Chemistry and technology of synthetic rubber. Textbook

Cumene hydroperoxide - is an oily liquid with ozone like odor. Heat sensitive, decomposing explosively. It can undergo rapid decomposition under the influence of a wide range of trace compounds, such as transition metals, mineral acids, salts or metal oxides even at room temperature. It is soluble in organic solvents (alcohol, ether, benzene, acetone, etc.) and poorly soluble in water. It belongs to the second class of danger. Cumene hydroperoxide is directly adsorbed by skin, has a general toxic, irritant action. MPC=1 mg/m3.

Before working, study safety sheets for the hydroperoxide. Check the availability of first aid kits, fire fighting appliances and take individual protective measures, wear glasses or mask, working clothes, gloves, protect the unit with organic glass screen, check the state of gas mask and ventilation.

The work area should be operated only with the required equipment (including pipette with bulb for hydroperoxide dosing). Promote good housekeeping practices in the laboratory or work area. The necessary resources like equipment, glassware, cooling mixtures and solutions for neutralization should be available for use.

Never perform works anywhere near an open flame or source of heat. When installing the setup, you must eliminate the possibility of sparking from electrical appliances, ingress of foreign substances, the use of ground glass joints.

Operating conditions, cooling and temperature should be monitored during lab procedures.

When working with hydroperoxides, remember that they are explosive. Therefore work with small amounts of these substances. When used in relatively large amounts, take them in the form of pastes or suspensions in water if possible.

If distillation temperature jumps rapidly, flask should be cooled immediately with prearranged cooling mixture (ice + salt). At the flask break and the peroxide or hydroperoxide spillage it is urgently needed to remove the heat device, to cover the spill area with sand, impregnated with an alkaline solution. When pipetting the peroxide compounds with a rubber bulb, prevent air bubbling and blowing products from the pipette - it's dangerous, because shaking can cause an explosion.

First aid

Immediately wipe off the products with tampon moistened with alcohol, then wash the affected area with copious amounts of water soap.

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If in contact with the eye(s), hold eyelids apart and flush the eye, wash with copious amounts of water continuously. Take care not to rinse contaminated water into the non-affected eye. Continue flushing for at least 15 minutes and seek medical aid immediately. This reinforces the damaging effect.

If inhalation, remove the source of contamination or move the victim to a well ventilated area - avoid becoming a casualty. Ensure airways are clear and have a qualified person administer oxygen through a face mask if breathing is difficult.

Work under a hood, prevent skin contact.

3 Production of Synthetic Rubber by Ionic Polymerization

Ionic polymerization occurs under the influence of catalysts of ionic nature. Depending on the conditions of polymerization the growing active centers can be in the form of covalent compounds (polarized molecules), contact ion pairs, associated ion pairs, solvate-separated pairs, free ions. The most active in this sequence are free ions. Depending on the charge at the terminal atom of the growing chain the ionic polymerization is divided into cationic and anionic.

3.1 Cationic Polymerization

Cationic (electrophilic) polymerization is a process of macromolecules formation, where the active growing chains are positively charged, and cationic polymerization initiator is a source of positive charge.

In the synthetic rubber industry cationic initiation is used in the production of butyl rubber, polyisobutylene, and silicone rubber. Cationic polymerization catalysts are acidic substances.

When obtaining butyl rubber and polyisobutylene the aprotic Lewis acids are used (AlCl3, BF3, Al(C2H5)1,5Cl1,5). When using aprotic acids to form the active site it is necessary to use cocatalysts (promoters). They are the substances of proton-donor nature - proton acids and aprotic bases.

As a result of the interaction of Lewis acids with proton acids and aprotic bases the complexes are formed:

AlCl3 HOH H+[AlCl3OH]-

C Cl + AlCl3 C+[AlCl4] -

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Protons or carbocations as a part of complexes initiate the polyaddition reaction:

Regardless of the nature of the catalytic complex the active center of polymerization after the interaction with the unsaturated monomer is carbonium ion:

Because of the numerous reactions of chain transfer in cationic isobutylene polymerization the lifetime of active centers is small. The common feature of the active centers of MnM+ type is the tendency to chain termination and transfer:

CH3

The activation energy of chain transfer reaction is superior to the activation energy of the chain growth reaction. Therefore, lowering the temperature to a certain critical point leads to the fact that the chain growth reaction rate constant starts to dominate the rate constants of chain transfer reactions (Figure 1.1):

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Tcr.

Fig.1.1 Rate constants of growth and chain transfer reactions changes behavior depending on the reaction temperature

Receiving butyl rubber and polyisobutylene, this temperature is quite low. To achieve the required values of molecular weights of polyisobutylene and butyl rubber the polymerization is carried out at -95°C. In modern production processes the temperature of synthesis could be raised up to -70°C by increasing the activity of the catalyst.

