Polymerization of Lactones by Coordination Mechanism

The mechanism of coordination polymerization was pictured by Yong, Malzner, and Pilato [90] as being an intermediate between the above two modes of polymerization (a cationic and anionic one):

Initiation

where, MT means metal.

Propagation

The above shown mechanism, however, is incorrect when caprolactone is polymerized with tin compounds [95]. Yet, it appears to be correct for polymerizations of propiolactones with an ethylzinc monoxide catalyst [95].

The bimetallic oxoalkoxides are useful catalysts for the polymerizations of ε-caprolactone. The general course of the reaction is quite similar to one for oxiranes. A typical coordination mechanism is indicated from kinetic and structural data [97]. The molecular weight increases with conversion and the reaction exhibits a "living" character, because there is a linear relationship between DP and conversion. When the monomer is all used up, addition of fresh monomer to the reaction mixture results in increases in DP. By avoiding side reactions it is possible to achieve high molecular weights (up to 200,000) with narrow molecular weight distribution (Mw/Mn≥ 1.05) [97]. The reaction proceeds through insertion of the lactone units in the Al-OR bonds. The acyl-oxygen bond cleaves and the chain binds through the oxygen to the catalyst by forming an alkoxide link rather than a carboxylate one:

There are potentially four active sites per trinuclear catalytic molecule. The number of actual sites, however, depends upon the aggregation of the oxoalkoxides. Two different types of OR groups exist, depending upon the bridging in the aggregates. Only one is active in the polymerization. This results in a catalytic star-shaped entity. The fact that the dissociated catalysts generate four growing chains per each Al2(CH2)5CO2(OR)4 molecule [97] tends to confirm this.

The commercially available aluminum triisopropoxide was reported to be a very effective initiator for the "living" ring-opening polymerizations of ɛ-caprolactone, lactides, glacolide, and cyclic anhydrides [98]. Based on kinetic and structural data, the ring-opening polymerization is believed to take place by a coordination-insertion mechanism. While the molecules of aluminum triisopropoxide are coordinatively associated in toluene, in the presence of lactones single isolated monomeric species form and are believed to remain unassociated during the propagation reaction [98].

Actually, ring-opening polymerizations of ɛ-caprolactone were achieved by various catalysts. Only a few, however, initiate "living" polymerizations. Among these are the aluminum alkoxides described above, bimetallic μ-alkoxides [99], porphynatoaluminum [100], mono(cyclopentadienyl) titanium complexes [101], and rare earth alkoxides [102, 103]. Examples of rare earth alkoxides are Ln, Nd, Y, or Nd isopropoxy diethyl acetoacetates and (C₅H₅)₂LnOR and [Cs(CH₃)₅]2LnCH₃ (donor) complexes. It was suggested that the steric effect of bulky groups of these catalysts is to suppress an interfering transesterification reaction by screening linear polymeric chains from the active centers during the reactions and yield "living" polymerizations [104]. These catalysts also are useful in formation of various block copolymers of lactones with other monomers [104, 105]. Among other lactones that were polymerized with the help of such rare earth catalysts are lactide [106-108], 8-valerolactone [109], B-propiolactone [109], and ẞ-butyrolactone [107].

Polymerization of ε-caprolactone with a catalyst system consisting of tris(2,6-di-tert-butylphenoxy) yttrium and 2-propanol is first order with respect to the monomer and initiator [105]. This led to the conclusion that the reaction proceeds via a three-step mechanism that can be illustrated as follows [105]:

It was also reported that "living" e-caprolactone polymerization can be carried with bis(acryloxy-) lanthanide (II) complexes based on samarium [110]. Thus, (ArO)2Sm(THF)4, (where ArO = 2,6-di- tert-butyl-4-methyl-phenoxy) yielded 98% conversion in toluene at 60°C in 1 h. The central ions and ligands appear to have an effect on the activity of the catalyst [110].

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مواضيع ذات صلة

Spontaneous Alternating Zwitterion Copolymerizations

Copolymerization of Cyclic Monomers

Ring-Opening Polymerizations of Cyclic Sulfides

Cationic Polymerization of Lactams

Polymerizations of Lactams

Special Catalysts for Polymerizations of Lactones

Special Catalysts for Polymerizations of Lactones

Polymer-Based Solar Cells

Photo-Conducting Polymers That Are Not Based on Carbazole

Photoconductive Polymers Based on Carbazole

Photo-Conducting Polymers

Liquid Crystalline Alignment