library(stringr)
parts <- c("6 4- to 5-mm-diameter plugs",       #1  6     4- to...diameter
           "6 large bricks",                    #2  6     large
           "1 1/3 shipment concrete",           #3  1.33  shipment
           "1 (14- to 15-oz) gold bars",        #4  1     (14- to 15-oz)
           "16 3 1/2- to 4-inch-diameter caps", #5  16    3 1/2- to...eter
           "1 1/2 tons sand",                   #6  1.5   tons
           "2 1 1/4- to 3-inch diameter caps",  #7  2     1 1/4- to...eter
           "1/3 shipment cement")

m <- str_match(parts, '^(\\d+(?:\\s+\\d+/\\d+)?|\\d+/\\d+)\\s+((?:\\d+(?:-?in(?:ch)?|")?\\s+)*\\S+(?:\\s+to\\s+(?:\\d+(?:-?in(?:ch)?|")?\\s+)*\\S+)?)')
qty <- m[,2]
unit <- m[,3]
qty
unit