Crosslinkable reactive polymers comprise -A- and —B— recurring units, arranged randomly along a backbone. The -A- recurring units comprise pendant aromatic sulfonic acid oxime ester groups that are capable of providing pendant aromatic sulfonic acid groups upon irradiation with radiation having a λmax of at least 150 nm and up to and including 450 nm. The -A- recurring units are present in the reactive polymer in an amount of greater than 50 mol % and up to and including 98 mol % based on total reactive polymer recurring units. The —B— recurring units comprise pendant groups that provide crosslinking upon generation of the aromatic sulfonic acid groups in the -A- recurring units. The —B— recurring units are present in an amount of at least 2 mol %, based on total reactive polymer recurring units. These reactive polymers can be used in various pattern-forming methods.