However, under anhydrous conditions at room temperature, it undergoes many addition reactions at the double bond

$ \mathrm{H}_{2} \mathrm{C}=\mathrm{CH}-\mathrm{OCH}_{3} \stackrel{\mathrm{H}^{+}}{\longrightarrow}$  $\begin{array}{*{20}{c}}
  {{{\text{H}}_2}{\text{C}} - \mathop C\limits^ \oplus  {\text{H}} - {\text{OC}}{{\text{H}}_3}} \\ 
  {\,|\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,}\\ 
  {H\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,\,} 
\end{array}$ $\xrightarrow{{B{r^ - }}}$ $\begin{array}{*{20}{c}}
  {Br\,\,\,\,\,} \\ 
  {|\,\,\,\,\,\,\,\,} \\ 
  {{{\text{H}}_3}{\text{C}} - {\text{CH}} - {\text{OC}}{{\text{H}}_3}} 
\end{array}$