When R3 represents the preferred (CH2 CH2 O)p H group, p is a positive number representing the average degree of ethoxylation, and is preferably from 1 to 10, especially 1.5 to 6, and most preferably from about 2 to 4, such as 2.5. n and m are each integers of from 1 to 5, preferably 2 to 4, especially 2.