B. Mechanism of Stepwise Polymerization

1. Carbonyl Addition-Elimination Mechanism

a. Direction interaction:   RCOOH + R’OH -- > RCOOR’ + H2O

For example: Nylon 66, the largest produced nylon, is prepared using adipic acid.    Nylon 66 is synthesized by the reaction of diamine and dibasic acid. It is necessary to have an exact stoichiometric equivalence of the reactants. To achieve this, a "nylon salt" can be prepared, where the dibasic acid donates a proton to the diamine. The nylon salt is purified, and when the salt is converted back to acid and salt, there is one diamine molecule for every dibasic acid molecule, and thus, a perfect stoichiometric balance (assuming perfect purification.)

 

The nylon salt solution is fed into an autoclave together with a small amount of acetic acidto limit molecular weight The reactor is sealed and the temperature raised to 220 deg C and a pressure of 20 atm. After 1 to 2 hours the temperature is increased to 270 to 280 deg C and steam is bled off to maintain the pressure of 20 atm.

The heating is continued for two more hours and the pressure is allowed to fall to atmospheric pressure.

The molten polymer is extruded from the reactor by pumping in nitrogen (to protect the melt from oxygen.)

 

The extrudate is fed onto a water cooled drum to for a ribbon which is then chopped.

 

      b. Interchange (between a glycol and an ester to produce polyesters)

      ROH  +  R’COOR”  -- >  R’COOR  +  R”OH

      It is often used to produce polyesters, especially where the dibasic acid has low solubility, such as in

      the production of poly(ethylene terephthalate) from ethylene glycol and dimethyl terephthalate.

      c. Acid Chloride or Anhydride

      RCOCl + R’OH -- > RCOOR’ + HCl   

      Two examples of anhydride reaction:

                A dianhydride and a tetraamine react to form a polyimide that is an example of a ladder polymer.

      d. Interfacial Condensation

This polymerization process occurs when a water soluble monomer and an organic soluble monomer are brought together at the interface between water and a water immiscible organic solvent. The method has been applied to the formation of polyamides, polyurethanes, polyureas, polysulfonamides, and polyphenyl esters.

Example of interfacial polymerization: Nylon can be prepared from NH2CH2CH2CH2NH2 dissolved in water; there is sodium hydroxide (NaOH) in the water to convert the diamine into a salt, which has a higher water solubility.

And ClCOCH2CH2CH2CH2CH2CH2CH2CH2COCl dissolved in the organic solvent.

A nylon fibre is pulled, and attached to a rotating spool. As the spool turns, and pulls the fibre, more polymerization occurs, generating more fibre, until all the monomer is reacted.

Physical properties of nylons:

• High Impact Strength
• Toughness
• Flexibility and abrasion resistance
• the mechanical properties are considerably affected by the amount of crystallinity
• The principle structure difference between different fibres is the length of the aliphatic chain separating the amide groups in the polyamide.
• Crystallinity readily occurs and gives materials of high melting point and tensile strength.
• Increasing the length of the aliphatic segment reduces the melting point, the tensile strength, the heat distortion temperature, and the water absorption, but increases the impact strength and elongation (to break, I believe.)

Elongation is by what percentage you can stretch a polymer before it breaks. As an example if a 10 cm length breaks after stretching to 15 cm, then it is said to have a 50% elongation.

• Textile fibres are usually prepared from nylon 66 or nylon 6 because these polymers have the highest tensile strengths.
• Brushes, sports equipment and surgical sutures are normally nylon 6, nylon 10 or nylon 11, which have greater flexibility and water resistance.
• Nylon fibres have satisfactory electrical insulating ability under low frequencies and dry conditions.
• Nylon fibres are soluble in acetic acid, formic acid, and phenols.
• The oxidation of nylons occurs in air exposure to UV light and or temperature above 70 deg C. This oxidation leads to a distortion in mechanical properties.

2. Carbonyl Addition-Substitution Mechanism. It is general used to synthesize polyacetals.

         RCHO + R’OH -- > RCH(OR’)2

3. Nucleophilic Substitution Reactions (mainly polymerization of epoxides)

For example:

4. Double bond Addition Reaction (mainly heteroatom containing double bond, such as R-N=C=O)

C. Molecular weight control:

One easy way to control the molecular weight of stepwise polymerization is to adjust the composition of the reaction mixture slightly, by adding a slight excess of one bifunctional reactant.

Assume N_A < N_B, the extra B part is called stabilizing group.

Then, after the reaction is completed,    X_n =  (1+r) / (1-r) ,    X_w = 2 X_n 

X_w is the weight average degree of polymerization, X_n is the number-average degree of polymerization, r = N_A / N_B

For example:  If a 1 mol. % of stabilizing groups is added,

X_n =  (1+100/101) / (1-100/101) = 201

X_w = 402

Chapter 5. Copolymerization

A. Kinetics of Copolymerization

Early Experiments in Copolymerization

For copolymerization including M1 and M2, there are four possible ways in which monomer can add:

RM₁· + M₁  -- > R’M₁ .        rate = K₁₁[RM₁.] [ M₁]

RM₁· + M₂  -- > R’M₂ .        rate = K₁₂[RM₁.] [ M₂]

RM₂· + M₁  -- > R’M₁ .        rate = K₂₁[RM₂.] [ M₁]

RM₂· + M₂  -- > R’M₂ .        rate = K₂₂[RM₂.] [ M₂]

The copolymer equation can be deduced as

r₁ and r₂ are called monomer reactivity ratios, r₁=k₁₁/k₁₂, r₂=k₂₂/k₂₁.

So r₁>1 means that the radical RM₁. prefers to add M₁,

     r₁<1 means that the radical RM₁. prefers to add M₂.

In the system styrene (M₁)-methyl methacrylate (M₂), r₁=0.52 and r₂=0.46, each radical adds the other monomer about twice as fast as its own.

A few typical values of monomer reactivity ratios are given in Table 5-1. (see page 103).

Two types of conditions are:

1. Ideal copolymerization—when the two radicals show the same preference for adding one of the monomers over the other.

k₁₁/k₁₂ = k₂₁/k₂₂,   or   r₁=1/r₂,  or  r₁* r₂ = 1

      In this case, the end group has no influence on the rate of addition, and the two types of units are arranged at random along the chain in relative amounts determined by the concentration of the monomers and the relative reactivities of the two monomers. The copolymer equations reduced to:

2. Alternating—each radical prefers to react exclusively with the other monomer

r₁ = r₂ = 0, the copolymer equation simplifies to:

3. If both r₁ and r₂ are > 1, that means the tendency to form block copolymers.

Most polymerizations lie between the ideal and alternating systems: 0 < r₁r₂ < 1.

Homework:

5. Describe briefly in step polymerization a) interchange reactions    b) the advantages of molecular-weight control by added stabilizer    c) how to obtain high molecular weight    d) why an ester is used as a starting material in the preparation of poly(ethylene terephthalate).

8. A Laboratory preparation of nylon-610 ( -[-NH(CH2)6NHCO(CH2)8CO-]- was made with 0.2 mol. % acetic acid (based on carboxyl groups) present as a viscosity stabilizer. The reaction was carried to completion. Calculate Mw and Mn.