Coordination polymerization monomer types

One of the major advantages of radical polymerization over most other forms of polymerization, (anionic, cationic, coordination) is that statistical copolymers can be prepared from a very wide range of monomer types that can contain various unprotected functionalities. Radical copolymerization and the factors that influence copolymer structure have been discussed in Chapter 7. Copolymerization of macromonomers by NMP, ATRP and RAFT is discussed in Section 9.10.1. 

Chain gro tvth polymerization begins when a reactive species and a monomer react to form an active site. There are four principal mechanisms of chain growth polymerization free radical, anionic, cationic, and coordination polymerization. The names of the first three refer to the chemical nature of the active group at the growing end of the monomer. The last type, coordination polymerization, encompasses reactions in which polymers are manufactured in the presence of a catalyst. Coordination polymerization may occur via a free radical, anionic, or cationic reaction. The catalyst acts to increase the speed of the reaction and to provide improved control of the process. 

First, new "living" initiators have been discovered (although not always as efficient), which respond to other mechanisms, i.e. cationic (5) or even radical ones (6), and can accordingly accomodate other types of monomers. A recent typical example is the coordination polymerization of butadiene by bis (n3-allyl-trifluoro-acetato-nickel) to yield a "living" pure 1.4 cis-poly-butadienyl-nickel, able to initiate in turn the polymerization of monomers like isoprene or styrene (7). 

This method exclusively yields macrocyclic polyesters without any competition with linear polymers. Furthermore, the coordination-insertion ROP process can take part in a more global construction set, ultimately leading to the development of new polymeric materials with versatile and original properties. Note that other types of efficient coordination initiators, i.e., rare earth and yttrium alkoxides, are more and more studied in the framework of the controlled ROP of lactones and (di)lactones [126-129]. These polymerizations are usually characterized by very fast kinetics so as one can expect to (co)polymerize monomers known for their poor reactivity with more conventional systems. Those initiators should extend the control that chemists have already got over the structure of aliphatic polyesters and should therefore allow us to reach again new molecular architectures. It is also important to insist on the very promising enzyme-catalyzed ROP of (di)lactones which will more likely pave the way to a new kind of macromolecular control [6,130-132]. 

Kinetics of Addition Polymerization. As the name suggests, addition polymerizations proceed by the addition of many monomer units to a single active center on the growing polymer chain. Though there are many types of active centers, and thus many types of addition polymerizations, such as anionic, cationic, and coordination polymerizations, the most common active center is a radical, usually formed at... 

Various types of well-defined block copolymers containing polypropylene segments have been synthesized by Doi et al. on the basis of three methods (i) sequential coordination polymerization of propylene and ethylene 83-m>, (ii) transformation of living polypropylene ends to radical or cationic ones which initiate the polymerization of polar monomers 104, u2i, and (iii) coupling reaction between iodine-terminated monodisperse polypropylene and living polystyrene anion 84). In particular, the well-defined block copolymers consisting of polypropylene blocks and polar monomer unit blocks are expected to exhibit new characteristic properties owing to the effect of microphase separation. 

In the coordinated anionic polymerizations with Group I—III metal alkyls alone, monomer coordination involves overlap of the olefinic jr-electrons with vacant sp3 hybrid orbitals. Since this interaction is very weak it is most effective with easily polarized monomers. In the coordination polymerizations with Ziegler type catalysts, stronger monomer coordination is obtained by overlap of jr-electrons with vacant -orbitals of the transition metal component. The complexes have structures of the type proposed by Dewar (199b) and by Chatt and Duncanson (200) and applied to Ziegler type catalysts by Cossee (201) (Fig. 6). The olefin yr-electrons overlap with the orbital of... 

In coordinated polymerizations with alkyl metal and Ziegler-type catalysts, vacant p- or d-orbitals of a metal component coordinate with the jr-electrons of olefins, diolefins and non-polar monomers. When the polymer chain end is fixed in position and partially stabilized by its metal-containing gegen-ion, repetitive insertion of the polarized and oriented monomer between the chain end and gegen-ion yields stereoregular polymers. Of the various factors which affect polymer stereoregularity, the most important appears to be the gegen-ion structure and its ability to coordinate and orient the monomer. 

According to this classification, the polymerization type can usually be easily determined. The structure of the initiator, the manner of its reaction with the monomer, the effects of the medium and last, but not least, sensitive spectroscopic or resonance methods usually, but not always, provide sufficiently convincing information. We know systems containing radical ions. Several years ago it was sometimes assumed that stereospecific polymerizations (now classified as coordination polymerizations) proceed by a radical or cationic mechanism. 

Monomer complexes play an important role even in non-radical processes. In coordination polymerizations, the interactions of monomers with catalysts are evidently of greatest importance without them this type of addition would not be possible. The formation of unstable complexes between the electrophilic initiator and nucleophilic monomer is also necessary in cationic polymerizations. The idea that under certain conditions the formation of stable complexes between initiator and monomer may prevent polymerization [171] is now frequently accepted [172-174]. 

