Chemistry and technology of synthetic rubber. Textbook

As a result the crotyl lithium compounds have -allyl structure unlike -allyl structures for other metals.

Lithium -complexes have mainly cis-configuration. On the base of organolithium active centers 1,4-cis-structures for both polybutadiene, and polyisoprene are formed in non-polar medias.

Organocyclosiloxane polymerization catalysts are strong bases.

The mechanism of octamethylcyclotetrasiloxane anionic polymerization can be represented as the following scheme:

1) Initiation:

3) Chain transfer reaction. The main reason for chain termination is the interaction of potassium-siloxanolate groups and water:

Therefore, in the industry to produce low molecular weight (liquid) silicone rubbers highly concentrated aqueous alkaline solutions are used. The high molecular silicone rubbers are obtained on the basis of a dry alkali D4 (potassium oligosiloksanolate) as a catalyst.

Anionic polymerization has several advantages over the polymerization by means of acidic catalysts. It allows obtaining high-

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molecular silicone rubbers (with a molecular weight of up to 106 without maturing) and liquid rubbers in a short time with the catalyst concentration 10-2-10-4 % (by weight). It is able to regulate the molecular weight of the synthesized rubber. In contrast to the use of an acid catalyst using alkaline catalysis does not require neutralization of the terminal active sites by water washing. During alkaline catalysis neutralization of active sites is carried out by their stabilization with aerosil or phosphoric acid.

Lab 3. Polymerization of Octamethylcyclotetrasiloxane via anionic polymerization

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, at 15-20 Mmhg bp = 74 C, 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;

6)phosphoric acid (H3PO4) MM = 98.0, bp=158 C, mp = 42.35 C, ρ =

1.685 (liq) g/cm3;

7) toluene (С6Н5СН3) 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

The silicone rubber is obtained at different concentrations of the catalyst of the same nature and its effect on the molecular weight is investigated.

25 ml of octamethylcyclotetrasiloxane is loaded into three dry threenecked flasks equipped with backflow condenser and glass stirrer. The flasks are then blown with dry nitrogen and heated with stirring to 140°C. Then, when stirring 0.01% (by weight) of 50% aqueous KOH solution is loaded into one of the flask, 0.02% (by weight) of 50% aqueous KOH solution is loaded into the other, and 0.03% (by weight) of 50% aqueous KOH solution is injected into the third one. The reaction completes, when the growth of the polymer viscosity visually becomes constant. Terminal

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active centers of polymerization are deactivated by adding 0.5% (by weight) of aerosil at 140°C to the reaction system. Then the reaction mixture is stirred. After that the weighed sample is taken from each flask, dissolved in a solvent and the viscosity average molar mass of the polymer is determined (see Lab 2). The solvent is selected by the teacher. After measuring the molar mass, the comparative analysis is performed and the valid conclusion is made.

3.3 Ionic-Coordination Polymerization

Ionic-coordination polymerization is a form of an anionic polymerization, in which monomer adds to a growing macromolecule accompanied by coordination with the components of the growing chain end (organometallic active center).

First organometallic compounds of transition metals were used by K. Ziegler and G. Natta as catalysts for olefins in the early 1950s. ZieglerNatta catalysts are mostly heterogeneous, however, homogeneous catalysts are also known. In some cases, it depends on the order of mixing the reagents. For example, in case of insertion of TiI2Cl2 into solution containing triisobutylaluminum (TIBA) and butadiene, the catalyst will be homogenous. When TiI2Cl2 is entered into the TIBA solution without butadiene, the catalyst is heterogeneous.

The interaction of transition metal halides such as TiCl4 and organoaluminum compounds (triisobutylaluminum Al(i-C4H9)3) underlies the synthesis of Ziegler-Natta catalysts:

TiCl4+ (i-C4H9)3Al + H2CCH CHCH CH3

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i-C4H9

The Ziegler-Natta catalysts formation process includes two main reactions. They are the alkylation of transition metal ion and the formation of an octahedral complex with a vacant (coordinating) orbital. The result of the alkylation is the formation of carbanion - transition metal ion bonding. The insertion of monomer molecules and the chain growth occurs along carbanion - transition metal ion bond. Coordination of diene monomers by the transition metal ion stipulates the formation of stereoregular polymer.

