The Notes for
Polymer and Coatings Science-
Chapter One- part three



This is polystyrene:



For polystyrene, the argument is made that head-to-tail predominates because placing a radical on the "head" carbon is more stable than placing the radical on the "tail" carbon, because when the radical is placed on the "head" carbon it can delocalize over the entire benzene substituent group.


Initiators

The most common is benzoyl peroxide:

             O           O
             ||         ||
    phenyl - C - O - O - C - phenyl
which undergoes cleavage between the two oxygens (two very electronegative atoms don't like to be placed next to each other.)

If there are substituent groups on the phenyl rings of a benzoyl peroxide, they affect the temperature at which cleavage of the peroxide into two radicals occurs. If the substituent groups are electron withdrawing, the cleavage occurs at a slower rate for a given temperature. If the substituent groups are electron donating, the cleavage occurs at a faster rate for a given temperature.

Decomposition of benzoyl peroxide rate as a function of solvent at 80 deg C:


Solvent                1 hour      4 hours

Tetrachloroethylene    13          35
Cyclohexene            14
Toluene                17          50
Dioxane                82
Cyclohexane            51          84
I'm not sure what the units here are ?? --percent decomposition? If this were the case, then the 50 for Toluene/4 hours would indicate that the half-life of benzoyl peroxide in toluene at 80 deg C is 4 hours.

Species which have saturation in them tend to increase the rate of decomposition (i.e., faster production of the radical.) This is due to the stabilization of the electron cloud (e-) for the free radical species.

With regard to temperature, one must consider the problem of a "positive feedback loop." It is possible that the temperature needed to cause an initiator to cleave is sufficient to cause the polymerization to generate heat faster than the heat exchangers can remove it. The temperature increases, and the reaction procedes still faster, etc. until reactor containment is breached. Just something to think about.

Nitrosoacylanilide is another initiator: 
             O
             ||
phenyl - N - C - R
         |
         N
         ||
         O


"Nitrogen type" initiators are also possible, of which azobisisobutylnitride, shown below, is an example.


      CH3         CH3
      |           |
CH3 - C - N = N - C - CH3
      |           |
      CN          CN


Inorganic salts as initiators: The notes make this comment, but don't show anything. Later in the semester, the topic of room temperature peroxides that require the use of inorganic accelerators will come up.

Molecular weight control

In a polymerization process, the aim is to control the molecular weight. The following variables influence the molecular weight:

  1. Temperature
  2. Solvent
  3. Concentration of Initiator
  4. Concentration of Monomer
  5. Type of Initiator

The solvent medium

If ionic catalysis is used in a heterogeneous system, where there is an insoluble catalyst, a soluble monomer, and a soluble polymer, the rate of diffusion of the monomer to the catalyst surface may control the overall polymerization rate. If this is the case, the intensity of solution agitation (stirring :), the catalyst particle size, and the viscosity of the solvent are important.

Rate of Initiation- K(i) [catalyst] [monomer]

Rate of Termination- K(t) [Monomer ion+]


K(i) [catalyst] [monomer] = K(t) [monomer ion+]

The rate of reaction depends on:
  1. The concentration of monomer
  2. The concentration of catalyst
The carbanion process

This terminology refers to an anionic mechanism. An anion exists in basic conditions. An example would be 
        O                                            
        ||  -   +                                      
  H C - C - O   Na  
   3
The monomer must be capable of sustaining a negative charge. Anionic polymerization is possible for step reactions.

The notes show 
                              -
  Bu: + C=C  --->  Bu - C - C:
          |                 |
          X                 X
and states that X must be an electron withdrawing group. Examples include ester, - C (triple bond) N, -NO2, -C - O-CH2-CH3.

Catalysts for anionic processes include NaNH2, BuLi, RMgX (Grignard), NaH, all of which are very strong basic catalysts.

Anionic polymerization produces a very narrow molecular weight polystyrene that can be used as a gel permeation chromatography (gpc) standard. See also polydispersity

Organometallic catalysis- It appears from the notes that organometallic catalysts are used to promote the formation of carbanion species for anionic polymerization. A magnesium is shown with a positive charge, which originated from an R-M, stabilizing a negative charge on what was previously a carbon from a C=C, and that the vacant orbitals of magnesium are available for this interaction. Stereochemical consideration- For the free radical process, the intermediate could be attacked with an equal probability (recall racemization.) Now, the presence of the magnesium ion on the "back" side of the reaction site makes a "front" side attack more likely and an isotactic polymer (as opposed to an atactic polymer ) is more likely.

To terminate an anionic polymerization, you add some (any) positive species to act as the termination agent: water, alcohol, acids, etc. Acrylic acid does not work for anionic polymerization because it has an active hydrogen. (Editor note: I believe the term active hydrogen is synonymous to "labile hydrogen" or "acidic hydrogen", and it refers to a hydrogen that can "fall off" the organic molecule and act as an acid. Alcohols and carboxylic acids have active hydrogens.)

