i have the heroclix rules ap for my phone and under breaking away is says

"regardless of any game effects, a d6 roll of 1 automatically results in a character failing to break away."

plasticity= add 2 to the break away roll
giant reach = add 1 to break away roll

could a giant with plasticity still fail a break away roll on a 1

A roll of 1 always fails.

page 8 2012 rulebook

Regardless of any game effects, a d6 roll of 1 automatically results in a character failing to break away, while a d6 roll of 6 automatically results in a character succeeding in breaking away.

So even +20 would fail on a roll of 1.

Yes, a D6 result of one always fails. No matter what.

i'm curious on why give the giant plasticity and not something like volstag to make it harder for people to get away

Plasticity does make it harder for other figures to break away, and easier for the figure with Plasticity to break away. Look at the power description again.