Butter a 15 1/2 x 11 inch jelly roll pan and a large bowl.

In heavy saucepan over medium heat, cook sugar, molasses, butter and vinegar until mixture reaches 244 degrees to 248 degrees on candy thermometer, or until small amount of mixture forms firm ball when dropped into very cold water.

Place 1 1/2 quarts popcorn in large bowl. With spoon, beat baking soda into molasses mixture until well combined; pour half into popcorn.

Stir quickly until popcorn is coated. With buttered hands, spread mixture onto half of jelly roll pan; repeat with remaining popcorn and molasses mixture; cool.

When cool break mixture into pieces.