The kinetic equations will have k(i), k(p), and k(t) terms for chain transfer, disproportionation, and termination just as the free radical mechanism kinetic equations did.

Examples of Complex System Catalysis- Inorganic catalyst systems that use 
AlR  - TiCl     or    AlR  - TiCl
   3       3             3       4
are called Ziegler-Nalta systems.



Later on, we will want to put some pictures here. The CHEM 381 student is advised to look up the pictures of ZN systems in a textbook and to know them for the test.

in 1955, Giulio Nalta demonstrated that stereo-specific polymers could be produced with synthetic catalysts (co-ordination compounds). The systems polymerized were primarily X-olefins (someone ask Stoffer what the X stands for) polymerized by Ziegler catalysts.

Ziegler-type catalysts originally involved the formation of a complex precipitate of some unspecified color from aluminum triethyl and titanium tetrachloride. Ziegler-Natta catalysts made it possible to synthesize isotactic polypropylene.

              hexane
AlEt  + TiCl  -----> TiCl Et + AlEt Cl
    3       4            3         2
The reaction also produces some volatile by-products.

Under the Ziegler-Natta (ZN) process, ethylene is bubbled into the suspended catalyst at room temperature, and it rapidly polymerizes to a high molecular weight, linear polyethylene. Linear polyethylene (as opposed to branched polyethylene) has a higher crystallinity, so that the product differs significantly from polyethylene prepared from a high pressure, free radical process (I guess we can infer from this that the high pressure, free radical process produces a branched polymer.

Pictures of the initation, propagation, and termination steps for ZN polymerization are available in the notes, but from these it is not really clear what is happening so it will be necessary for the CHEM 381 student to look for another reference.

Diolefins (monomers with two double bonds) such as butadiene (CH2=CH-CH=CH2) and isoprene yield both cis- and trans- 1,4 polymers from Ziegler Natta catalyzed processes. Variations in the metal halide and/or the ratio of alkyl to halide give different structures.

the following two statements are not logical, and require a subsequent explanation: ?? From just looking at the above two sentences, you would assume that you were using a TiCl4-AlR3 molecule, and that from stoichiometric balance, there would always be as many Ti atoms as Al atoms. It is speculated here (this could be wrong!) that for a Ziegler Natta system, you have titanium tetrachloride (TiCl4) and aluminum trialkyl (AlR3), and that you mix them together on site and that you most likely WILL NOT be mixing together molar equivalents. Ask Stoffer to confirm in class if this is what is done. The notes do say that the final Ziegler Natta product is a complex mixture of organo-aluminum and titanium compounds, that depends on the relative proportions of starting materials, time and temperature of the reaction.

The essential feature of the Ziegler Natta process is that monomers are inserted, one after the other, into a polarized titanium carbon bond. The polymer grows out of the active center, rather as a hair grows from the root.

The termination step (I assume the notes are still talking about Ziegler Natta): The propagating end of the polymer chain is negatively charged and therefore, the reaction may be regarded as an anionic polymerization. Chain growth may be terminated by several different processes: A chain transfer type reaction may be brought about by the addition of an active hydrogen compound such as an alcohol: Cat-CH2CHR-[-CH2-CHR-]n-CH2CH3 + R'-OH --> Cat-OR' + CH3-CHR-[-CH2-CHR-]n-CH2CH3

The primed alkyl (R') could be the same or different from the unprimed alkyl. The above chain transfer breaks a metal bond.
In general, if an organometallic bond is broken, a proton goes to the carbon where the organometal was previously attached (you saw this in your first semester of organic chemistry when you were dealing with the Grignard, R-Mg-X.) A disadvantage of Zeigler Natta catalysts is that the catalyst is dispersed in the final product.

Metal oxides have been used to give linear polyolefins, but they have not been used for isotactic structures. Nickel, cobalt, chromic, or vanadium oxides supported on silica-alumina have been effective for ethylene and olefin copolymers at low temperatures. An inhibitor prohibits a reaction from occuring, and is typically used at concentrations of 10% or less. Usually 0.025 is enough to stop a process from happening. An example includes butylated hydroxy toluene. This is essentially a benzene ring with a methyl group on it, a hydroxy group opposite to the methyl group, and a methyl group on each side of the hydroxyl group.

A retarder slows down a reaction.

(Williams- 84)

This is a replication of a figure in the notes. The y axis might be the extent of polymerization. We night call the x-axis time, but that really isn't true. This ties back in to that Organic I idea that you should call the x-axis the "reaction coordinate."

Questions to ask:
You can add enough initiator to overcome the inhibitor concentration. You could think of the initiator as deactivating the inhibitor.





Last Update- July 8, 1995- wld