When receiving silicone rubber by cation initiation there are used strong protonic acid. Major industrial monomer is octamethylcyclotetrasiloxane (D4).

Organocyclosiloxanes polymerization mechanism in the presence of sulfuric acid can be represented by the following scheme.

1) Initiation:

2) Propagation:

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3) Chain transfer reaction. Main reaction of chain transfer during cationic initiation caused by a formation of sulfate bridges:

While interacting the polymer and water, formed sulfo ether groups and terminal sulfate groups are converted into hydroxyl groups:

Lab 2. Cationic Polymerization of Octamethylcyclotetrasyloxane

Objective: to study the influence of nature and concentration of the catalyst on the molecular weight of silicone rubber.

Materials:

1)octamethylcyclotetrasiloxane (D4) MM = 296, bp 175 C, bp 74 C at 1520 mmHg, d = 0.957520;

2)distilled water;

3)50% aqueous KOH solution;

4)concentrated sulfuric acid (H2SO4) MM = 98.07, bp 279.6 C, mp 10.3 C, ρ = 1.830520 g/cm3;

5)silicon dioxide (aerosil) MM = 60.08, ρ = 2.651 g/cm3;

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7) toluene (C6H5CH3), MM = 92.14, bp 110°C, d = 0.8669420; Apparatus: 3-necked flask, 50 ml; glass stirrer; thermometer to

250°C; hot plate; backflow condenser.

Procedure

25 ml of octamethylcyclotetrasiloxane is loaded into the threenecked flask and the stirrer is turned on. While vigorous stirring, 1% (by weight) of concentrated sulfuric acid is added through the side neck of the flask with a pipette into the reaction mixture and the polymerization goes at 25°C. The process is carried out with stirring, until liquid transforms into thick polymer twisted on the stirrer. Usually this is just after 1.0-2.0 hours. After the polymer formation the stirrer is stopped, 1 ml of distilled water is addeded into the flask by pipette and then it is gently stirred preventing from liquid scattering when diluting of the polymer. The polymer is rapidly diluted (the molecular weight slumps), and then begins to thicken. After 20 minutes of stirring, it turns again into a thick mass (cures), twisted on the stirrer. The flask is disconnected from the setup and the polymer is poured into a beaker. To pour out the polymer, the flask is fixed in a retort stand with neck down over the cup. The rubber is left in a beaker for one to two weeks for further curing. The polymer is then thoroughly washed with water till neutral. The neutralization is determined by the color constancy of the wash water after adding to them an alcoholic solution of methyl orange. Then the polymer is dried in a vacuum hot air oven at 50°C, and the viscosity-average molar mass of the polymer is determined.

Determination of average molecular weight of obtained polymers

The viscosity average molar mass of the polymer Mv is related

with an intrinsic viscosity of polymer solution [η] with the Mark-Kuhn- Houwink equation:

[ ] К М ,

where K and α - constants characteristic for a given pair of polymer - solvent. For the calculation of molecular weight, it is more convenient to transform the equation:

М (1К)1/ [ ]1/

In order to find the intrinsic viscosity, it is necessary to determine the viscosities of the pure solvent and diluted polymer solutions of several

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concentrations. Usually there is no need to determine the absolute values of the viscosity of solutions, therefore their relative viscosities are found:

r i

0

where τi and τ0 - the flow time of the solution and the solvent (in the same conditions and in the same instrument).

Determination of the viscosity of solutions is carried out in an Ubbelohde viscometer at constant temperature T=20±0.1°C, that is controlled by a thermostat.

20 ml (V0) of the polymer solution with concentration about 0.5-0.7 g/dl (C1) is poured through a tube of dry and clean viscometer placed vertically in thermostat, and stood for 20 minutes until the sample reaches the specified temperature.

The rubber bulb is attached to the middle tube of viscometer, the inlet of this tube is closed and the liquid is sucked until the meniscus of the sample rises to the indicator line of the measuring bulb. Then the efflux time of the solution is determined.

Use a stopwatch to measure the time between the meniscuses crossing the first and second markers, and record efflux time (t1). Repeat the above determination at least 3 times until no more than 0.2 s deviation is found. Record average flow time. Then 1.5-2 ml (V1) of the solvent is added to the viscometer and after stirring and thermostating, the efflux time2 of the solution is determined, the concentration C2 of the last is calculated from the formula:

С2=(С1 V0)/(V0+V1);

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Similarly the efflux times of solutions, obtained by sequential addition of new portions of 1.5-2 ml of solvent are defined:

3 at C3=(C1*V0)/(V0+V1+V2), etc.

To obtain reliable data, the efflux time for 5-6 dilutions is determined. Then the investigated solution is poured out, the viscometer is thoroughly washed and dried.

The blank experiment is carried out by measuring the efflux time of the pure solvent 0. Measurements are carried out at least three times, the difference between the measurements should not exceed 0.2-0.3 sec. The results of measurements and calculations are recorded in the table 2.2.