Well... no, margarine doesn't contain small particles of plastic. Butter in the US is at least 80% fat, about 18% water, and 1-2% milk solids. Higher quality butter, like European and Irish butter is 82% fat, making it creamier and more spreadable. Amish butter, by the way, is 84% fat.
Margarine also contains at least 80% fat (by law), and if it's less than 80% it is called "spread."
The fat in butter is animal fat, which remains solid at room temperature, whereas the fat in margarine is vegetable fat, which, like all vegetable oils, remains liquid at room temperature.
To make margarine remain solid at room temperature (i,e, more spreadable, in addition to reducing the like likelihood of turning rancid, thereby increasing the shelf life) they use an industrial process that adds hydrogen to vegetable oil, which causes the oil to become solid at room temperature. The hydrogenation process also creates trans fat (trans-fatty acids) as a byproduct. There are minuscule amounts of trans fats that occur naturally, mostly in meats, and are called, I kid you not, cis trans fats. The molecules have a bent shape bonded with carbon atoms that our bodies can at least deal with and break down. Hydrogenated oils replace the carbon atom bonds with hydrogen atoms, creating a very straight, long-chain molecule that doesn't exist in nature and our bodies don't know how to deal with them.
These unnatural long-chain molecules dramatically decrease the good cholesterol and increase the bad cholesterol. They're so bad for the body that nearly every country has banned the use of added trans fats in foods.
Even margarine today contains no trans fats as a result of hydrogenation, so it's safer to eat. Nevertheless, margarine is still an invented food. A good general rule of thumb is, don't eat invented food, eat natural food instead.