The Notes for
Polymer and Coatings Science-
Chapter Two
Bulk Polymerization of Poly(methylmethacrylate)
An example:
- Stir in the monomer with 5% benzoyl peroxide at 90 deg C for 10 min, which
results in a syrup which can then be cooled to room temperature.
- Add colorant, pasticizer, and UV absorbers if necessary
- Pour the syrup into casting cells
- Pass the fill cell through a heating tunnel, with a temperature maintained
at 40 deg C for 15 hours, then finish the polymerization at 95 deg C for 1
hour.
- Cool and remove from the cell
- Bubbles may form because of the exothermic reaction if the casting has
a thickness greater than 2 cm. If bubbles are a problem, the polymerization
may be carried out under pressure to increase the temperature at which the
monomer boils (so that the monomer won't boil during the polymerization.)
- Molecular weights on the order of millions (10^6) may be obtained.
Solution Polymerization of Poly(methylmethacrylate)
- used mainly for the production of polymer for surface coatings
- solvents used include methyl ethyl ketone, benzene and toluene
- initiator- peroxide (benzoyl peroxide)
- carried out at 90 to 110 deg C
- monomer is added gradually over 1 to 4 hour period
- a polymer with a wide molecular weight distribution is produced, with
an average around 90,000
- The final product is 40- 60% solution (I'm not sure what this means:
possibly that for every 100 lbs of solution, you have 40 to 60 pounds
of dissolved PMMA.
Suspension Polymerization of Poly(methylmethacrylate)
- used mainly for production of injection molding with extrusion grades of
polymer
- carried out batchwise in a stirred reactor; the reactor is jacketed for
heating and cooling
Example:
- 100 part by weight of water
- 200 part by weight of MgCO3
- 0.2 part by weight of benzoyl peroxide (the initiator)
- carried out under nitrogen atmosphere
- polymerization time is rapid (1 hour)
point of order- I disagree on calling a 1 hour polymerization "rapid."
Interfacial polymerizations can occur on the order of a fraction of
a second--now that deserves the term "rapid."
- suspension is cooled and acidified in sulfuric acid to remove the
suspending agent. The beads of polymer are filtered off, washed, and
dried to give an average molecular weight of 60,000.
Emulsion Polymerization of Poly(methylmethacrylate)
- nonaqueous emulsion techniques are used for the preparation of surface
coatings
- a very low viscosity media-suitable for coatings (??this sentence fragment
does not make sense)
- not suitable for automobile lacquers--this may be overdone (overcome??)
by adding an organosol in which the polymer is in the form of a stable
dispersion in a suitable organic liquid.
Properties of PMMA
- hard, rigid, transparent (very clear to see through)
- softening point at 125 deg C
- tougher than polystyrene but less tough than ABS (acrylobutylstyrene)
polymer
- absorbs very little visible light but there is a 4% reflection at
each polymer-air interface for normal incident light.
Strictly speaking, the term "absorbance" applies only to radiation that
is trapped and then usually re-emitted at a lower frequency (heat rather
than light.) If you are standing on the other side of a polymer sheet, and
you want light to come through the polymer so you can see what is
on the other side, than the problem of reflectance must be considered.
Since 4% of the light reflects back at the air/polymer interface, and
then another 4% is lost at the polymer air interface, only 92% of the
light makes it through (so says the notes, actually, since 4% of 96
is 3.84, and 96 - 3.84 = 92.16, you get 92.16 % of the light, assume the
4% number is accurate to 4.00%, anyhow)
- PMMA is a polar material and has a rather high dielectric constant
and power factor (what is a power factor??)
- a good electrical insulator at low frequencies but less satisfactory at
higher frequencies
- good water resistance
- PMMA prepared by free radical polymerization is amorphous and is therefore
soluble in solvents with similar solubility parameters such as benzene,
toluene, chloroform, methylene chloride, esters, ethyl acetate,
and amyl acetate.
- PMMA has good resistance to alkalis (sodium hydroxide, etc.), aqueous
inorganic salts (the Pacific Ocean) and dilute acids.
- PMMA has a better resistance to hydrolysis than PMA probably by
virtue of the shielding of the methyl group.
- PMMA's outstanding good outdoor weather resistance is marketably
superior to other thermoplastics.
- When heated about 200 deg C, decomposition becomes appreciable and at
350- 450 deg C, a nearly quantitative yield of monomer is readibly
obtained. Thus, the recovery of monomer from scrap is feasible.
