Polymers from vinyl monomers, CH2=CHX, and vinylidene monomers, CH2=CY2.
Another synthesis of acrylic acid
A. Polystyrene and related polymers
Polystyrene is an inexpensive and hard plastic, and probably only polyethylene is more common in your everyday life. The outside housing of the computer you are using now is probably made of polystyrene. Model cars and airplanes are made from polystyrene, and it also is made in the form of foam packaging and insulation (StyrofoamTM is one brand of polystyrene foam). Clear plastic drinking cups are made of polystyrene. So are a lot of the molded parts on the inside of your car, like the radio knobs. Polystyrene is also used in toys, and the housings of things like hairdryers, computers, and kitchen appliances.
Polystyrene is a vinyl polymer. Structurally, it is a long hydrocarbon chain, with a phenyl group attached to every other carbon atom. Polystyrene is produced by free radical vinyl polymerization, from the monomer styrene.
Production is 7 to 9 billion pounds per year with a cost of 50 cents a lb (1991.)
CH2=CH2 + benzene ----> benzene-CH2CH3 -------> benzene-CH=CH2
boiling pt. 139 oC boiling pt. 145 oC
A Friedel-Crafts reaction (AlCl3 as catalyst) is carried out by treating benzene with ethylene in the liquid phase.
Bulky polymerization has a broader molecular weight distribution than polymer prepared at one temperature. Suspension polymerization Advantages-
· The heat transfer problem associated with bulk polymerization is simplified.
· There isn't a problem with solvent removal and recovery that is associated with solution polymerization.
Disadvantages- There is an added drying step and this process does not
readily lend itself to a continuous operation.
The polymerization is carried out batch-wise in a stirred reactor which is jacketed for eating and cooling. A typical formulation might be as follows
(quantities shown as parts-by-mass):
Styrene (inhibitor free) 100
water (demineralized) 70
tricalcium phosphate 0.8 suspending agent
dedecylbenzene sulphonate 0.003 suspending agent
benzyl peroxide 0.2 initiator
Reaction temperature: 90 deg C.
The size of the beads produced depends on:
· on how fast the solution is stirred
· temperature
· suspending agent
· surfactant agent
· ratio of styrene to water
When the polymerization is complete, the product is in the form of slurry, is washed with HCl and water to remove the suspending agent,
centrifuged, dried in warm air (about 60 deg C), extruded, and chopped.
The tacticity is predominantly head-to-tail. Expanded polystyrene is very important as a thermal insulating material. Most commercial methods make use of expandable beads.
Styrene is polymerized as it is in solution polymerization except that a low boiling point hydrocarbon such as n-pentane is added to the system.
This modification results in the formation of polystyrene beads containing 5- 8% volatile hydrocarbon.
The alternative is to treat formed PS beads under heat and pressure with the volatile
hydrocarbon. The impregnated beads are then expanded commonly by treatment with steam. When the beads are heated in the steam, they
soften and volatilization of the low boiling point hydrocarbon and diffusion of steam into the beads causes the beads to expand to about 40
times their original size. At this stage the beads aren't fused together.
Pressure is reduced back to atmospheric pressure, and temperature is decreased back to room temperature, and this causes air permeation
into the beads. The beads are loaded into a mould through which steam is passed, and they expand again by a small amount, and the enclosure
of the mold consolidates the beads into a block.
Styrene-Acrylonitrile Copolymers (SAN)- SAN copolymers contain 20 to 30% acrylonitrile copolymerized by solution polymerization.
Compared to PS homopolymer, SAN copolymers have a higher softening point and improved impact strength. SAN copolymers have a higher resistance to hydrocarbons and oils than PS homopolymer because of the polar nature of the acrylonitrile.
This ties into the ideas of solubility parameters and compatibility. The inclusion of the acrylonitrile monomers makes the difference in solubility
parameters between the polymer and the solubility parameters of hydrocarbons and oils more significant.
The higher the acrylonitrile content, the greater the heat resistance, impact strength, and chemical resistance but the ease of molding declines.
Higher mechanical strength usually corresponds to an increase in temperature resistance. Molding requires that the polymer flow at temperatures
below the temperature where degradation occurs.
Styrene Butadiene Rubber was synthesized during World War II as a substitute after the Japanese took control of the natural rubber plantations.
It is the most important of all copolymers of styrene in terms of volume.
Acrylonitrile-Butadiene-Styrene terpolymers (ABS resins)- The impact strength of SAN is higher than the impact strength of PS, but it is
still sufficiently low to make it a limiting factor in many applications. SBS rubber is a thermoplastic elastomer.
