POLYMER CHEMISTRY SEM-6, DSE-B3 PART-9, PPT-9

POLYMER CHEMISTRY

SEM-6, DSE-B3 PART-9, PPT-9

Dr. Kalyan Kumar Mandal

Associate Professor

St. Paul’s C. M. College

Kolkata

Polymer ChemistryPart-9

Contents

• Polydienes

• Polycarbonates

Polydienes: Introduction

• Polydienes are polymers which have their monomer as diene, which is essentially two

carbon-carbon double bonds (C=C) occurring in the same molecule. This group of polymers

comprises natural rubber (and related natural materials) and its derivatives together with the

products of polymerization and copolymerization of conjugated dienes. The importance of

the polydienes lies in the fact that they encompass the bulk of the commercial elastomers

currently in use.

• The most important diene elastomers (synthetic rubbers) are styrene-butadiene rubber

(SBR), polybutadiene (PBD), nitrile-butadiene rubber (NBR), polychloroprene (CR) and

polyisoprene (IR). Among these rubbers, polybutadiene and butadiene copolymers (SBR)

have the largest sales volume of the synthetic rubbers.

• Diene elastomers contain unsaturated sites and are often vulcanized (lightly cross-linked) to

create an elastomeric network, which resists irreversible deformation below the break load.

This Lecture is prepared by Dr. K. K. Mandal, SPCMC, Kolkata

Polydienes: Polymers from 1,3-Dienes

• Polybutadiene [PBD; butadiene rubber BR] is a synthetic rubber. PBD rubber is a polymer

formed from the polymerization of the monomer, buta-1,3-diene (CH2=CH-CH=CH2) that is

a simple conjugated diene.

• Polybutadiene has a high resistance to wear and is used especially in the manufacture of tires,

which consumes about 70% of the production. Another 25% is used as an additive to improve

the toughness (impact resistance) of plastics such as polystyrene (Styrene-butadiene rubber,

SBR) and acrylonitrile-butadiene-styrene (ABS).

• Diene elastomers are amorphous polymers having a glass transition temperature below room

temperature, usually ranging between 170 and 250 K (-100 °C and -25 °C). Due to its very

low vinyl content, PBD has a very low glass transition temperature (-90 °C), very high

resilience and is probably the most elastic rubber. PBD also has good abrasion and tear

resistance as well as low heat buildup, which is important for tire applications. Compared to

SBR, it has a lower glass transition temperature, which gives it a lower rolling resistance.


Preparation of Buta-1,3-diene• From dehydrogenation of n-butane: Buta-1,3-diene is commercially produced (Houdry

Catadiene process) by the catalytic dehydrogenation of normal butane (n-butane). This

entails treating n-butane over alumina and chromium trioxide at high temperatures.

• From ethanol: Two processes were in use. In the single-step process (developed by Sergei

Lebedev), ethanol is converted to buta-1,3-diene, hydrogen, and water at 400–450 °C over

any of a variety of metal oxide catalysts (Method 1; Figure 1).

• In the other, two-step process (developed

by Ivan Ostromislensky), ethanol is

oxidized to acetaldehyde, which reacts with

additional ethanol over a tantalum-

promoted porous silica catalyst at 325-

350 °C to yield buta-1,3-diene (Method 2).


Polymerization of Buta-1,3-diene

• Polybutadiene forms by linking many buta-1,3-diene monomers to make a much longer

polymer chain molecule. In terms of the connectivity of the polymer chain, buta-1,3-diene

can polymerize in three different ways, called cis, trans and vinyl.

• The cis and trans forms arise by connecting the buta-1,3-diene molecules end-to-end, so

called 1,4-polymerisation. The properties of the resulting isomeric forms of polybutadiene

differ. For example, “high cis”-polybutadiene has a high elasticity and is very popular,

whereas the so-called “high trans” is a plastic crystal with few useful applications.

• The vinyl content of polybutadiene is typically no more than a few percent. In addition to

these three kinds of connectivity, polybutadienes differ in terms of their branching and

molecular weights.

• Polybutadiene rubber is produced by a solution based polymerization and using suitable

transition metal catalyst.


