The following methods have been suggested [130] to improve the
polymerizability or copolymerizability of polar monomers by Ziegler-type
catalysts:
• The polymerizable double bond is isolated from the heteroatom by a spacer,
typically consisting of a long sequence of methylene units or a rigid cyclic unit
• The heteroatom is shielded by sterically demanding groups
• The electron-donating character of the heteroatom is decreased via attachment of
electron-withdrawing substituents on or adjacent to it
• The polar monomers are precomplexed with a Lewis acid, typically an
organoaluminum compound
• The catalyst components are chosen so that the deleterious effect of the heteroatom is minimized
Interest in homogeneous olefin polymerization catalysts, especially group
4 metallocenes has caused a dramatic increase in the number of publications
describing the synthesis of functionalized polyolefins by direct copolymerization.
Many soluble metallocenes, such as bridged zirconocenes, have much better ability
to incorporate higher α-olefins than do Ti-based Ziegler–Natta catalysts. This also
makes them better suited for copolymerizations involving, often very bulky, functional comonomers.
The copolymerization of a polar comonomer with nonpolar olefins by coordination polymerization is thought to be possible if the insertion of the polar comonomer takes place on the same active catalyst center as the nonpolar olefin according
to the Cossee–Arlman mechanism [131, 132]. The prerequisite for this is that the
polar comonomer coordinates to the metal center by its C¼C double bond rather
than by its polar group [133].
The most studied polar comonomers are those containing a functional group
directly bound to the olefinic carbon. These include alkyl acrylates, vinyl ethers,
and vinyl halides. Experimental and computational studies have elucidated the
characteristic difficulties in copolymerizations of those comonomers with earlyor late-transition-metal catalysts [134–138]. The coordination of the polar
comonomers tends to be sterically and electrically unfavorable compared with the
coordination of nonpolar olefins, and thus the incorporation rates are slow. An even
more severe hindrance to successful copolymerization is the high barrier to
subsequent monomer insertion after, for example, an inserted methyl acrylate
unit, owing to the strong binding of the monomer polar end to the catalyst, i.e.,
formation of a stable chelate. On the other hand, in investigations of the reactions of
vinyl chloride with group 4 metal catalysts [136] and a tantalum hydride model
complex [133], 1,2-insertion of the vinyl chloride was found to be followed by
selective β-Cl elimination. This means that in copolymerizations with nonpolar
olefins, vinyl chloride acts as a chain transfer agent and no chlorine is incorporated
into the polymer [139].
In our studies, functional long-chain alkene comonomers with different electronic and steric environments were selected for copolymerization experiments.
Most of these comonomers can be considered derivatives of 10-undecenoic acid,
Functional Polyolefins Through Polymerizations by Using Bis(indenyl). . .
211
polymerizability or copolymerizability of polar monomers by Ziegler-type
catalysts:
• The polymerizable double bond is isolated from the heteroatom by a spacer,
typically consisting of a long sequence of methylene units or a rigid cyclic unit
• The heteroatom is shielded by sterically demanding groups
• The electron-donating character of the heteroatom is decreased via attachment of
electron-withdrawing substituents on or adjacent to it
• The polar monomers are precomplexed with a Lewis acid, typically an
organoaluminum compound
• The catalyst components are chosen so that the deleterious effect of the heteroatom is minimized
Interest in homogeneous olefin polymerization catalysts, especially group
4 metallocenes has caused a dramatic increase in the number of publications
describing the synthesis of functionalized polyolefins by direct copolymerization.
Many soluble metallocenes, such as bridged zirconocenes, have much better ability
to incorporate higher α-olefins than do Ti-based Ziegler–Natta catalysts. This also
makes them better suited for copolymerizations involving, often very bulky, functional comonomers.
The copolymerization of a polar comonomer with nonpolar olefins by coordination polymerization is thought to be possible if the insertion of the polar comonomer takes place on the same active catalyst center as the nonpolar olefin according
to the Cossee–Arlman mechanism [131, 132]. The prerequisite for this is that the
polar comonomer coordinates to the metal center by its C¼C double bond rather
than by its polar group [133].
The most studied polar comonomers are those containing a functional group
directly bound to the olefinic carbon. These include alkyl acrylates, vinyl ethers,
and vinyl halides. Experimental and computational studies have elucidated the
characteristic difficulties in copolymerizations of those comonomers with earlyor late-transition-metal catalysts [134–138]. The coordination of the polar
comonomers tends to be sterically and electrically unfavorable compared with the
coordination of nonpolar olefins, and thus the incorporation rates are slow. An even
more severe hindrance to successful copolymerization is the high barrier to
subsequent monomer insertion after, for example, an inserted methyl acrylate
unit, owing to the strong binding of the monomer polar end to the catalyst, i.e.,
formation of a stable chelate. On the other hand, in investigations of the reactions of
vinyl chloride with group 4 metal catalysts [136] and a tantalum hydride model
complex [133], 1,2-insertion of the vinyl chloride was found to be followed by
selective β-Cl elimination. This means that in copolymerizations with nonpolar
olefins, vinyl chloride acts as a chain transfer agent and no chlorine is incorporated
into the polymer [139].
In our studies, functional long-chain alkene comonomers with different electronic and steric environments were selected for copolymerization experiments.
Most of these comonomers can be considered derivatives of 10-undecenoic acid,
Functional Polyolefins Through Polymerizations by Using Bis(indenyl). . .
211