Coordinative Polymerization of Styrenes

Subsequent studies revealed that a variety of cyclopentadienyl-based titanium complexes may be used to catalyze the production of s-PS and this area has been extensively reviewed.350-353 Indeed, most complexes of the general formula (Cp )TiX3 generate s-PS when activated with a 

In an attempt to combine the syndioselectivity of half-sandwich titanium catalysts with the living characteristics of anionic polymerization initiators, the use of half-sandwich calcium-based catalysts has been described.363 364 In neat styrene complex (152) affords 76% rr triad PS. However, polydispersities are still quite high (Mw/Mn 2.2) 

Further evidence for a TiIII-centered pathway has been provided from model studies comparing the activity of analogous TiIV and Ti precatalysts for example, (C5Me5)Ti(OMe)2/MAO was reported to be more active than (C5Me5)Ti(0Me)3/MA0.371 More recently it has been shown that Tiiv catalysts only produce atactic-PS when the polymerization is performed in the absence of 

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Proto et al. [434] living isoselective coordination polymerization of styrene to form isoselective block copolymers. This was accomplished by sequential monomer addition. 

Schellenberg, J. Effect of impurities on the syndiospecific coordination polymerization of styrene. Macromol. Mater. Eng., 290,833-842 (2005). 

The Introduction gives a historical overview of SPS from the first discovery through developmental stages to the fuU commercialization of this polymer based on an inexpensive monomer (Chapter 1). Because the transition metal catalysts for the coordination polymerization of styrene are of high importance for the properties of the polymers, these catalysts are comprehensively covered in the section on the preparation of SPS. 

Industrialization of coordination polymerization of styrene using metallocenes and leading to syndiotactic polystyrene is in progress. 

TABLE 8.10 Effect of Catalyst on the Course of the Coordination Polymerization of Styrene and Propylene at Temperature of 313 K... 

The reactive nature of compound 22 is illustrated by the series of transformations shown in Scheme 7.12, in which its Zr—C bond reacts selectively with electrophilic reagents to produce a-haloboronates 36—38. Compound 22 also catalyzes the polymerization of styrene. The polymers thus obtained had weight-average molecular masses in the range 75000—100000 with polydispersities of 1.8—2.1. An X-ray analysis of 22 confirmed it to be a four-coordinate Zr complex with two cyclopentadienyl rings, chlorine, and the aliphatic C-l carbon atom as the ligands (Fig. 7.4). 

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). 

As was found for the polymerization of styrene, CpTiCT/M AO and similar half-sandwich titanocenes are active catalysts for the polymerization of conjugated 1,3 dienes (Table XX) (275). Butadiene, 1,3-pentadiene, 2-methyl-l,3-pentadiene, and 2,3-dimethylbutadiene yield polymers with different cis-1,4, trans-1,4, and 1,2 structures, depending on the polymerization temperature. A change in the stereospecificity as a function of polymerization temperature was observed by Ricci et al. (276). At 20°C, polypen-tadiene with mainly ds-1,4 structures was obtained, whereas at -20°C a crystalline, 1,2- syndiotactic polymer was produced. This temperature effect is attributed to a change in the mode of coordination of the monomer to the metallocene, which is mainly cis-rf at 20°C and trans-rj2 at -20°C. 

Zambelli et al. reported on the mechanism of styrene polymerization [36]. They showed that the main chain of the syndiotactic polymer has a statistically trans-trans conformation. It was established then the double-bond opening mechanism in the syndiospecific polymerization of styrene involves a cis opening. The details in the control of the monomer coordination for this polymerization mechanism were examined by Newman and Malanga using detailed, 3C NMR. It was shown through the analysis of tacticity error (rmrr) that the tacticity in the polymer is chain-end controlled and that the last monomer added directs the orientation and coordination of the incoming monomer unit prior to insertion [37]. 

According to Natta , the rate-determining step of the polymerization of styrene and its derivatives is the coordination (kp = ki) since the nature of the last unit of the... 

Diamine compounds such as L-15 coordinate to copper species, but their use for MA, MMA, and styrene results in slower polymerizations and broader MWDs (MJMn = 1.3—2.5) than those with bipyridine-based bidentate ligands.81108 An increase of the number of amine linkages and bulkier substituents further broadened the MWDs.81 Sparteine (L-16), a bicyclic diamine, was found to be an efficient ligand for homogeneous living radical polymerization of styrene and MMA with CuBr and CuCl, respectively, to give better-controlled polymers (MJMn = 1.1 —... 

Functional groups can also be introduced in the spacer units. Bifunctional initiators with bipyridine units such as MI-17 and MI-18 induced the living radical polymerizations of styrene and MMA, respectively, with copper catalysts to give polymers that carry a coordination site at the middle of the chain.87,333 These polymers can be connected together into star polymers with a ruthenium cation at the core, where the arm numbers are varied among three, four, five, and six in combination with the polymers obtained from the monofunctional initiator with a bipyridine unit (FI-21 and FI-22 Figure 13).416 A... 

Rare earth borohydrides obtained from the chlorides [Eq. (9)] [83] have been used in salt metathesis reactions and were found to be attractive for the generation of cationic species [84]. The presence of more weakly coordinated BF4 anions in [Eu(MeCN)3(BF4)3]x which can be synthesized according to Eq. (10) promotes several catalytic transformations of non-heteroatom-substituted organic substrates, including the polymerization of styrene [85]. 

Polymerization of styrene can occur by a free-radical, cationic, anionic, or coordination mechanism. What mechanism will be followed when each of the compounds shown is used to initiate polymerization ... 

The presence of heteroatoms like oxygen or nitrogen in the monomer causes rapid complexation with the organometallic compounds. For instance, when butyllithium initiates polymerization of styrene in hydrocarbon solvents, there is an induction period and the overall reaction is slow and sluggish. When, however, it initiates polymerization of o-methoxystyrene under the same conditions, there is no induction period and the reaction is rapid. This is due to the initiator coordinating with the oxygen atom... 

Polystyrene was first prepared by E. Simon in 1839 (7). At the time, he thought that he had produced an oxidation product fiom styrene which he called "styrol oxide." Since then, the polymerization of styrene has been studied extensively. Today, because of the stabilization effect of the phenyl group on styrene, styrene can be polymerized using fi ee radical, anionic, cationic, and even coordination initiatois to make polystyrene with dilferent properties. In general, a polymerization reaction consists of the following steps ... 

New block copolymers of polystyrene and poly(e-caprolactone) have recently been prepared with a combination of coordination catalyst and anionic polymerization( An amorphous styrene block was prepared by conventional anionic polymerization at room temperature, and terminated with ethylene oxide. After hydrolysis the terminal - OH groups were substituted by oxo-Al-Zn alkoxide. A crystalline block was then added by coordination polymerization of e-caprolactam. 

Figure 7. Weight average molecular weight of polystyrene ( x 10 ) from free-radical polymerization of styrene in carbon dioxide + sulfur hexafluoride mixtures. Left. Influence of SF6 content at 56 MPa and 51 °C, 5 hr polymerization time. Right Influence of pressure in the solvent mixture containing 30 % by mass SFe at 51 °C. Pressure dependence of the density (in g/cm ) of the solvent mixture is shown on the right coordinate. [1 MPa = 10 bar],...

SCHEME 14.1 The role of TOA in the proposed mechanisms for the formation of the active species for atactic and syndioselective polymerizations of styrene with the Cp TiMe3/B(C6F5)3 catalyst system at low temperature. Dashed lines represent weak coordination interactions. 

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