Adding a rubbery material such as butadience would improve impact properties. ABS polymers are prepared by either 1) blending or 2) grafting.
Blending: SAN copolymers and ANB (acrylonitrile butadiene) copolymers are blended together.
Example:
70 parts of a 70:30 styrene : acrylonitrile copolymer
40 parts of a 35:65 acrylonitrile : butadiene copolymer
The material must be crosslinked with peroxide type free radical reaction in order to obtain the desired properties (the mixture is not completely
soluble in the copolymer), which are high impact strength and high softening points.
Another way is to mix the solids on a two-roll mill. The non-crosslinked butadiene rubber may be used as a starting material. The rubber is
crosslinked with peroxide by milling and then SAN copolymer is added.
Grafting ABS: acrylonitrile and styrene are polymerized in the presence
of a polybutadiene. Mechanically blend (parts by weight):
polybutadiene latex solids: 34
acrylonitrile 24
styrene 42
water 200
surfactant 2
mercaptants 1 (transfer agent)
potassium persulfate 0.2 (initiator)
carried out at 50 deg C a solid product is isolated
The two methods mentioned above differ in the following:
· Blending is a blend of two copolymers done physically followed by crosslinking.
· Grafting starts with a backbone of polybutadiene to which SAN is grafted and then copolymerized.
The two end products don't differ much after they have been crosslinked.
Once crosslinked, high impact strength and high softening points are achieved. The ABS prepared by grafting consists of mixtures of polybutadiene, polybutadiene grafted with acrylonitrile and styrene, and SAN copolymer. Grafted ABS is more branched than blended ABS. Grafted ABS is superior to blended ABS in that moulded specimens commonly have a
better surface appearance.
Applications for ABS include automobile armrests and panels, suitcases, shoe heels, and sewer pipes.
There's a new kind of polystyrene out there, called syndotactic polystyrene. "Normal" or atactic polystyrene has no order with regard to the
side of the chain on which the phenyl groups are attached.
The new syndiotactic polystyrene is crystalline, and melts at 270 oC. But it's a lot more expensive!
Syndiotactic polystyrene is made by metallocene catalysis polymerization.
B. Acrylic polymers
PMMA is also found in paint. But PMMA is more than just plastic and paint. Often lubricating oils and hydraulic fluids tend to get really viscous and even gummy when they get really cold. This is a real pain when you're trying to operate heavy equipment in really cold weather. But when a little bit PMMA is dissolved in these oils and fluids, they don't get viscous in the cold, and machines can be operated down to -100 oC, that is, presuming the rest of the machine can take that kind of cold!
PMMA is a vinyl polymer, made by free radical vinyl polymerization from the monomer methyl methacrylate.
PMMA is a member of a family of polymers which chemists call acrylates, but the rest of the world calls acrylics.
Another polymer used as an unbreakable glass substitute is polycarbonate. But PMMA is cheaper!
Properties of PMMA
Also, sometimes we make copolymers of acrylonitrile and vinyl chloride. These copolymers are flame-retardant, and the fibers made from them are called modacrylic fibers.
But the slew of copolymers of acrylonitrile doesn't stop there. Poly(styrene-co-acrylonitrile) (SAN) and poly(acrylonitrile-co-butadiene-co--styrene) (ABS), are used as plastic.
ABS is more complicated. It's made by polymerizing styrene and acrylonitrile in the presence of polybutadiene. Polybutadiene has carbon-carbon double bonds in it, which can polymerize, too.
ABS is very strong and lightweight.
ABS is a stronger plastic than polystyrene because of the nitrile groups of its acrylonitrile units. The nitrile groups are very polar, so they are attracted to each other. This allows opposite charges on the nitrile groups to stablize each other like you see in the picture above. This strong attraction holds ABS chains together tightly, making the material stronger. Also the rubbery polybutadiene makes ABS tougher than polystyrene.
Polyacrylonitrile is a vinyl polymer, and a derivative of the acrylate family of polymers. It is made from the monomer acrylonitrile by free radical polymerization.
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.
PAN can be polymerized in aqueous solutions of concentrated inorganic salts such as calcium thiocyanate, sodium perchlorate, and zinc chloride.
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.
When this monomer meets trace amounts of moisture, it polymerizes by anionic vinyl polymerization quickly.
Acrylamide- Acrylamide can be prepared by the hydrolysis of acrylonitrile.
