US 9,703,193 B2  101  102  Besides the aforementioned structures, a monomer having 10  a plurality of hydroxyl groups which are acetal-protected  with one ketone compound as shown below is also exem› plary of the monomer providing the recurring unit having  formula (el).  -continued  ~o ~o  0  0  ~  ?}yo  oxo  fr>› oyo  -continued 