Metal-based initiators

There is a large group of metal-based compounds capable of initiating the free radical photopolymerization of unsaturated compounds (see Table 10.4) [23, 24]. 

By virtue of their absorption characteristics, many of the compounds listed in Table 10.4 can be employed in conjunction with visible light sources. As the research in organometallic chemistry gained momentum, the potential advantages of organometallic complexes as photoinitiators were also explored, and two such compounds, a ferrocenium salt and a titanocene, were commercialized (see Chart 10.3). 

Transition metal inorganic complexes L2VOCI, LjMn, LjFeiSCNjj, LjRu  

Transition metal organometallic complexes, including ferrocenium salts and titanocene derivatives In conjunction with a co-initiator such as CCI4 Mn2(CO)io, Fe(CO)s, Cr(CO)6, W(CO)6, Mo(CO)6, Mo(CO)5Py, CpMn(CO)3 

Chart 10.3 Chemical structures of (// -arene)(// -cyclopenta-dienyl)iron(ll) hexafluorophosphate (left) and bis(// -cydo-pentadienyl)-bis[2,6-difluoro-3-(lH-pyrr-l-yl)phenyl]titanium (right). 

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The active center I, which first initiates and then propagates the chain, can be a free radical, an anion, a cation or a transition-metal based initiator (Ziegler-Natta systems or metathesis reactions). 

Croup VIIA Transition Metal-Based Initiators... 

B. Group VIII Transition-Metal-Based Initiators 1573... 

Well-Defined Transition Metal-Based Initiators for ROMP... 

A most famous metal-based initiator is the Ziegler-Natta catalyst, which is made from a titanium-based catalyst and an electron-deficient A1(C2H5)3 cocatalyst. Since this used to be patented, naturally, there are now many Ziegler-Natta-type catalysts based on different metals. The Ziegler-Natta process is used today to produce poly alkene plastics and elastomers, which are produced on a scale of over 100 million tons a year. Karl Ziegler and Giulio Natta (Figure 6.2) were awarded the Nobel Prize in Chemistry in 1963 for their invention, which altered our world for better and for worse. 

Another popular metal-based initiator-catalyst is aluminium tri-isopropoxide. While aluminium-based compounds are less active compared to other catalysts, they offer good control over the polymerisation. This occurs through a living polymerisation, a chain polymerisation where no termination or chain transfer occurs, resulting in well-defined polymers (Quirk and Lee, 1992). 

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