CH2=CHCN + H2O à CH2=CHCONH2
Poly(acrylamide) (PAM) is water soluble to infinite molecular weight.
PAM is prepared by a free radical reaction, but it also can be prepared via alkoxide (RO-) initiation:
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
Thermosetting resins
(polymers) are those that change irreversibly under the influence of heat
from a fusible and soluble material into one which is infusible and insoluble
through the formation of a covalently crosslinked, thermally stable network.
Usually thermoset acrylics are terpolymers (made from three different monomers.)
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.
Applications of thermoset acrylics range from very flexible
coatings needed for strip metal to the hard chemical resistance needed for
domestic appliances.
C. Poly(vinyl acetate) and derived polymers
Poly(vinyl acetate), or PVA for short, is one of those low-profile
behind-the-scenes polymers. It isn't blatantly obvious where it's found, as is
the case with polyethylene or polystyrene. PVA is used to make wood glues, as well as other adhesives.
Paper and textiles often have coatings made of PVA and other ingredients to make
them shiny.
PVA is a vinyl polymer, as if you could guess from the name. It's made by free radical vinyl polymerization of the monomer vinyl acetate.
PVA is the latex in acrylic latex paint.
When we do this we clip off all those acetate groups to end up with another polymer, poly(vinyl alcohol). But for paint we don't really want to clip all of the acetate groups. We can control this reaction so that when we're done we still have about 20% of the acetate groups left on the polymer. what we have then is a copolymer of poly(vinyl alcohol) and poly(vinyl acetate) called poly(vinyl alcohol-co-vinyl acetate), appropriately enough. It's a random copolymer, that looks like poly(vinyl alcohol), except with vinyl acetate repeat unit every now and then, like this:
But why do we want to do this? It has to do with how acrylic paints work. Acrylic paints contain poly(methyl methacrylate) in a solvent which evaporates as the paint dries.
Poly(methyl methacrylate), or PMMA, is a hard, tough, and shiny plastic, and if when it forms in the wet paint, it makes the paint surface, hard, tough and shiny. This is good. We want paint to do this. But there's a problem. PMMA is hydrophobic. It doesn't dissolve in water, and a lot of paints are water based.
Poly(vinyl alcohol)
PVA is not prepared via addition polymerization because of the following
keto-enol tautomerism:
CH2=CHOH ßà CH3CHO
Poly(vinyl alcohol) is prepared by the hydrolysis of poly(vinyl acetate.)
PVAc is hydrolyzed by treating an alcoholic solution of
PVAc with aqueous acid or alkali.
Acid hydrolysis results in traces of the acid in the polymer which are difficult
to remove and which promote polymer instability.
Alkali hydrolysis results in the contamination of the product by large amounts
of sodium acetate which are hard to remove and have little intrinsic value.
These difficulties are avoided by using small amounts of base as catalyst.
Poly(vinyl alcohol) has substantial head-to-tail tacticity. The treatment of
poly(vinyl alcohol) with HIO4 shows
Poly (vinyl alcohol) is soluble in highly polar solvents such as DMSO, DMF, and water.
D. Poly(vinyl chloride) and related polymers
Poly(vinyl chloride) is the plastic known at the hardware store as PVC. This is the PVC from which pipes are made, and PVC pipe is everywhere. The plumbing in your house is probably PVC pipe, unless it's an older house. PVC pipe is what rural high schools with small budgets use to make goal posts for their football fields. But there's more to PVC than just pipe. The "vinyl" siding used on houses is made of poly(vinyl chloride). Inside the house, PVC is used to make linoleum for the floor. In the seventies, PVC was often used to make vinyl car tops.
PVC is useful because it resists two things that hate each other: fire and water. Because of it's water resistance it is used to make raincoats and shower curtains, and of course, water pipes. It has flame resistance, too, because it contains chlorine. When you try to burn PVC, chlorine atoms are released, and chlorine atoms inhibit combustion.
Structurally, PVC is a vinyl polymer. It is similar to polyethylene, but on every other carbon in the backbone chain, one of the hydrogen atoms is replaced with a chlorine atom. It is produced by the free radical polymerization of vinyl chloride.
Vinyl chloride is prepared from acetylene and ethylene. The preparation of poly(vinyl chloride) is mainly done by suspension polymerization, carried out batchwise in a stirred reactor, jacketed for heating
and cooling. The reaction is run at 50 to 100 deg C and the pressure must be 100 psi. Once the reaction is completed, the pressure is essentially
gone (i.e., the monomer is all polymerized.)
