Hydrolases in Polymer Chemistry: Chemoenzymatic Approaches to Polymeric Materials - Enzymatic Polymerisation - page 80

catalysts or methodologies, each specific for one conversion step. Depending on the
compatibility of the individual reaction steps, this can either be done with or without
isolation of the reaction intermediates (cascade reaction). This is not different for
biocatalysts such as enzymes, which are tailored to very high substrate specificity
in the natural environment. In many cases, the specificity of enzymes is retained in
vitro and has led to improved (i.e., more efficient in terms of energy and raw mate-
rials) synthetic procedures (e.g., for pharmaceutical intermediates) in both stepwise
and multienzyme cascade approaches [ 1 ]. However, not all chemical conversions
can be achieved enzymatically, hence the full exploitation of multistep synthetic
strategies will require the development of novel organic and biosynthetic methods,
so-called chemoenzymatic procedures. Although a large number of chemoenzy-
matic procedures already exist in organic synthesis (both stepwise and cascade),
this is a relatively new concept in polymer chemistry. Obviously, one reason is
that enzymatic polymerization as such is a relatively new field and a basic under-
standing of its scope and limitations had to be developed first [ 2 , 3 ] . On the other
hand, polymer chemistry knows many examples in which two or more catalytic
systems are combined to realize structures and materials not available from one
technique alone [ 4 ] . Although enzymatic polymerizations, in particular those using
hydrolases (lipases), are very successful for making a large variety of polymers by
ring-opening polymerization (ROP) and polycondensation, a general shortcoming
of lipase-catalyzed polymerizations is the lack of control over the molecular struc-
ture. This prohibits the synthesis of complex molecular architectures like block and
graft copolymers, which are possible with controlled polymerization techniques.
Chemoenzymatic polymerizations have the potential to further increase macro-
molecular complexity by overcoming these limitations. Their combination with
other polymerization techniques can give access to such structures. Depending on
the mutual compatibility, multistep reactions as well as cascade reactions have been
reported for the synthesis of polymer architectures and will be reviewed in the first
part of this article. A unique feature of enzymes is their selectivity, such as regio-,
chemo-, and in particular enantioselectivity. This offers opportunities to synthesize
novel chiral polymers and polymer architectures when combined with chemical
catalysis. This will be discussed in the second part of this article. Generally, we
will focus on the developments of the last 5-8 years. Unless otherwise noted, the
term enzyme or lipase in this chapter refers to Candida antarctica Lipase B (CALB)
or Novozym 435 (CALB immobilized on macroporous resin).

2 Chemoenzymatic Synthesis of Polymer Architectures

2.1 Crosslinked Structures

Crosslinking of polymers is usually applied to stabilize the macroscopic mor-
phology or shape of a material. In most cases, it results in insoluble polymeric
materials, e.g., for polymeric coatings. In the chemoenzymatic strategies towards
polymer networks, the enzymatic step is exclusively applied to synthesize the