The results of 8yr R-band photopolarimetric data of blazar Mrk 421 collected from 2008 February to 2016 May are presented, along with extensive multiwavelength observations covering radio to TeV {gamma}-rays around the flares observed in 2008 May, 2010 March, and 2013 April. The most important results are found in 2013, when the source displayed in the R band a very high brightness state of 11.29+/-0.03mag (93.60+/-1.53mJy) on April 10 and a polarization degree of 11.00%+/-0.44% on May 13. The analysis of the optical data shows that the polarization variability is due to the superposition of two polarized components that might be produced in two distinct emitting regions. An intranight photopolarimetric variability study carried out over seven nights after the 2013 April maximum found flux and polarization variations on the nights of April 14, 15, 16, and 19. In addition, the flux shows a minimum variability timescale of {Delta}t=2.34+/-0.12hr, and the polarization degree presents variations of~1%-2% on a timescale of {Delta}t~minutes. Also, a detailed analysis of the intranight data shows a coherence length of the large-scale magnetic field of l_B_~0.3pc, which is the same order of magnitude as the distance traveled by the relativistic shocks. This result suggests that there is a connection between the intranight polarimetric variations and spatial changes of the magnetic field. Analysis of the complete R-band data along with the historical optical light curve found for this object shows that Mrk 421 varies with a period of 16.26+/-1.78yr.