When preparing the catalyst, a low-activity diene monomer - piperylene - is introduced in the reaction metal complex system. It is required to occupy the coordination site by the compound similar to diene monomers and to replace it easily on this molecule after the feeding of the monomers (beginning of polymerization).

Using transition metal ions of variable oxidation level - Ti(IV) or V(V) for the preparation of the catalytic complex, there are redox reactions accompanied by reduction of metal ion: Ti(IV) → Ti(III) → Ti(II); V(V) → V(IV) → V(III) → V(II). When metal ion transits in lower oxidation degree, the catalyst loses its catalytic activity. This process is called the aging of catalyst. Especially fast aging of the catalytic complex goes with the vanadium catalysts. To extend the catalyst activity rating time, the reactivators are loaded in catalytic reaction system. The scheme shows an oxidation mechanism of the ion V(II) (reactivation of the catalyst) under hexachlorocyclopentadiene.

CombinationРекомбинация

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According to the currently accepted mechanism, the chain propagation is preceded by coordination of the monomer. Regardless of the type of transition metal and the nature of the ligands, before the introduction stage of the monomer on the C-Mt-bonding in the metal complex catalyst the π-allyl complex is formed. The formation of 1,4-cis- polydiene is preceded by the anti-configuration of the -allyl complex. According to the Cossey-Allemand mechanism, the growing polymer chain and the coordinating orbital interchange in each act of growth:

Chain termination is mainly related to the spontaneous reaction of hydride transfer, in which from the end of the growing chain the complex comes off with the formation of a transition-metal hydride, which due to the interaction with the molecule of the monomer regenerates the -allyl group. It is also possible the transfer of hydride-ion per molecule of the monomer at the moment of its coordination. Hydride transition metal compound interacting with the molecule of the monomer gives an active catalyst complex over, and both reactions can be regarded together as a chain transfer to monomer.

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CH2

HC MtXn

CH

CH3

Due to the fact that the diene fragment is formed at the end of the residual polymer chain the reaction of chain transfer to polymer becomes possible. The result is branch points in polymer.

In process of polymerization with Ziegler-Natta catalysts the transfer chain is also possible by means of the trialkylaluminum, provided that it is taken in excess when preparing the catalyst complex.

~~CH2-MtXn + AlR3 → ~~CH2-AlR2 + R-MtXn.

In anionic coordination polymerization the chain can be transferred through hydrogen, whose introduction into the reaction mixture can control the molecular weight of polydiene:

CH3

CH2

CH

CH

CH

CH2

CH2

Lab 4. Synthesis of Synthetic Isoprene Rubber in the Presence of

Ziegler-Natta Complex

Objective: practical introduction to the method of polymerization in the presence of metal complex catalysts, the synthesis of stereoregular polyisoprene rubber and the study of its properties.

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Materials:

1)isopentane (CH3CH2CH(CH3)2), MM = 72.2, mp -129.72 C, bp 36.07 C, d420 = 0.6262;

2)isoprene (С5Н8), MM = 68, mp -145 C, bp 34.1 C, d = 0.6809;

4)titanium tetrachloride (TiCl4), MM = 189.69 mp -24.1 C, bp 34.1 C, ρ = 1.73 g/cm3;

5)triisobutylaluminum (Al(i-C4H9)3), MM = 198.33, mp -6°C, bp 86°C/10 mmHg, d = 0.7876;

6)diphenyl oxide (C6H5)2O, MM = 170.21 g/mol, mp 28-29°C, bp 259.3°C, d420 = 1.0748;

7)ethanol (C2H5OH), MM = 46.07 g/mol, mp -114.15 C,

bp 78.39 C, d = 0.7851325.