- Because it's a thermoplastic, it can be molten and molded (at 100 to
150 deg C) into anything we want.
- a syndiotactic polymer can be polymerized. At lower temperatures,
the stereochemistry of the polymer can be controlled by means of the
solvent.
Poly(acrylonitrile) (PAN) can be polymerized from acrylonitrile
using a monosuspension process. ??So hows does monosuspension compare
to general suspension??
The major use for poly(acrylonitrile) is textile (clothes) fibers.
PAN is toxic only if burned, in which case HCN (the "cyanide gas" used
for gas chambers) is produced. When PAN is solution polymerized,
organic solvent, the solution can be used for fiber spinning. Suitable
organic solvents include dimethylacetamide, DMF, DMSO. Fibers prepared
from pure PAN are difficult to dye, so a minor portion (~10%) of
comonomers such as methyl methacrylate, vinyl acetate, or N-vinyl pyridine
are added to improve dyeability.
H CH3 H H H H
| | | | | |
C = C C = C C = C
| | | | | |
H C=O H O H N
| | / \
O-CH3 C=O C C=O
| | /
CH3 C-C
Methyl Vinyl N-Vinyl
Methacrylate Acetate Pyridine
PAN can be polymerized in aqueous solutions of concentrated inorganic salts
such as calcium thiocyanate, sodium perchlorate, and zinc chloride.
Emulsion polymerization of PAN is problematic because the monomer has
an appreciable water solubility and the formation of polymer in the
aqueous phase (for emulsion polymerization, polymerization is supposed
to occur in the surfactant droplets!) can lead to coagulation of the
latex.
(I'm not certain, but I think this is trying to say that from
emulsion polymerization, you are supposed to get these nice little
beads, but since the polymerization occurs in the water phase, you can
end up with huge chunks of solid polymer.)
The molecular weight of PAN molecules ranges from 80,000 to 170,000.
PAN is extremely soluble in polar solvents such as THF and DMOS.
In PAN, appreciable electrostatic forces occur between the dipoles of
adjacent nitrile groups on the same polymer molecule. This intramolecular
interaction restricts bond rotation and leads to a stiff chain. As a result,
PAN has a high crystalline melting point (Tm) of 317 deg C and is soluble
only in the solvents mentioned above.
C C C C C C C C
/ \ / \ / \ / \ / / \ / \ / \ / \ /
C C C C C C C C
| | | | -------> | O | O | O |
C C C C C C C C
\\\ \\\ \\\ \\\ / \ / \ / \ /
N N N N N N N
: : : : : : :
I believe the above reaction is some kind of oxidation because the notes
show oxygen over the reaction arrow. The reaction leads a ladder polymer
with aromatic rings.
The heating of this polymer to temperatures of 1500- 3000 deg C (hellacious
temperatures which disrupt most organic matter into water and carbon dioxide)
result in a graphite carbon skeleton structure with great strength.
Poly(acrylamide) (PAM) is water soluble to infinite molecular weight.
To me, this expression "soluble to infinite molecular weight" implies that
as long as there is enough solvent, you can keep adding to the molecular
weight a a single chain, and it will not phase separate. I think though,
that for a give mass of polymer, there is critical mass of solvent that
must be present so that the entire polymer molecule can be covered, otherwise,
in the places where there was no water, the polymer would bind to itself
just the same as a precipitated polymer. Anyhow...
PAM is prepared by a free radical reaction, but it also
can be prepared via alkoxide (RO-) initiation:
H H H H
| | | |
RO- + C = C ----> C - C- (the second carbon has a negative charge)
| | | |
H C=O O-R C=O
| |
NH2 NH2
H H And the negatively charged -NH is going to act like
| | the RO- above, ripping the hydrogen away from
----> C - C-H ----> the second carbon of another acrylamide monomer,
| | and leaving behind a pair of electrons, etc.
H C=O
|
NH
(-)
PAM is hard and brittle. It is readibly soluble in cold water, and
is slightly soluble in organic compounds because of its polarity. It
undergoes reactions characteristic of the amide group.
Applications:
PAM is used as a flocculant in the processing of minerals, the treatment
of industrial wastes. Copolymers which incorporate acrylamide increase
the dry strength of paper.
Thermosetting Acrylic Copolymers
Usually thermoset acrylics are terpolymers (made from three different monomers.)
