Biodegradable Shape - Memory Polymers - Biodegradable Polymers - page 202

Table 8.1 Overview over the four categories of SMP.

Type of netpoints
of polymer network

Switching
domains

Thermal
transition

Example

Covalent Crystallizable T m Polymer networks from

poly( - caprolactone)

dimethacrylate [38]

ε

Covalent Amorphous, not

crystallizable

T g Polymer networks from

oligo[( rac - lactide) - co - glycolide]
tetrol and diisocyanate [39]

Physical Crystallizable T m Polymer networks from

oligo( ε - caprolactone)diole,
oligo( p - dioxanone)diole and
diisocyanate [9]

Physical Amorphous, not

crystallizable

T g Poly( l , l - lactide - co - glycolide - co -

trimethylene carbonate) [40]

8.3.1
Covalent Networks with Crystallizable Switching Domains, T trans = T m

This type of polymer network consists of chain segments of homo- or copolymers
and covalent netpoints. They can be prepared by (co)polymerization/poly(co)con-
densation of several monomers (Figure 8.6 ).
As a linear polyester poly( - caprolactone) ( PCL ) is hydrolytic degradable. It can

be synthesized by ring-opening polymerization (ROP) of

ε

- caprolactone. When
diols are used as initiators, macrodiols can be obtained. Covalent polymer net-
works can be created from these macrodiols after subsequent functionalization
with polymerizable end groups, for example, dimethacrylates. These polymer
networks were shown to be hydrolytically degradable and capable of an SME [38].
By the addition of a comonomer, for example, n -butyl acrylate, the elasticity of
such polymer networks can be increased, resulting in AB copolymer networks. At
the same time, T trans of the network can be adjusted from 51 °C for the PCL dimeth-
acrylate homonetwork to 44 °C for a copolymer network having 70 wt% n - butyl
acrylate [41]. The degradability of such AB copolymer networks could be increased
by the introduction of glycolide into the macrodimethacrylates [42]. The AB copoly-
mers were prepared from poly(

ε

- caprolactone - co - glycolide) dimethacrylate and n -
butyl acrylate as photosets. The macrodimethacrylates had a number average
molecular weight ( M n ) up to 13,500 g mol − 1 and a maximum glycolide content of
21 mol%. The polymers were semicrystalline at room temperature and displayed
a T m between 18 and 53 °C. In the polymer networks, the oligo(butyl acrylate)
formed the amorphous soft segment. Degradation experiments showed good
hydrolytic degradability at pH 7 and 37 °C. The presence of glycolate accelerates

ε