Introduction to polymers
Introduction to polymers

This free course is available to start right now. Review the full course description and key learning outcomes and create an account and enrol if you want a free statement of participation.

Free course

Introduction to polymers

2.3.5 Repeat unit placement

A final kind of isomerism in homopolymers is possible when monomer units are added to a growing chain in reverse rather than in their normal position. Because monomer molecules have a particular shape in space, they will normally approach a growing chain end to minimise any spatial interaction, and a regular chain structure results from head-to-tail joints. A defective joint can sometimes occur, however, when heads combine to form a head-to-head joint (Figure 21). Although the chain units are fully chemically bonded together, it represents a weak link in the chain because less energy is needed to break the chain here than elsewhere. So in failure problems, such as thermal degradation, breakage will start here rather than in the normal chain. The occurrence of head-to-head joints is less than 1 per cent in normal polymers.

Figure 21
Figure 21 (a) Regular head-to-tail repeat units in polystyrene; (b) irregular head-to-head joint in polystyrene chain

For further information, take a look at our frequently asked questions which may give you the support you need.

Have a question?
Free Courses

OpenLearn has over 800 free courses across all our subjects designed by our academic experts. From 1 to 100 hours, introductory to advanced, you can start learning at any time and all for free.