GB2070613A - Method of producing O-trimethylsilyl celluloses, and utilisation thereof - Google Patents

Abstract

In a method of producing O-trimethylsilyl celluloses and the utilisation of that method, cotton or wood cellulose is treated with a cellulose swelling agent, reacted with trimethylchlorosilane, the product obtained is dissolved in an organic solvent, a basic reagent is added to the crude reaction product dissolved in the organic solvent for neutralising slight amounts of acidic salts present, solids are removed from the solution and, if desired, trimethylsilyl cellulose is precipitated from the solution and processed to a solid preparation. Suitable basic reagents include alkali metal and alkaline earth metal carbonates and hydrogen carbonates and alkaline earth metal oxides.

Description

SPECIFICATION Improvements in or relating to a method of producing O-trimethylsilyl celluloses, and utilisation thereof The invention relates to a method of producing O-trimethylsilyl celluloses by treating cotton or wood cellulose with cellulose swelling agents, in particular nitrogen-containing bases, reacting with trimethylchlorosilane, and dissolving the product obtained in an organic solvent, as well as to the utilisation of the method when producing novel O-trimethylsilyl celluloses with a degree of substitution (DS) of below 2.

Various experiments have already been carried out in order to silylate the free hydroxy groups of cellulose in the positions 2, 3 and 6 of the anhydroglucose units.

The methods known so far were directed at obtaining trimethylsilyl and other di- and triorganosilyl derivatives of cellulose, which are soluble in apolar solvents. This aim can be achieved only when preparing products that have a mean DS as high as possible, i.e. by silylating possibly all three hydroxy groups per anhydroglucose unit.

G. Keilich et al. (Makromol. Chem. 120 (1968), 87-95) for the first time produced, among others,2,3,6-tris-0-trimethylsilyl cellulose of native cotton cellulose having a mean-weight average degree of polymerisation (DPw) of about 5,800. The cellulose was reacted after bucking under nitrogen with a 0.5% aqueous NaOHsolution with an excess of trimethylchlorosilane in pyridine. The resulting tris-0-trimethylsilyl cellulose (O-TMS cellulose, I) was soluble only in apolar solvents, such as n-hexane and cyclohexane, and precipitable from these solutions with ethanol. Tetrahydrofuran (THF), dioxane, benzene, toluene and chloroform merely caused a swelling of (I).Keilich et al. also already found out that this O-TMS cellulose is substantially less sensitive to hydrolysis than are, for instance, the O-TMS glucoses. Thus, the hydrolysis of a TMS group per unit in O-TMS cellulose is effected by K2CO3 in methanol only after 75 minutes at 500C.

A complete desilylation takes place under these conditions only after a three-hour boiling under reflux.

O-TMS celluloses which are also soluble in aliphatic and aromatic, if desired chlorinated, hydrocarbons and which contain 2 to 3 O-TMS groups per anhydroglucose unit are products from the method according to U.S. patent No.

3,418,312, according to which cellulose is added to a mixture of trimethylchlorosilane, a mono or disaccharide, a solvent, and a tertiary amine.

U.S. patent No. 3,418,313 relates to the production of O-TMS celluloses with the same solubility properties, by the addition of pure trimethylchlorosilane -- prepared by reacting hexamethyldisilazane with anhydrous hydrogen chlorideto a mixture of cellulose, a tertiary amine, and a solvent.

J. Nagy et al. (Period. Polytechn. Chem. Eng.

18, 91-98 (1972)) chose hexamethyldisilazane in pyridine in the presence of catalytic amounts of trimethylchlorosilane for the silylation of Cellophane (Registered Trade Mark) used as a model substance. The product obtained was soluble in toluene. In this publication the discrepancy of bibliographical data referring to the solubility behaviour of cellulose derivatives silylated with trialkylchlorosilanes in organic solvents was pointed out.

In the course of separate investigations it was found that O-trimethylsilyl celluloses are rapidly hydrolysed and thus become insoluble by slight amounts of acidic salts that are still present due to their way of production, in the presence of atmospheric humidity. Such products therefore are not suited for storage and thus cannot be used on an industrial scale. Soiutions of the silylated celluloses, which may for instance be used for the production of shaped articles of regenerated cellulose, are too unstable and subject to changes.

The invention has as its object to prevent the deficiencies described and to provide O-trimethylsilyl celluloses which have reproducible solubility properties and which are easy to handle.

