Secondary insertion, propylene

We considered the polymer chain could be formed by either primary or secondary insertion. Primary insertion is when the monomer forms a bond with the methylene group to the catalyst metal and secondary insertion is where the metal and monomer form a bond with the propylene methine group. Ethylene must always add by primary insertion, but propylene can add either way. And, in fact, the presence of the ag secondary carbon shows the presence of a two carbon methylene sequence which can occur only if both types of insertion are in force. 

The (co)polymerization of dienes can be a good method for the preparation of polymers with reactive vinyl groups, a method that enables the preparation of polymers possessing plural vinyl groups per polymer chain. A fluorinated bis(phenoxy-imine) Ti complex was shown by Coates and co-workers to convert 1,5-hexadiene to poly(methylene-l,3-cyclopentane-fti-3-vinyl tetramethylene), which contained multiple vinyl groups. As already discussed, Saito et al. and others revealed that bis(phenoxy-imine) Ti complexes favored secondary insertion. " This is probably responsible for the formation of 3-vinyl tetramethylene units. Likewise, the same catalyst system can form sPP-/ -poly(methylene-l,3-cyclopentane-z -3-vinyl tetramethylene) from propylene and 1,5-hexadiene. Very recently. 

The reactivity ratio rQ is almost independent of the catalyst used, whereas r, is strongly dependent upon the catalyst and lowers with a decrease in the syndiotactic-specificity of catalyst. Thus, the regioselectivity of secondary insertion of propylene into a secondary vanadium-carbon bond (rj = ku/k10) correlates closely with the syndiotac-... 

On the basis of 13C NMR analysis on ethylene-propylene copolymers obtained with the soluble VCiy A1(C2H5)2C1 catalyst, Zambelli et al. 90,91> have shown that the syndiotactic propagation of propylene occurs by secondary insertion, i.e. in the 1 unit sequence blocks, whereas the primary insertion in the 0 unit blocks results in either atactic or isotactic propagation. 

Polymerisation experiments run with catalysts prepared with 14C-labelled AlEtiCl [74] and NMR analysis of syndiotactic polymers obtained from 1 — ( 3C)-enriched propylene [386] show the secondary insertion of monomeric units to occur into a V C bond [317], i.e. according to the 2,1 mode [scheme (40)] [387,388]. 

It must be emphasised that the propagation step according to scheme (65), which involves the secondary insertion of propylene in a V-CH(CH3) bond, is... 

The same conclusion as in the case of propylene homopolymerisation has been drawn considering IR [396] and NMR [389,395] spectra of ethylene/propylene copolymers obtained with vanadium-based syndiospecific catalysts. The type of propylene insertion depends on the kind of last inserted monomer unit secondary insertion [scheme (40)] occurs more frequently when the last monomeric unit of the growing chain is propylene, while primary propylene insertion [scheme (39)] is more frequent when the last monomeric unit of the growing chain is ethylene [2]. The above explains the microstructure of ethylene/propylene copolymers obtained with vanadium-based Ziegler-Natta catalysts. These copolymers contain both m and r diads when the sequence of propylene units is interrupted by isolated ethylene units i.e. a propylene insertion after an ethylene insertion is substantially non-stereospecific [327,390,397], The existence of a steric interaction between the incoming monomer molecule and the last added monomer unit is also confirmed by the fact that the propagation rate for the secondary insertion of propylene in syndiospecific polymerisation is lower than for primary insertion in non-stereospecific polymerisation [398],... 

Figure 3.32 Supposed four-centred activated complexes for secondary insertion of a propylene molecule...

A detailed molecular mechanics analysis of model catalytic sites has rationalised these observed behaviours for homogeneous propylene isospecific polymerisation catalysts based on bridged rac.-metallocene [357], It is interesting that, in contrast to the discussed case of enantioselectivity of the models for the primary insertion of propylene, the enantioselectivity of the models for the secondary insertion of propylene is due to direct interactions of the... 

Figure 3.40 Models for the secondary insertion of propylene into a polypropylene growing chain, when the catalyst precursor is (R, K)-rac.-(ThindCFF ZrCh. Complex A with an re coordinated monomer corresponds to higher energy, while complex B with an si coordinated monomer corresponds to lower energy and is suitable for secondary insertion. For the sake of clarity, only the C-C bonds are sketched for the n ligands. O - Zr O - C o - H...

Generally, metallocenes favor consecutive primary insertions as a consequence of their bent sandwich structures. Secondary insertion also occurs to an extent determined by the structure of the metallocene and the experimental conditions (especially temperature and monomer concentration). Secondary insertions cause an increased steric hindrance to the next primary insertion. The active center is blocked and therefore regarded as a resting state of the catalyst (138). The kinetic hindrance of chain propagation by another insertion favors chain termination and isomerization processes. One of the isomerization processes observed in metallocene-catalyzed polymerization of propylene leads to the formation of 1,3-enchained monomer units (Fig. 14) (139-142). The mechanism originally proposed to be of an elimination-isomerization-addition type is now thought to involve transition metal-mediated hydride shifts (143,144). 

