Coordination-Insertion Polymerization

Coordination-insertion polymerization has been extensively used for preparing polymers such as polyesters and polyphosphoesters with well-defined molecular profiles. The most widely used polyesters such as polylactic acid (PLA) and poly (lactic-co-glycohc acid) (PLGA) are produced on industrial scale by following the coordination-insertion polymerization method catalyzed by tin(II)bis (2-ethyUiexanoate) (Stannous octoate, Sn(Oct)2). Other metal alkoxides containing free p-, d-, or/-orbitals such as Mg-, Ti-, Zr-, Fe-, A1-, Y-, Sm-, and Zn-alkoxides are also widely used as the catalyst for this type of polymerization [61]. 

Detailed mechanistic investigation of the coordination-insertion polymerization has been done by a number of researchers [62-65]. A generalized mechanism for the synthesis of PLGA catalyzed by stannous octoate is presented in Fig. 2.18, which involves the acyl-oxygen cleavage of the lactone with the insertion of the monomer into the metal-oxygen bond of the catalyst [66]. 

Polyphosphoesters (PPE) is another important class of biomedical polymers [70] that can be prepared by stannous octoate catalyzed coordination-insertion polymerization method. A representative example is the synthesis of polyfethylene ethyl phosphate) (PEEP) as reported by Xiao et al. by the polymerization of the cyclic phosphoester 2-ethoxy-2-oxo-l,3,2-dioxaphospholane (EEP) using stannous octoate and dodecanol [71] (Fig. 2.20). 

Hybrids with Inorganic Biopoiymsrs Functional Cdtoids Modified Siefaoes 

ATRP technique provided one of the most efficient synthetic tools for preparing many polymer libraries with weU-defined molecular profile and high degrees of functionalities. The unique capability of ATRP to synthesize polymers from inorganic/organic hybrid materials, surfaces, nanoparticles, and proteins makes this process well suited for preparing various polymeric materials for numerous biomedical applications [77, 78]. 

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Depending on the nature of the active center, chain-growth reactions are subdivided into radicalic, ionic (anionic, cationic), or transition-metal mediated (coordinative, insertion) polymerizations. Accordingly, they can be induced by different initiators or catalysts. Whether a monomer polymerizes via any of these chain-growth reactions - radical, ionic, coordinative - depends on its con-... 

Paul, M-A., Alexandre, M., Degee, P., Calberg, C., Jerome, R., and Dubois, P. Exfoliated polylactide/clay nanocomposites by In-Situ coordination-insertion polymerization, Macromol. Rapid Commun. (2003), 24, 561-566. 

Coordination/Insertion polymerization employed less reactive metal car-boxylates, oxides and alkoxides. Polymerization takes place in the presence of tin, zinc, aluminum and other heavy metal catalysts with tin (II) and zinc yielding the purest polymers. The mechanism is shown in Figure 7.6 [12]. 

Coordination-insertion, anionic, cationic, and nucleophilic polymerization are the most frequently reported controlled ring-opening polymerization (ROP) of cyclic monomers in the literature [37, 38]. The coordination-insertion and nucleophilic polymerization are undoubtedly the most efficient and general methods reported so far for the ROP of lactones, with cationic and anionic polymerization being much less investigated. While coordination-insertion polymerization uses metal-alkoxides and related complexes as catalysts, the organocatalytic nucleophilic polymerization is a metal-free approach to ROP. 

Role of the Catalyst and Initiator in Lactide Polymerization The theoretical description of the Sn(Oct)2 Catalyzed ROP of cyclic esters has been studied by many authors, but there does not appear to be a theory that consistently explains all experimental results of the coordination-insertion polymerization [3, 4, 82-84]. Different polymerization mechanisms may dominate, depending on polymerization conditions, catalyst and initiator concentration, and the presence of a solvent. 

Comparatively, the mechanism of Zn(Lact)2-catalyzed ROP has been much less studied. However, the combination of Zn(Lact)2 with a primary alcohol is demonstrated to increase its activity and allows for a better control of the polymerization, as in the case of Sn(Oct)2 [83]. Thus, it seems rather likely that the polymerization proceeds with Zn(Lact)2 in a similar way to that discussed above for Sn(Oct)2. In such coordination-insertion polymerizations the efficiency... 

Nanocomposites of syndiotactic polystyrene (sPS) employing MMT-hexadecyltributylphosphonium [40, 41] and high-impact polystyrene (H1PS)/MMT-hexadecyltriphenylphosphonium [42] were prepared by melt-blending and in situ coordination-insertion polymerization. Partially exfoliated or intercalated materials were obtained in all cases, and a decrease of crystallinity of sPS was observed. However, the presence of clay did not have a strong influence on the sPS thermal transitions. Thermal decomposition of the material was slowed and mechanieal properties were improved in the presence of low organoclay content. Intercalated HIPS nanocomposites were obtained, with improved thermal and flame retardant properties compared to pure HIPS (Figure 3.8). 

S. Bruzaud, Y. Grohens, S. Ilinca, and J.-F. Carpentier, Syndiotactic polystyrene/ organoclay nanocomposites Synthesis via in situ coordination-insertion polymerization and preliminary characterization. Macromolecular Materials and Engineering, 290 (2005), 1106-14. 

Through the controlled degradation of the polyethers to diol dimers using n-butyllithium, the stereochemistry of the monomer units in the polymer chain was determined. The decomposition of all four polymers showed that inversion of configuration at the site of attack on the epoxide ring occurred in both cationic and coordination-insertion polymerization mechanisms. The cis epoxides (RS stereocenters) produce monomeric units in the polymer chain with RR and SS stereocenters, and the trans epoxides (either RR or SS) produce monomeric units in the polymer chain with only RS units. 

Scheme 41 Mechanisms of coordination-insertion polymerization of six-membered cyclic carbonates.

The mechanistic aspects of the coordination-insertion polymerization of six-membered neopentyl carbonate (DTC) initiated by tin(II) alkoxide-based catalysts were discussed by Rokicki et NMR and MALDI-TOF mass spectrometric... 

Scheme 30 Mechanism of the coordination-insertion polymerization of 5-HDON in the presence of Sn(0ct)2.

To date, polyolefins - spedfically polyethene (PE) and poly-propene (PP) - are the most important commerdal polymers. Due to the combination of their excellent chemical and physical properties and their low production costs, these materials find extremely broad application. Since the discovery of a coordinative insertion polymerization by Ziegler in 1953,... 

In addition to the wide variety of true coordination/insertion polymerization reactions with polar fimaional monomers, the phosphine sulfonate catalyst system is capable of the nonalternating copolymerization of olefins and CO. First discovered by Drent a with in situ formed catalysts, Nowack et al later prepared the first single component catalysts 104. In the course of this development, the reaction was studied in detail concerning the mechanism that leads to the additional incorporation of ethene emits (Figure 45). 

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