reshape data frame when one column has unequal number of entries

Hello!

I have my data frame x with 2 character columns:

x <- data.frame(a = numeric(), b = I(list()))
x[1:3,"a"] = 1:3
x[[1, "b"]] <- "a, b, c"
x[[2, "b"]] <- "d, e"
x[[3, "b"]] <- "f"
x$a = as.character(x$a)
x$b = as.character(x$b)
x
str(x)

I need to produce this data frame:

1  a
1  b
1  c
2  d
2  e
3  f

Is it possible without looping?
Thank you!

Dimitri Liakhovitski