Polymerization of Buta-1,3-diene

• The trans double bonds formed during

polymerization allow the polymer chain to stay

rather straight, allowing sections of polymer

chains to align to form microcrystalline regions

in the material.

• The cis double bonds cause a bend in the

polymer chain, preventing polymer chains from

aligning to form crystalline regions, which

results in larger regions of amorphous polymer. It

is found that a substantial percentage of cis

double bond configurations in the polymer will

result in a material with flexible elastomer

(rubber-like) qualities.This Lecture is prepared by Dr. K. K. Mandal, SPCMC, Kolkata

Polymerization of Butadiene

Table 1: Typical polybutadiene composition by

catalyst

Catalyst Molar Proportion (%)

cis trans vinyl

Neodymium 98 1 1

Cobalt 96 2 2

Nickel 96 3 1

Titanium 93 3 4

Lithium 10-30 20-60 10-70

• In free radical polymerization, both cis

and trans double bonds will form in

percentages that depend on temperature.

• The catalysts influence the cis vs trans

ratio. The catalyst used in the production

significantly affects the type of

polybutadiene product.

• High cis polybutadiene: This type is characterized by a high proportion of cis (typically

over 92%) and a small proportion of vinyl (less than 4%). It is manufactured using

Ziegler-Natta catalysts based on transition metals. Depending on the metal used, the

properties vary slightly. Using cobalt gives branched molecules, resulting in a low

viscosity material that is easy to use, but its mechanical strength is relatively low.

Neodymium gives the most linear structure (and therefore higher mechanical strength) and

a higher percentage of 98% cis. Other less-used catalysts include nickel and titanium.


Types of Polybutadiene

• Low cis polybutadiene: Using an alkyllithium (e.g. butyllithium) as the catalyst produces a

polybutadiene called “low cis” which typically contains 36% cis, 59% trans and 10% vinyl.

Despite its high liquid-glass transition, low cis polybutadiene is used in tire manufacturing

and is blended with other tire polymers, also it can be advantageously used as an additive in

plastics due to its low contents of gels.

• High trans polybutadiene: Polybutadiene can be produced with more than 90% trans using

catalysts similar to those of high cis: neodymium, lanthanum, nickel, by varying

temperatures. This material is a plastic crystal (i.e. not an elastomer) which melts at about

80 °C. It was formerly used for the outer layer of golf balls.

• High vinyl polybutadiene: High-vinyl polybutadiene (over 70%), despite having a high

liquid-glass transition, could be advantageously used in combination with high cis in tires.

This material is produced with an alkyllithium catalyst.


Synthesis of Polybutadiene• Most polybutadienes are made by a solution process, using either a transition metal (Nd, Ni,

or Co) complex or an alkyllithium, like butyllithium, as catalyst. Since the reaction is very

exothermic, and can be explosive, particularly with alkyllithium catalysts, the reaction is

normally carried out in non-polar solvents like hexane, cyclohexane, benzene or toluene.

• The solvents are used to reduce the rate of reaction, control the heat generated by the

polymerization and to lower the viscosity of the polymer solution in the reactor. A typical

polybutadiene polymerization would be run at about 20% monomer and 80% solvent.

Polymerization using Batch and Continuous Processing

• In batch mode, monomer, solvent and catalyst are charged to the reactor, heated to initiate

the process, and then allowed to continue to completion. The polymer solution is then

transferred to another vessel or process unit to remove the solvent.


Production Using Batch and Continuous Processing

• In continuous mode, monomer, solvent and catalyst are continuously fed into the bottom of

the first of a series of reactors at a temperature suitable for polymerization. The

polymerization progresses as the solution flows through the reactors and polymer solution is

taken off at the top of the last reactor without stopping the process.

• The continuous process is the most economical. In both processes, the finished product is

usually in the form of bales which weigh from 50 to 75 pounds each.


• High Cis Polybutadiene: The alkyllithium and transition metal catalysts make very different

products. The transition metal, or so called Ziegler catalysts produce very “stereoregular”

polybutadienes with one type having the main polymer chain on the same side of the carbon-

carbon double bond contained in the polybutadiene backbone. This is called the cis

configuration.



