问题 问答题 简答题

如何选择引发剂?

答案

参考答案:

首先要根据聚合反应实施的方法来选择,一般情况下:

(1)本体聚合、悬浮聚合和有机溶液聚合选择偶氮类和过氧化物类油溶性引发

剂;

(2)乳液聚合和水溶液聚合则要选择过硫酸盐类的水溶性引发剂。

其次要根据聚合温度选择活化能或半衰期适当的引发剂,使聚合时间适宜。

单项选择题

Questions l-5 are based on Passage One:Passage OneLong bus rides are like television shows. They have a beginning,a middle, and an end-with commercials thrown in every three or four minutes. The commercials are unavoidable. They happen whether you want them or not. Every couple of minutes a billboard glides by outside the bus window. “Buy Super Clean Toothpaste”“Drink Good’n Wet Root Beer.”“Fill up with Pacific Gas.”Only if you sleep, which is equal to turning the television set off,are you spared the unending cry of "You Need It! Buy It Now!"The beginning of the ride is comfortable and somewhat exciting, even if you’ve traveled that way before. Usually some things have changed-new houses, new buildings, sometimes even a new road. The bus driver has a style of driving and it’s fun to try to figure it out the first hour or so. If the driver is particularly reckless or daring, the ride can be as thrilling as a suspense story. Will the driver pass the truck in time Will the driver move into the right or the left-hand lane After a while, of course, the excitement dies down. Sleeping for a while helps pass the middle hours of the ride. Food always makes bus rides more interesting.But you’ve got to be careful of what kind of food you eat. Too much salty food can make youvery thirsty between stops.The end of the ride is somewhat like the beginning. You know it will soon be over and there’s a kind of expectation and excitement in that. The seat, of course, has become harder as the hours have passed. By now you’ve sat with your legs crossed, with your hands crossed behind your head. The end comes just at the right time. There are just no more ways to sit.

What is the purpose of this passage ( )

A.To give the writer’s opinion about long bus trips

B.To persuade you to take a long bus trip

C. To explain how bus trips and television shows differ

D.To describe the billboards along the road

单项选择题