Transition Metal-Catalyzed Polymerization in Aqueous Systems - Late Transition Metal Polymerization Catalysis - page 232

7.1.2
General Aspects of Transition Metal-Catalyzed Polymerization in Aqueous Systems

The previous section demonstrated the utility of polymerization reactions in aque-
ous systems. Carrying out transition metal-catalyzed polymerizations in aqueous
systems is of great interest as they can afford a variety of new materials not acces-
sible by established free radical techniques. Thus, various monomers subject to
catalytic polymerization can not be polymerized by free radical polymerization,
and catalytic polymerization also offers access to other polymer microstructures.
In addition, control of free radical polymerization (e.g. with respect to molecular
weight distributions) can be brought about by metal complexes.
Transition metal catalysis in aqueous media has developed into a broad field
over the past two decades, and such reactions can by no means be regarded as
exotic [11]. The Ruhrchemie-Rhone Poulenc process is applied commercially on a
large scale. In this process, propene is hydroformylated using a rhodium catalyst
in a biphasic aqueous system. Hereby, a simple separation of the water-soluble
catalyst from the apolar products is enabled. To date, the majority of investiga-
tions have focused on the synthesis of low-molecular-weight compounds. How-
ever, as shown in the preceding section, polymerization reactions in aqueous sys-
tems are of particular interest [12]. A major reason why aqueous catalytic polymer-
izations had received relatively little attention for a long time is the notorious
water sensitivity of the early transition metal-based Ziegler or Phillips catalysts
[13-17] used for industrial olefin polymerization.
Although encapsulation techniques, e.g. in a hydrophobic polymer ( vide infra ),
can restrict the access of water to the metal centers, a certain stability towards
water of the different metal species involved in the polymerization reaction is
desirable. Scheme 7.4 summarizes the mechanisms of various metal-catalyzed
polymerizations.
The majority of reactions involve organometallic species, most often metal-alkyl
complexes. While such complexes are generally considered to be prone to hydroly-
sis [Eq. (1)], examples of quite stable alkyl complexes of late transition metals

1

(that is Group 8 to 10 metals) also exist (Section 7.2.2).
Also for species not involving metal-carbon bonds, such as in ATRP, possible
hydrolysis of coordinating ligands (L) must be considered. Another reaction of
general importance is the possibility of coordination of water as a ligand. Hereby,
coordination sites can be blocked reversibly for the substrate [Eq. (2)].

L x M L x M

2