• High cis polybutadiene will usually have cis content > 95% which

gives rise to better “green strength” and increased cut growth

resistance in the cured product. Green strength, which is the strength

of the uncured rubber compound, is important for the tyre building

process and cut growth resistance is necessary for tyre performance.

• The neodymium catalyst system produces the highest cis content of about 99% and also

makes the most linear chain structure (no branching) producing a polymer with the best

tensile and hysteresis (low heat build-up) properties of all the high cis types.

• The cobalt system produces a highly branched BR with a low solution viscosity that makes a

good polystyrene and ABS modifier. The nickel catalyst makes polybutadiene with an

intermediate level of branching.

• Vinyl tends to increase the Tg of the polymer. The low vinyl content and low Tg makes high

cis polybutadiene ideal for golf ball cores. Golf ball cores are cured with peroxides, which

tend to “over cure” the vinyl units making a very hard golf ball.


High Trans Polybutadiene

• High trans polybutadiene is a crystalline plastic material. In the

trans configuration the main polymer chain is on opposite sides

of the internal carbon-carbon double bond.

• Trans polybutadiene has a melting point of about 80°C. It is made with transition metal

catalysts similar to the high cis process (La, Nd, and Ni). These catalysts can make polymers

with >90% trans again using the solution process.

• Polybutadiene of high trans content is a hard, horny material at room temperature. It is

thermoplastic and thus can be molded without addition of vulcanization agents. The

softening point, hardness, and tensile strength increase with increasing trans content.

• High-trans polybutadiene can be readily vulcanized with sulphur or peroxides. It remains a

hard, partially thermoplastic material when lightly vulcanized but can be made rubbery by

increasing the curative level. Tensile strength is increased by the addition of reinforcing

pigments (carbon black, silica, or clay).


Copolymers of Buta-1,3-diene

• Buta-1,3-diene is normally copolymerized with other types of monomers such as styrene and

acrylonitrile and to form rubbers or plastics with various qualities. The most common form is

styrene-butadiene copolymer, which is a commodity material for car tires. It is also used in

block copolymers and tough thermoplastics such as ABS plastics. This way a copolymer

material can be made with good stiffness, hardness, and toughness. Because the chains have

a double bond in each and every repeat unit, the material is sensitive to ozone cracking.

• Other copolymers of butadiene are BUNA-S and BUNA-N. BUNA-S is a copolymer of

butadiene and styrene. It is formed by the polymerization of buta-1,3-diene and styrene in the

ratio 3:1 in presence of sodium. BUNA-N is a copolymer of butadiene and acrylonitrile. It is

formed by the polymerization of buta-1,3-diene and acrylonitrile in presence of sodium.

BUNA-S is used for making automobile tyres and rubber soles whereas BUNA-N is used for

making synthetic leather and footwear.


Uses of Polybutadiene

• Tyres: Polybutadiene is largely used in various parts of automobile tires; with a majority of

it being high cis. The polybutadiene is used primarily in the sidewall of truck tires, this helps

to improve fatigue to failure life due to the continuous flexing during run. As a result, tires

will not blow out in extreme service conditions. It is also used in the tread portion of giant

truck tires to improve the abrasion, i.e. less wearing, and to run the tire comparatively cool,

since the internal heat comes out quickly.

• Its main competitors in this application are styrene-butadiene rubber (SBR) and natural

rubber. Polybutadiene has the advantage compared to SBR in its lower liquid-glass transition

temperature, which gives it a high resistance to wear and a low rolling resistance. This gives

the tires a long life and low fuel consumption.

• Golf balls: Most golf balls are made of an elastic core of polybutadiene surrounded by a

layer of a harder material. Polybutadiene is preferred to other elastomers due to its high

resilience.


Uses of Polybutadiene

• The core of the balls are formed by compression moulding with chemical reactions. First,

polybutadiene is mixed with additives, then extruded, pressed using a calender (A calender

is a series of hard pressure rollers used to finish or smooth a sheet of material such as paper,

textiles, or plastics.) and cut into pieces which are placed in a mold. The mold is subjected to

high pressure and high temperature for about 30 minutes, enough time to vulcanize the

material.

