How Do you export only R code from Rnw file

I am working with knitr, polymode, ess and I would like to export just the R code from my .Rnw into a .R file. A basic file might look something like this:

\documentclass[a4paper]{article} My Rnw file <<>>= ##This is some R code 1+1 @ \end{document} 

In the end, I would like an R file with just

##This is some R code 1+1