Apparatus:

Dilatometer - glass device, consisting of two tanks, joint by the graduated tube 1.5-2.0 ml (division value is 0.05 ml). The volume of the lower tank with flat bottom is 28-30 ml, the upper one - 40-60 ml. The upper tank ends with the standard tube with borders (ground glass joint 14). The volume of the reaction mixture reduces during polymerization, which is fixed on a scale of graduated tube. When loading a batch the upper meniscus of the reaction mass corresponds to the zero point.

Magnetic stirrer, a glass tube sealed at both ends with a diameter of 3-4 mm in length of 12-15 mm with a steel rod inside.

Vessels for the reference isoprene and isopentane storage, consisting of two glass cylinders, connected by glass tube. The large cylinder (400-500 ml) is closed with the three-way stopcock on the standard ground glass joint 14, and the smallest one (50-70 ml) ends with the standard tube 14.

Isoprene, isopentane and catalytic complex sampling can with threeway stopcock and standard tube 14.

Syringes inert gas blowing device consists of two bubbles counter with dry mineral oil. The upper part of the device ends with the standard tube 14.

Sleeve nut made of ebonite or textolite with self-tightening rubber stopper, used for penicillin packaging. It is used for hermetic closing standard tubes of devices and vessels.

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Syringes with glass piston and tip with a volume of 5-10 ml (division value is 0.2 ml) and 20-50 ml (division value is 1.0 ml), a syringe with glass or metal piston with a volume of 1-2 ml (division value is 0.02- 0.25 ml).

Needles for syringes № 1 with a length of 150 mm, an outer diameter of 1 mm; № 2 with a length of 40-70 mm, an outer diameter of 0.8-1.0 mm.

Desiccator for syringes storage, which has freshly calcined aluminum oxide at the bottom.

Water bath with a stirrer. Electromagnetic stirrer.

Cylinder and inert gas - nitrogen drying and cleaning system. Cylinder and Dewar vessel for carbonic acid (dry ice).

Procedure

Polymerization of isoprene in the dilatometer using a syringe technique. In this method the dosage of catalytic complex, isopentane and isoprene into dilatometer is carried out using syringes. Used glass instruments, syringes, needles, etc., should be prepared in strict accordance with these instructions.

Preparation and storage of syringes and needles

Clean and dried needles and syringes are stored in desiccator in parts. No more than one set of syringes needed for the job can be stored in one desiccator. The syringes should be periodically boiled in distilled water in a disassembled form. Syringe parts are immersed in water at room temperature and then it gradually heated to boiling. Syringes are boiled for 10-15 minutes, then they are rinsed with alcohol, dried and placed in desiccator.

Preparation and storage of the batch and catalytic complex components

Isoprene. Specified rectified isoprene is used for the polymerization. Rectified isoprene is stored with the stabilizer (1% hydroquinone) under inert gas in a cool, dark place for up to 3 months. It is subjected to additional purification before using. For this purpose, with the help of a siphon with valve 250-300 ml of isoprene is poured into a roundbottomed flask with ground glass joint with a capacity of 0.5 liter from the storage container. Distillation off the stabilizer is performed in the setup, assembled using ground glass joints, with the speed of 3-4 ml per minute, stopping the process, when it is at least 20% of the initial amount in the

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flask. Distilled isoprene is again distilled over the metallic sodium in the setup, assembled using ground glass joints. The receiver to store dried isoprene after the distillation setup assembly is subjected to vacuum "training" at 180-200°C in a cylindrical electric furnace for 20 min, and the subsequent cooling to room temperature and vacuum releasing with inert gas. Simultaneously the distilling flask "trains" also before filling with isoprene and sodium. Isoprene is heated for distillation by weak heating mantle with closed heating, the distillation rate is 3-43 ml/min. Distilled isoprene is analyzed for moisture content, which should not exceed 0.0005% (wt). The sampling is carried out by syringe, the shelf life before polymerization is no longer than a day.

