Generally, in the synthesis of hydroxypropyl methylcellulose, the refined cotton cellulose is treated with alkali solution at 35-40°C for half an hour, squeezed, the cellulose is pulverized, and appropriately aged at 35°C, so that the obtained alkali fibers are averagely polymerized degree is within the required range. Put the alkali fiber into the etherification kettle, add propylene oxide and methyl chloride in turn, and etherify at 50-80 ℃ for 5 hours to a high pressure of about 1.8 MPa. Then add an appropriate amount of hydrochloric acid and oxalic acid to the hot water at 90 ° C to wash the material to expand the volume. Dehydrate with a centrifuge. Wash until neutral, when the water content in the material is less than 60%, dry it with a hot air flow at 130°C to less than 5%.

Alkalization: The powdered refined cotton after opening is added into an inert solvent, and activated with alkali and soft water to swell the crystal lattice of the refined cotton, which is conducive to the penetration of etherifying agent molecules and improves the uniformity of the etherification reaction. The alkali used in the alkalization is a metal hydroxide or an organic base. The amount of alkali added (by mass, the same below) is 0.1-0.6 times that of refined cotton, and the amount of soft water is 0.3-1.0 times that of refined cotton; the inert solvent is a mixture of alcohol and hydrocarbon, and the amount of inert solvent added is refined cotton. 7-15 times: the inert solvent can also be an alcohol with 3-5 carbon atoms (such as alcohol, propanol), acetone. It can also be aliphatic hydrocarbons and aromatic hydrocarbons; the temperature should be controlled within 0-35°C during alkalization; the alkalization time is about 1hr. The adjustment of temperature and time can be determined according to the requirements of the material and product.

Etherification: After alkalization treatment, under vacuum conditions, etherification is carried out by adding an etherifying agent, and the etherifying agent is propylene oxide. In order to reduce the consumption of the etherifying agent, the etherifying agent was added twice during the etherification process.