US 2020/0133122 Al  73  Apr. 30, 2020  (L4-1)  (L4-2)  (L4-3)  (L4-4)  [0117]  In formulas (L4-1) to (L4-4), the broken line  denotes a bonding site and direction.  RL $^{41 }$is each indepen› dently a C$_{1 }$-C$_{10 }$ monovalent hydrocarbon group. The mon› ovalent hydrocarbon group may be straight, branched or  cyclic, and examples thereof include alkyl groups such as  methyl, ethyl, propyl, isopropyl, n-butyl, sec-butyl, tert› butyl, tert-pentyl, n-pentyl, n-hexyl, cyclopentyl and cyclo› hexyl.  [0118]  For formulas (L4-1) to (L4-4), there can exist  stereoisomers ( enantiomers or diastereomers ). Each of for› mulae (L4-1) to (L4-4) collectively represents all such  stereoisomers. When the acid labile group  XA  is of formula  (L4), there may be contained a plurality of stereoisomers.  [0119]  For example, the formula (L4-3) represents one or  a mixture of two selected from groups having the following  formulas (L4-3-l) and (L4-3-2).  Herein  RL 41  is as defined above.  [0120]  Similarly, the formula (L4-4) represents one or a  mixture of two or more selected from groups having the  following formulas (L4-4-1) to (L4-4-4).  Herein  RL 41  is as defined above.  [0121]  Each of formulas (L4-1) to (L4-4), (L4-3-l) and  (L4-3-2), and (L4-4-1) to (L4-4-4) collectively represents an  enantiomer thereof and a mixture of enantiomers.  [0122]  It is noted that in the above formulas (L4-1) to  (L4-4), (L4-3-l) and (L4-3-2), and (L4-4-1) to (L4-4-4), the  bond direction is on the exo side relative to the bicyclo[2.  2.l]heptane ring, which ensures high reactivity for acid  catalyzed elimination reaction (see JP-A 2000-336121). In  preparing these monomers having a tertiary exo-alkyl group  of bicyclo[2.2.l]heptane skeleton as a substituent group,  there may be contained monomers substituted with an  endo-alkyl group as represented by the following formulas  (L4-1-endo) to (L4-4-endo). For good reactivity, an exo  proportion of at least 50 mo! % is preferred, with an exo  proportion of at least 80 mo!  % being more preferred.  (L4-4-1)  (L4-4-2)  (L4-4-3)  (L4-4-4)  (L4-3-1)  (L4-3-2)  (L4-1-endo)  (L4-2-endo) 