of chain transfer is faster. Heat effects are much better.

Solution polymerization and bulk polymerization are carried out at the highest temperature (i.e, the highest temperature within reason- you don't want decomposition of monomer or polymer.) because condensation reactions can occur on the timescale of hours or days.


 

Howework:

1. Define the reactivity ratios r1 and r2, and indicate their values for a) ideal  b) alternating
2. Consider the copolymerization of methyl acrylate (1) and vinyl chloride (2). The following compositions were found:

Calculate the reactivity ratios.