This reaction requires monomer, water, surfactant, and stirring of the initiator. The shape of the polymer particles depends on the suspending agent
used. The syndiotacticity of PVC may be increased by polymerizing below -40 deg C, using highly active In (indium) and the result is a more brittle
product. Atactic PVC has better qualities for commercial applications.
Properties of Poly(vinyl chloride)
· PVC is colorless and rigid
· The high chlorine content of PVC renders it flame retarding
· The glass transition temperature of 50-60 deg C precludes the use of PVC for hot water pipes (the hot water would take the polymer into
the rubbery transition and over time there could be dimensional distortion.)
· PVC is soluble in proton acceptor solvents such as THF, esters
· PVC is unaffected by mild acids, basic materials and water.
· exposure to temperatures above 70 deg C and exposure to UV light
· have adverse effects on its properties.
· PVC is attacked by strong oxdizing agents.
Unplasticized PVC is outstanding for its anti-corrosion ability.
Exposure to UV light or heat leads to degradation vi dehydrochlorination
or oxidation.
Dehydrochlorination can occur until all chlorine atoms are gone. The above
reaction leads to a yellowing of the plastic. A Stabilizing agent is added to interfere with the degradation process.
Low temperature chlorination of PVC-
If PVC is chlorinated, via exposure to chlorine gas in a 50 deg C chloroform
solution, and illumination
The extra chlorines increase the glass transition temperature to 100 deg C, and there is an increase in the melt viscosity. Chlorinated PVC is
suitable for industrial and domestic (in your house) plumbing for hot effluents (hot water.)
High temperature chlorination of PVC
· conducted in solution at 100 deg C
· substitution occurs extensively at the
· soluble in low cost solvents such as acetone, butylacetate, and methylene chloride
· useful for adhesives, protective coatings, spinning fibres, and chemical filter cloth
· low softening point, low impact strength
· poor color stability
Copolymers of PVC- vinyl acetate comonomer is added to increase the
solubility and to improve the moulding characteristics by lowering the
temperature necessary for polymer flow.
Poly(vinylidene chloride) is polymer that's only used for one thing, but it's an important thing. PVDC, as we call it for short, is the plastic wrap
that food comes in at the grocery store, and that you put over your dish of casserole that you're taking to the pot luck supper. Dow Chemical
makes this stuff, and calls it Saran.
Poly(vinylidene chloride) is a vinyl polymer. As you might be guessing, it is made from the monomer vinylidene chloride, using free radical vinyl polymerization like this:
Vinylidene chloride copolymers (5-12% vinylidene chloride)
The properties are similar to the poly(vinyl chloride-co-vinyl acetate)
copolymer.
Poly(vinyl choride-co-vinylidene chloride) is used for calendering applications
and filler polymer in rigisols.
Acrylonitrile copolymer (60% VC and 40% AN)- used for nonflammable
fibres, and industrial garments. Noted for good chemical resistance.
Olefin Copolymers (3-10% ethylene or propylene)- used for blow-moulded
bottles.
Vinylidene chloride production
The liquid phase chlorination of vinyl chloride at 30 to 50 deg C under
pressure (the first arrow reaction shown above--the second arrow reaction
probably requires heat.)
The liquid phase chlorination of ethylene dichloride (the first arrow
reaction shown above--the second arrow reaction probably requires heat.) The properties
high crystallinity and high crystalline melting point (220 deg C.) Applications include filaments, car upholstery, garden chair fabrics, toughness, chemical resistance, and packing films. The main uses of poly(vinylidene chloride-co-acrylonitrile) copolymers include coatings for materials such as cellophane, paper, and polyethylene.
The coatings confer moisture and gas impermeability (due to the chlorines) and they are heat-sealable. Prepared by the liquid phase chlorination
of vinyl chloride at 30 to 50 deg C under pressure.
PTFE is a vinyl polymer, and its structure, if not its behavior, is similar to polyethylene. Polytetrafluoroethylene is made from the monomer tetrafluoroethylene by free radical vinyl polymerization.
Unique properties associated with fluoropolymers
Homework:
2. Write chemical equations for (a) the polymerization of poly(vinyl acetate), (b) the formation of poly(vinyl alcohol), (c) the crosslinking of poly(vinyl alcohol) fibers, and (d) the formation of poly(vinyl butyral)
3. Discuss the relative physical properties of polystyrene,
impact polystyrene and ABS resins, and account for differences in terms of
molecular structure.