Before using isopentane is purified from unsaturated hydrocarbons with concentrated sulfuric acid. To do this, 1 liter of isopentane and 100150 ml of concentrated sulfuric acid are poured into a thick 2-liter bottle, which then is covered tightly with stopper, wrapped with teflon film and fixed with metal wire. The bottle is stirred for 1-2 hours in the shaking machine. If the acid becomes yellow, the contents of the bottle is poured into a separatory funnel, the acid is poured and the washing repeats with a new batch of acid. Separated from acid isopentane is washed in a separatory funnel with 150-200 ml of water, shaking for 5 minutes, then it is washed with the same amount of water containing 10% alkali, and then with water until neutral according to the indicator.

Purified isopentane is poured into the bottle, pre-dried and blown with nitrogen and then it is filled with freshly calcined alumina keeping for at least 6 hours. For fine dehydration isopentane is siphoned to the roundbottomed flask, previously dried under vacuum and filled with nitrogen, create inside an excess pressure of nitrogen and push 3-5 g of sodium metal wire. The flask is attached to the backflow condenser and isopentane is boiled for 2-3 hours. Isopentane distillation over metallic sodium is carried out similarly to isoprene, stopping the process when there is at least 5% of the initial volume in the flask. Isopentane is analyzed for moisture content, which should not exceed 0.0005% (wt).

"Catalytic" oil brand toluene is hold over active aluminum oxide at least for a day before using, and then is boiled over metallic sodium for at least 2 hours and is distilled in a stream of inert gas over metallic sodium to the "trained" Schlenk vessel.

Specified "pure" titanium tetrachloride for additional purification is distilled in the setup with ground glass joints over the copper shavings in an

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inert gas using a sand bath (temperature of 150-200°C) at a rate of 3-4 ml/min. The first drops of distillate are taken to the flask, while the main fraction boiling at 136-137°C at a pressure of 760 mm Hg is run off to the pre-"trained" thick-bottle with a capacity of 100 ml.

Distilled titanium tetrachloride is kept in the bottle, covered with a rubber stopper wrapped with teflon film. The distillation residue in the flask should be at least 20% of the initial amount of TiCl4. Transparent colorless titanium tetrachloride is used for work, when yellowing the product is redistilled over the copper shavings. For preparation of (0.10±0.005) g/ml solution purified toluene and calculated amount of titanium tetrachloride is injected to the "trained" Schlenk vessel or a vessel with a bottle. All operations are carried out in an inert gas. Prepared solution is analyzed for the content of titanium tetrachloride by argentometry and can be stored for month.

Attention! Titanium tetrachloride is decomposed under the influence of moisture of the air. The glassware and reagents used must be thoroughly dried.

Triisobutylaluminum (TIBA) is used as a solution in toluene with TIBA concentration of (0.10±0.005) g/ml. This solution is obtained directly from aluminum, isobutylene and hydrogen, by direct synthesis at the polyisoprene producing plant. The concentrated solution of TIBA in a highboiling specified benzine, should be colorless or slightly coloured and purified of solids by sedimentation.

Dried under vacuum at 180-200°C and filled inert gas when cooling at room temperature, Schlenk vessel is fill with toluene in an inert gas. The end of Schlenk vessel's burette with concentrated TIBA is lowered through the upper tube of Schlenk vessel with toluene and the calculated amount of TIBA is injected in inert gas to obtain 10% of the solution. The valves are closed, then it thoroughly mixed, and the concentration of active TIBA is determined by iodometric method.

Diphenyl oxide (DPO) is used as a solution in toluene with a concentration of (0.10±0.005) g/ml, prepared by dissolving the sample of DPO in toluene in an inert gas.

Preparation of the catalyst complex

Catalyst complex is prepared in the bottle closed with a penicillin cork with sleeve nut and magnetic stirrer. The bottle is pre-"trained." The bottle is placed in a double-walled beaker, where the carbon dioxide and alcohol are put for cooling.

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