Switzerland-based privately-owned Ineos has given its 1350 employees until 6pm tonight to agree to new, less favourable terms and conditions or face redundancy.
The company, speaking after trade union Unite held a rally opposing the moves at the refinery gates yesterday, said more than 250 people had already signed the deal, which cuts their pension pots and holiday entitlement and imposes a three-year wage freeze.
First Minister Alex Salmond on Saturday told Ineos "to fire the plant up and fire it up now" and urged both sides to negotiate.
But - despite union and management welcoming his intervention - neither last night appeared ready to budge. Instead, the two sides are still fighting to convince employees each offers the best long-term solution to what is Scotland's single most important economic asset.
Tom Crotty, a former manager at Grangemouth and current director of Ineos, said many employees were coming around to the company's view and that yesterday's rally was "not particularly well attended." He added: "We are surprised we have had so many responses, so early."
Ineos, which says it is losing money at Grangemouth, shut the plant down last week. It will only reopen, it insists, if it gets a guarantee from Unite that there will be no strikes this year.
Mr Crotty would not say how many of the existing employees would have to sign the new contracts before the plant was re-opened
He said: "A lot would depend on who they are as well as how many there are. We would have to have enough people signing up to know we can start the site and run it safely and not have the risk of taking it down in a few weeks time."
Alex Flynn, Unite spokesman, claimed a "fantastic" turn-out of some 500 or 600 at the rally.
He said: "It was really heartening to see so many people sending the message to the company that they want a future for Grangemouth and they want quality jobs, but they won't be bullied into signing away their rights. The people who braved the rain certainly did not see themselves as being in a minority. They were a sizeable chunk of the Grangemouth workforce. Their voice should not be dismissed by the company. We need to get round the table and negotiate."
Mr Flynn said Unite had formally told Ineos it had no strike plans this year, but he repeated the threat to sack those who do not sign new contracts must be lifted before "constructive dialogue" could begin.
Ineos says it shut down Grangemouth because of strike action announced but then cancelled by Unite.
Mr Crotty said the firm would only start the refinery up again if it knew there was no risk of a further shutdown. Comparing the plant to an aeroplane that was safe when in the air but dangerous when taking off and landing, he said: "We can't keep turning this thing on and off again like a light switch."
Unions believe Ineos, which has numerous affiliates linked to its Swiss HQ, makes profits, not losses, at Grangemouth.
Mr Crotty said: "For us to cook the books would be crazy. We have publicly listed debt. There may be employees seeing union figures and thinking 'It's not as bad as they're saying'. Well, it is as bad."
Scottish Labour leader Johann Lamont last night joined calls for Ineos to drop its ultimatum. She said: "This is not the way we should be engaging in workplace disputes in the 21st century. Ineos should also undertake to immediately reopen the plant and return to production and both the union and the company should return to meaningful talks."