- the first monomer confers hardness and rigidity to the polymer (examples below)
- acrylonitrile
- methyl methacrylate
- styrene
- vinyl toluene
- the second monomer contributes flexibility to the polymer (examples below)
- ethyl acrylate
- 2-ethylhexyl acrylate
- butyl methacrylate
- the third monomer provides pendant reactive groups which are the sites for
crosslinking (examples below)
- acrylic acid
- butoxymethylacrylamide
- glycidyl acrylate
- hydroxyethyl acrylate
Most thermoset acrylics are prepared by solution polymerization. Solvents used
include butanol and xylene. In order for 1) the solution to have the right
viscosity and 2) the solution to have a satisfactory solides content (40 to 60% by
weight), the molecular weight of the thermoset acrylic must be kept down to
about
20,000 to 30,000 by the use of a relatively high initiator concentration and
high temperature.
To explain the above, if for a solution, the hold the amount of monomer "in the pot"
constant, but increase the concentration of initiator, then the number of
polymer
chains increases, and the degree of polymerization decreases. The part about
the
temperature increase assumes that increasing the temperature has a greater
increase on
the rate of initiator decomposition that the the rate of the polymer
propogation
step.
Applications of thermoset acrylics range from very flexible coatings needed for
strip metal to the hard chemical resistance needed for domestic appliances.
Poly(vinyl acetate)
vinyl acetate is prepared from acetylene and acetic acid.
H H
- | |
H - C - C - H + H C - C - OH --> C = C
- 3 | |
H O
acetylene acetic acid |
C=O
|
CH3
There is a problem with acetic acid reacting with the vinyl acetate to produce
ethylidene diacetate, shown below:
H H O
| | ||
H - C - C - O - C - CH
| | 3
H O
|
C=O
|
CH3
This side reaction is minimized by:
- using a molar axcess of acetylene
- short reaction times
- low temperature
Poly(vinyl acetate) (PVAc) can be prepared from ethylene, starting
with the oxidation of the ethylene. Below, [O] is a generic
term for 'oxidation' that appears again and again in the notes.
H H H O H H O
| | [O] | || [O] Co(Ac) | | ||
C = C -----> H - C - C - H ----->, --------> H - C - C - O - C - CH3
| | | | |
H H H H O
|
ethylene acetaldehyde C=O
|
H H CH3
H+ | |
-----> C = C ethylene diacetate
| |
O
|
C=O
|
CH3
vinyl acetate
In the above, ethylene is oxidized to acetaldehyde with aqueous
palladium chloride. This reaction is carried out as a liquid
phase at a temperature of 100 deg C and a pressure of 1 to 3 atm.
H H H O
| | | ||
C = C + PdCl + H O ----> H - C - C - H + Pd + 2HCl
| | 2 2 |
H H H
Vinyl acetate is a colorless liquid with a boiling point of 73 deg C,
and a pleasant sweet odor (note that it is an ester.) Before
shipment, the monomer is usually inhibited by cupric acetate or
hydroquinone.
Poly(vinyl acetate) may be polymerized by bulk, solution, suspension,
or emulsion techniques. Commercially, emulsion polymerization is
the predominant method.
The emulsion polymerization of PVAc is carried out batch wise in a
stirred reactor which is jacketed for heating and cooling. Below
is a typical formula (recipe.) Note that the numbers below, to
which no specific units have been assigned,
refer to mass.
Vinyl Acetate 100 monomer
water 100 carrier
hydroxyethylcellulose 2.5 protective colloid
poly(ethylene glycol ether
of lauryl alcohol 2.5 surfactant
sodium doelecylbenzenesulphonate 0.1 surfactant
sodium bicarbonate 0.5 buffer
potassium persulfate 0.5 initiator
reaction temperature: 75 to 80 deg C
reaction time: 2 hours
The reaction is highly exothermic, and it is common to polymerize first
only a portion of the monomer, and then to add the remainder of the
monomer slowly over the 2 to 4 hours.
The resulting latex is used "as is" (as opposed to decanting the
water and then drying the solid polymer.) The solid polymer is not
isolated, because the end use is for surface coatings and adhesives.
With regard to tacticity, a head to tail structure is expected.
PVAc is soluble at room temperature in a wide range of solvents:
- benzene
- toluene
- carbon tetrachloride
- methylene chloride
- methanol
- ethanol
- ethyl acetate
- butyl acetate
- acetone
- MIB ketone
Last Update- May 28, 1995- wld