For achieving this object, it is proceeded with a method of the initially defined kind in a manner that a basic reagent, preferably sodium carbonate, is added to the crude reaction product dissolved in the organic solvent, in order to neutralise slight amounts of acidic salts that are present, whereupon solids are removed from the solution and, if desired, trimethylsilyl cellulose is precipitated from the solution and processed into a solid preparation.

O-TMS celluloses having a DS of below 2 are novel. These novel O-TMS celluloses are produced by using the method according to the invention, by reacting swollen cellulose with less than 2.0 mole of trimethylchlorosilane per mole of anhydroglucose unit of the cellulose.

Suitable cellulose swelling agents are known, pyridine being preferably used as a nitrogencontaining base.

As the solvents for the crude silylated celluldses polar solvents, such as dimethyl formamide (DMF), dimethylacetamide (DMA), N-methylpyrrolidone (NMP), dimethylsulphoxide (DMSO), tetrahydrofuran (THF), but also apolar solvents, such as aliphatic and aromatic, if desired halogenated, hydrocarbons may be used. The higher the DS of the O-TMS celluloses, the better their solubility in apolar and slightly polar solvents; products having a DS of around 2 are just still soluble in slightly polar solvents, such as THF, whereas silylated celluloses that have a DS of about 1.5 and below are soluble only in strongly polar solvents.

In addition to sodium carbonate, also other basic reagents may be used for neutralising, such as the carbonates of the remaining alkali metals, alkali and earth alkali hydrogen carbonates as well as earth alkali carbonates or oxides.

As precipitating agents suitably alcohols, preferably alkanols, such as methanol, ethanol, n- and i-propanol, as well as butanols, are used. For precipitating high-silylated products acetone, for instance, is also suited.

In the following examples the method according to the invention will be explained in more detail.

EXAMPLE 1 Production of O-trimethylsilyl cellulose with DS 2.65 Into a three-necked flask with a capacity of 100 ml, equipped with a stirrer and a reflux cooler being in communication with the surrounding atmosphere via a drying tube, 2 g (12.34 mmole) of cellulose and 50 ml of anhydrous pyridine are fed and the mixture is heated under reflux till boiling for four hours. Then 6 g (55.6 mmole) of trimethylchlorosilane are dropped in at room temperature and the flask content is stirred for three hours. By the addition of methanol the swelling of the silyl cellulose is reduced and the silylated substance is washed with methanol after sucking off.The crude product thus obtained is dissolved in petroleum ether and mixed with about 0.1 g of sodium carbonate in order to neutralise slight amounts of pyridinium hydrochloride that are still present and which would otherwise suffice for hydrolysing the silylated cellulose. The solution is centrifuged or pressure-filtered, and O-TMS cellulose is precipitated with ethanol from the resulting clear solution. After drying in a high vacuum a white product that is soluble in apolar solvents, such as hydrocarbons, or in ethers, such as tetrahydrofuran, is obtained.' Yield: 3.75 g (85% of theory), analysis: Si: 21%, DS: 2.65.

EXAMPLE 2 2 g (12.34 mmole) of cellulose in pyridine are heated to boiling for four hours under reflux for the purpose of swelling. Subsequently, 2.2 g (20.37 mmole) of trimethylchlorosilane are added by dropping and allowed to react at room temperature for 3 hours. By the addition of methanol the swelling is repressed, the product being sucked off and washed with methanol. For purification, the product is dissolved in DMF.

Slight amounts of pyridinium hydrochloride that are still present are neutralised by the addition of potassium carbonate and the solution is centrifuged or pressure-filtered. From the clear solution, the silyl cellulose is precipitated with water and dried.

Yield: 2.9 g (88% of theory), Si: 14.7%, DS: 1.37.

Claims (4)

1. A method of producing O-trimethylsilyl celluloses, which method comprises treating cotton or wood cellulose with cellulose swelling agents, in particular nitrogen-containing bases, reacting with trimethylchlorosilane, dissolving the product obtained in an organic solvent, adding a basic reagent, preferably sodium carbonate, to the crude reaction product dissolved in the organic solvent in order to neutralise slight amounts of acidic salts that are present, removing solids from the solution and, if desired, precipitating trimethylsilyl cellulose from the solution and processing it to a solid preparation.

2. Utilisation of the method according to claim 1 for the production of novel trimethylsilyl celluloses having a degree of substitution (DS) of below 2.0, wherein swollen cellulose is reacted with less than 2.0 mole of trimethylchlorosilane per mole of anhydroglucose unit of the cellulose.

3. A method substantially as hereinbefore described with reference to the accompanying examples.

4. Utilisation as hereinbefore described with reference to the accompanying examples.