Clayden et al. (2012) mention that formaldehyde is rather prone to polymerisation. In an aqueous solution containing formaldehyde, it is known that the polymeric "paraformaldehyde" is formed. However, the book does not did not display any information regarding the mechanism of polymerisation. Upon searching the Internet, I was only able to find this image which shows a plausible mechanism for polymerisation:
However, this mechanism seems to be for polymerisation under acidic conditions. I was wondering if there is a general scheme for the polymerisation of formaldehyde.
Clayden, J., Greeves, N., & Warren, S. (2012). Organic Chemistry (2nd ed.). New York : Oxford University Press Inc.