Some reactions are best described as coordination polymerizations, since they usually involve complexes formed between a transition metal and the jt electrons of the monomer. Many of these reactions are similar to anionic polymerizations and could be considered under that category. These types of polymerizations usually lead to linear and stereoregular chains and often use so-... 

In polymerizations of this type, each monomer is inserted between the growing macromolecule and the initiator. Complexing of the monomer to the initiator frequently precedes the insertion process and this polymerization is therefore often called coordination polymerization. The most important group of initiators in this category are Ziegler-Natta catalysts. 

In addition to the binary catalysts from transition metal compounds and metal alkyls there 2ire an increasing number which are clearly of the same general type but which have very different structures. Several of these are crystalline in character, and have been subjected to an activation process which gives rise to lattice defects and catalytic activity. Thus, nickel and cobalt chlorides, which untreated are not catalysts, lose chlorine on irradiation and become active for the polymerization of butadiene to high cis 1,4-polymer [59]. Titanium dichloride, likewise not a catalyst, is transformed into an active catalyst (the activity of which is proportional to the Ti content) for the polymerization of ethylene [60]. In these the active sites evidently react with monomer to form organo-transition metal compounds which coordinate further monomer and initiate polymerization. 

The coordinate type catalysts are also effective for thiirane polymerizations. The types of systems used are also similar. Thus diethylzinc and in particular diethylzinc/water mixtures have been studied [44]. Other studies made using triethylaluminium and diethylcadmium indicated that these metal alkyls all behave similarly. The reactions seem to be rather complex, and, as also was the case with the epoxides, no well defined kinetic studies have appeared. The polymers produced are of high molecular weight and are often crystalline. Thus stereospecific polysulphides have been reported. Again the bulk of the studies involve PS. Stereoselective polymerization of racemic monomer has been accomplished [45, 46] using a catalyst prepared from diethylzinc and (+) borneol. The marked difference between PO and PS in their polymer-... 

Since the Ziegler-Natta catalyst systems appear to function via formation of a coordination complex between the catalyst, growing chain, and incoming monomer, the process is also referred to as coordination addition polymerization and the catalysts as coordination catalysts. Other types of complex catalysts that have... 

Solution Coordination Polymerization is a type of addition polymerization. It differs from free radical polymerization in that it produces linear polymers and permits stereochemical control in their formation. In these reactions organometallic catalysts such as triethylaluminum-titaniumtrichloride are employed. This catalyst adds on a monomer and then adds another one between itself and the other monomer and the process is then repeated. Therefore the reaction is restricted near the catalyst and branching is prevented. Thus only linear molecules are formed. 

This chapter describes the coordination polymerization of acyclic and cyclic vinylic monomers, conjugated dienes, and polar vinylic monomers with the most important catalytic systems known in this area. A chronological classitication for the development of the main coordination catalyst types is outlined, as well as polymerization kinetics and mechanisms and applications of polymers obtained through different metallic complexes. 

Definitions, chemical and physical properties, and general features of the most relevant catalyst types used in coordination polymerizations are described in Section 5.3, after the classification by type of monomers most frequently used and studied in this kind of polymerization reactions. 

Various catalytic systems both of ion [240] and ion-coordinated [174, 245] types were proposed for piperylene polymerization. Polymer with low MM is formed under cationic polymerization due to the high rate of chain transfer reaction on monomer (it rises when catalysts acidity increases) and solvent (it reduces with the increase of polarity of solvent). Some halogenides of metals of Ill-V groups were tested as catalysts of cationic polymerization. TiCU and SnCU turned to be the most useful. Application of SbCls and InCU doesn t provide acceptable polymerization rate, and in the case of AICI3 insoluble polymer was formed [246]. 

Free radical polymerization is undergoing a metamorphosis. We now have at our disposal ways of systematically selecting the polymerization conditions, i.e. the solvent, initiator, monomer types and concentrations. This gives rise to exciting new chemistry which could lead to oligomers of very low molecular weight and controlled functionality. The possibility also exists to combine the attractions of free radical systems with the control of polymer structure, usually associated with more complex ionic/coordination catalyzed systems. 

Coordination Polymerization. A third general polymerization type is coordination polymerization. Like addition polymerization, it occurs by addition of monomer units, one by one. Coordination polymerization is a form of addition polymerization, but differs in that the addition of the monomer involves a third molecular species besides the monomer and the growing polymer chain. In coordination polymerization, the addition step takes place with the monomer and polymer coordinated to the third species, which functions to promote the formation of the new bond. Usually, this third species is a metal complex. 

Figure 1.13 One possible mechanism for the polymerization of olefins using the Ziegler-Natta catalyst A 1( 2115)3.71014 which involves both metal atoms in the catalytic complex. The coordination of the monomer with the Ti atom in the first stage of the reaction leads to this type of polymerization being described as coordination polymerization.

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