KIEV, U.S.S.R. -- Soviet rubles became virtually worthless last week in the Ukraine, the Soviet Union's second largest republic, unless backed up by coupons issued by the Ukrainian government.

The introduction on Thursday of what amounted to a separate Ukrainian monetary system dramatized the plight of the ruble at a time when too much money is chasing too few goods. But it was also a vivid demonstration of a seemingly inexorable process of economic disintegration as the Soviet Union's 15 republics attempt to defend the living standards of their own people.

Up to now, the debate over whether the Soviet Union can survive in its present form has been couched largely in terms of political or ethnic breakup. The constitutional trial of strength between President Mikhail Gorbachev and the Baltic states over their declarations of independence has obscured the fact that the economic breakup of the world's second superpower is already underway.

"Gorbachev doesn't have enough energy to save the whole country from economic collapse, so the only way out is to allow the component parts to save themselves," said Ihor Yukhnovsky, a leading Ukrainian physicist and opposition member of the republic's legislature.

Many Soviet republics have already passed legislation forbidding the export of food, raw materials and consumer commodities to other parts of the Soviet Union. In order to enforce the new regulations, Lithuania, Estonia and Latvia are setting up their own customs service. Byelorussia is retaliating by restricting gasoline sales to motorists from neighboring republics.

The Ukraine has now gone one step further by making it practically impossible for residents of other Soviet republics to buy anything on the local market. To make any purchase of more than 20 kopecks (about 10 cents at the new commercial rate of exchange), customers must produce special booklets of coupons issued at their workplaces along with the full ruble price.

"This is a natural reaction," said Ukrainian Communist Party chief Stanislav Gurenko. "We have to defend our own people. Our stores are still somewhat better stocked than stores elsewhere in the Soviet Union. Without restrictions of this kind, they would be wiped clean."

For the Communist authorities, the introduction of the coupon system is an emergency measure to cope with growing shortages and get the republic through the coming winter. According to official statistics, only one-third of the total amount of the money now in circulation is matched by goods in the stores. The coupons are valid for an initial period of six months.

The opposition movement Rukh, which holds one-third of the seats in the Ukrainian legislature, regards the coupon system as the first step toward the creation of a separate Ukrainian currency and eventual independence. Rukh leaders have criticized the coupon system as insufficiently radical, but they have not objected in principle.

"We may disagree politically with the Communists, but economically we are forced to move in much the same direction," said Vladimir Pylupchuk, chairman of the legislature's economic reform commission and a Rukh supporter. "At the end of six months, we will be obliged to introduce our own currency. Without our own currency, it will be impossible to balance the market."

The pressure on the republics' legislatures to take unilateral measures to stabilize the economy has increased in recent weeks because of confusion over the central government's economic reform plans. After weeks of debate, the federal Supreme Soviet in Moscow adopted a vague set of guidelines without any clear program for cutting the budget deficit, widely regarded as the principal cause of the chronic imbalance between supply and demand.

So far, however, attempts to implement partial economic reforms on the level of the republics have not been very successful. Last month, the Estonian government announced that it was cutting state subsidies on a wide range of consumer items and foodstuffs. Prices immediately soared, but shortages persisted because Estonian farmers simply sold their produce across the border in Russia.

In the Ukraine, neither the Communist Party nor Rukh has yet come up with a detailed blueprint for a transition to a market economy. The Communists are reluctant to share power with a movement that has committed itself to Ukrainian independence. For its part, Rukh does not want political responsibility for the economic hardships that are likely to accompany the move to a free market.

Former British prime minister Winston "Churchill once accused Communist reformers of trying to cross a precipice in two leaps," said Les Taniuk, a leading Rukh legislator. "This new monetary system will not resolve our economic problems. It will just increase the level of chaos. The crisis will deepen."