• Plastics: About 25% of the produced polybutadiene is used to improve the mechanical

properties of plastics, in particular of high-impact polystyrene and to a lesser extent

acrylonitrile-butadiene-styrene (ABS). The addition of between 4 and 12% polybutadiene to

polystyrene transforms it from a fragile and delicate material to a ductile and resistant one.

• The quality of the process is more important in the use in plastics than in tires, especially

when it comes to color and content of gels which have to be as low as possible. In addition,

the products need to meet a list of health requirements due to its use in the food industry.


Polycarbonates

• Polycarbonates are characterized by repeat units having carbonate (-O-CO-O-) inter-unit

linkages. Linear polycarbonates are formed either by reaction between dihydroxy compounds

and phosgene or by means of ester interchange reaction involving dihydroxy compounds and

organic carbonates (R-O-CO-O-R).

• The polycarbonate based on bisphenol (C;

Figure 3) is readily produced by the

condensation of acetone and phenol under

acidic condition at temperatures below 70 °C.

An excess of phenol is used in order to achieve

a high yield.

• Phosgene used (in the direct process or for

preparing diphenyl carbonate) is prepared

commercially from chlorine and carbon

monoxide.


Polycarbonates

• The polycarbonate based on diphenyl carbonate (C; Figure 4) (C6H5-O-CO-O-C6H5) is

conveniently obtained by reacting phenol with phosgene in aqueous NaOH solution in

presence of a tertiary amine catalyst. The diphenyl carbonate is purified by redistillation.


Production of Polycarbonates

• The ester interchange reaction leading to polycarbonate formation is shown in reaction

(Figure 5). The reaction is normally accomplished at nearly 200 °C at a reduced pressure (25

mm Hg), until about 90% of the phenol liberated has been removed. The temperature is then

raised to about 290-300 °C and the pressure further reduced to below 1 mm Hg.


Production of Polycarbonates

• Further chain extension takes place and the melt viscosity increases significantly at this stage.

The reaction is arrested at an appropriate melt viscosity and the melt is forced out of the kettle

under nitrogen pressure.

• For convenience, the reaction is done

using very high molar proportions (≥ 2) of

the diphenyl carbonate so that the initial

reaction product is the bis(phenyl

carbonate) of bisphenol (Figure 5). On

further heating, polymerization proceeds

by splitting out of the diphenyl carbonate

thus producing the polycarbonate resin.

• Direct phosgenation of bisphenol may be

used for the production of high molecular

weight polycarbonates (Figure 6).


Polycarbonates

• The reaction is carried out in presence of solvents like pyridine (which act as HCl acceptors)

diluted with some other solvents as chloroform, methylene chloride or tetrachloroethane. The

polymer is isolated by precipitation with methanol.

• The ester-interchange method is commercially more favoured as it allows avoidance of the

handling of toxic and flammable solvents and thus allowing avoidance of costs of solvents

and solvent recovery. For processing, the resin must be thoroughly dried for several hours in

an oven at 110-120 °C and used for processing immediately thereafter.

• Bisphenol-A polycarbonates have densities of nearly 1.2 g/cm3. They exhibit tensile

strengths of the order of 9,000-10,000 psi. They are characterized by very good impact

resistance (12-18 ft.lb/in notch). Their creep resistance is also very remarkable and this makes

them much superior to polyamides and acetal resins.


Properties and Uses of Polycarbonates

• The polycarbonates appear in crystal-clear grades, if prepared from high purity ingredients.

The refractive index at 25 °C is 1.58. Their low water absorption, good insulation properties

(low dielectric constant, 2.9-3.2, and high volume resistivity, 2 x 1016 ohm cm), transparency

and toughness coupled with heat and flame resistance (Tm = 225-250 °C, Tg = 145 °C), make

them suited for many areas of application including electrical insulation.

• Polycarbonates can be processed by all the usual thermoplastic processing techniques-

injection moulding, extrusion, blow moulding, roto-moulding, thermoforming, etc. Major

applications include electric and electronic appliances, helmet, automotive lighting, protective

shields, safety glass/sun glass, bottles, domestic wares, structural foams, components of

camera, projector housings, etc.


POLYMER CHEMISTRY SEM-6, DSE-B3 PART-9, PPT-9

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