At —78°C, the soluble catalyst system VCLi/AlEt2Cl polymerizes propylene to a predominantly syndiotactic polymer. A secondary insertion of propylene into the bond was invoked to explain this phenomenon, a hypothesis subsequently confirmed experimentally. ... 

Figure 11 Favored transition states for the secondary insertion of propylene with (a) the isospecific Me2Si(1 -lnd)2Zr system and with (b) the syndiospecific Me2C(Cp)(9-Flu)Zr system. High-energy transition states for the secondary insertion of propylene with (c) the isospecific Me2Si(1 -lnd)2Zr system and (d) the syndiospecific Me2C(Cp)(9-Flu)Zr system.

When the catalyst is not fully regioselective, chain release by a /3-H transfer after a secondary insertion with formation of internal double bonds is often observed. This has been reported for ethylene/a-olefin co-poly-mers, PP, and other polyolefins, as well as for 1-hexene polymerization with dialkoxide catalysts. The reaction is shown in Scheme 14 for the case of propylene, where kinetic studies have shown it to be a bimolecular process, following the rate law s/J/ -h=s / -h[sZr][m].217,257 [sZr] refers to the concentration of active Zr centers bearing a growing chain having a secondary propylene unit linked to the metal. 

The catalyst system 192/MAO has been used for the synthesis of multi-block propylene-styrene co-polymers. The resulting co-polymers have a propylene content in the range 1-33 mol%, and the molecular masses are in the range d/w = 2100 — 60 000. Lower d/w values were observed for higher propylene content. Interestingly, NMR analysis indicated that the regiochemistry of insertion of the two monomers is opposite. In particular, the iPP blocks are formed by primary insertion of propylene, whereas the iPS blocks are formed by secondary insertion of styrene.1244... 

It should also be noted that the probability of secondary insertion becomes higher when occurring on a metal-secondary carbon bond(7,10). The interested reader is referred to th quoted papers for a detailed discussion. In the spectrum of isotactic poly-3- 30-propylene prepared with the catalyst, 8TiCl3-Al(CH3 (sample 3 Figure 1.4) only enriched isobutyl end groups are detected. Therefore in this case the insertion of the monomer is always primary. 

We would like to emphasize here that the branching of polypropylene is controlled by different factors to that of polyethylene [29]. In the case of ethylene the primary/secondary insertion ratio is crucial, whereas in the propylene polymerization catalyzed by diimine catalysts, the ratio between the two alternative insertion pathways (1,2- and 2,1-) is more important [27]. As a result, an opposite temperature effect has been observed for ethylene (increase in branching number with T) and propylene (decrease in branching number with T). 

Insertion of propylene with 1,2-regioselectivity is known as primary insertion or regioregular insertion because it is the most prevalent type of monomer enchainment for metallocenes. Some metallocenes do insert propylene in other, regioirregular ways (Scheme 1.4). The monomer may be inserted with 2,1-regioselectivity (Scheme 1.4a), known as secondary insertion, with the more substituted olefinic carbon bonded to the metal. Following 2,1-insertion, a unimolecular isomerization may take place to give a 3,1 -insertion (Scheme 1.4b). Alternatively, monomer insertion can... 

L = phenoxy-imine ligand). Two subscript characters attached to k stand for regiochemistry of the last-inserted propylene unit (left) and that of a coordinating propylene monomer (right). For example, ifcps is the rate constant of a secondary (2,1-) insertion to a primary chain end. 

Hustad, P. D. Tian, J. Coates, G W. Mechanism of propylene insertion using bisfphenoxyimine)-based titanium catalysts An unusual secondary insertion of propylene in a group IV catalyst system. J. Am. Chem. Soc. 2002,124, 3614-3621. 

Ho, S. C. H. Wu, M. M. Xiong, Y. Novel cyclopolymerization polymers from nonconjugated dienes and 1-alkenes. PCT International Patent Application WO 95/06669 (Mobil Oil Corp.), March 9,1995. Hustad, P. D. Coates, G W. Insertion/isomerization polymerization of 1,5-hexadiene synthesis of functional propylene copolymers and block copolymers. J. Am. Chem. Soc. 2062,124, 11578-11579. Hustad, P. D. Tian, J. Coates, G. W. Mechanism of propylene insertion using bis(phenoxyimine)-based titanium catalysts an unusual secondary insertion of propylene in a group IV catalyst system. J. Am. Chem. Soc. 2002,124,3614-3621. 

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