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HPMC is a semi-synthetic polymer derived from cellulose. It is made by treating cellulose with an alkali solution to form alkali cellulose. The alkali cellulose is then reacted with methyl chloride and propylene oxide to form HPMC. The HPMC chemical structure is complex, but it can be simplified as a cellulose backbone with methyl and hydroxypropyl substituents.
The degree of substitution (DS) of HPMC refers to the average number of hydroxypropyl and methyl groups per anhydroglucose unit in the cellulose backbone. The DS can range from 0.1 to 2.5, and it affects the properties of HPMC. A higher DS results in a more hydrophilic polymer with lower gelation temperature and better solubility in water.