1.1.7　Stars and Dendrimers

Star polymers（星形聚合物）have mutli-arms radiating from a common core（Figure 1.3）.The number of arms may vary from three to six or more.Star polymers are prepared either by growing the arms by polymerization from a multifunctional core，or by linking preformed polymer molecules（预聚物分子）to a core through reactive end groups on the polymer.In principle，there is no limit to the length of the arms in a star polymer，but in the process of actual synthesis，the arm length is restricted by the influence of the steric hindrance，group reaction activity and so on.

Dendrimeric polymers（枝状聚合物）have the arborization（树枝状）structures in the chain of polymer molecules，as shown in Figure 1.4.Except for usually three-dimensional structure，there is a sphere structure in the overall final outer shape.

Figure 1.3　Tri-star polymer

Figure 1.4　Dendrimeric polymer

The solution and solid-state properties of star polymers and especially dendrimers（枝状聚合物）are usually noticeably different from those of their linear correspondent polymer with the same molecular weight.The spherical shape of a dendrimer restricts intermolecular entanglements（缠绕）.Hence，both the solution and bulk viscosities are lower than expected.