Metallocenes olefin-functionalized metallocene polymerization

Unlike molecules containing electron-rich heteroatoms, boron compounds do not poison Ziegler-Natta or metallocene polymerization catalysts. Borane-containing olefin comonomers are therefore well suited to produce olefin copolymers while retaining good catalyst activity. The resulting polymers are suitable for subsequent conversion into a variety of functional groups. In principle, two approaches are possible (1) hydroboration of the terminal double bond (formed by typical chain transfer processes) of a preformed polyolefin, and (2) direct copolymerization of propylene or a 1-alkene with an alkenyl borane (Scheme 11.4). 

We have recently demonstrated that a variety of functionalized olefins can be polymerized with these new cationic metallocenes. Catalysts derived from the reaction of Cp 2ZrMe2 with B(C6F5)3 or [PhNHMe2]" [B(C6F5)4] are active for the homopolymerization of the functionalized monomers 4-(TMSO)-l,6-heptadiene (5) (TMSO = trimethylsiloxy), l-(TBSO)-4-pentene (6) (TBSO = rm-butyldimethylsiloxy) and N,N-diisopropyl-4-penten-1-amine (7) (Scheme 6). 

Titanocene and zirconocene dichlorides (Cp2MtCl2 with Mt = Ti, Zr) were the first metallocenes studied [Breslow and Newburg, 1957 Natta et al., 1957a], The metallocene initiators, like the traditional Ziegler-Natta initiators, require activation by a Lewis acid coinitiator, sometimes called an activator. AIRCI2 and A1R3 were used initially, but the result was initiator systems with low activity for ethylene polymerization and no activity in a-olefin polymerization. The use of methylaluminoxane (MAO), [A1(CH3)0] , resulted in greatly improved activity for ethylene polymerization [Sinn and Kaminsky, 1980], The properties of MAO are discussed in Sec. 8-5g. MAO has two functions alkylation of a transition metal-chloride bond followed by abstraction of the second chloride to yield a metallocenium... 

Regarding the co-polymerization of hydrocarbon and polar monomers, late transition metal catalysts have provided the most significant advances to date because of their lower oxophilicity and thus greater functional-group tolerance than early transition metal catalysts, although group 4 metallocene catalysts are known to promote the co-polymer-ization of olefins and non-vinyl polar monomers with masked functional groups. 

The cocatalyst has various functions. The primary role of MAO as a cocatalyst for olefin polymerization with metallocenes is alkylation of the transition metal and the production of cation-like alkyl complexes of the type Cp2MR+ as catalytically active species (91). Indirect evidence that MAO generates metallocene cations has been furnished by the described perfluorophenyl-borates and by model systems (92, 93). Only a few direct spectroscopic studies of the reactions in the system CP2MCI2/MAO have been reported (94). The direct elucidation of the structure and of the function of MAO is hindered by the presence of multiple equilibria such as disproportionation reactions between oligomeric MAO chains. Moreover, some unreacted trimethylaluminum always remains bound to the MAO and markedly influences the catalyst performance (77, 95, 96). The reactions between MAO and zirconocenes are summarized in Fig. 8. 

The dual function of the precatalysts 4 opened the way to well-controlled block polymerization of ethylene and MMA (eq. (5)) [89, 90]. Homopolymerization of ethylene (Mn = 10000) and subsequent copolymerization with MAA (Mn 20000) yielded the desired linear AB block copolymers. Mono and bis(alkyl/silyl)-substituted flyover metallocene hydride complexes of type 8 gave the first well-controlled block copoymerization of higher a-olefins with polar monomers such as MMA or CL [91]. In contast to the rapid formation of polyethylene [92], the polymerization of 1-pentene and 1-hexene proceeded rather slowly. For example, AB block copolymers featuring poly( 1-pentene) blocks (M 14000, PDI = 1.41) and polar PMMA blocks (M 34000, PDI = 1.77) were obtained. Due to the bis-initiating action of samarocene(II) complexes (Scheme 4), type 13-15 precatalysts are capable of producing ABA block copolymers of type poly(MMA-co-ethylene-co-MMA), poly(CL-co-ethylene-co-CL), and poly(DTC-co-ethylene-co-DTC DTC = 2,2-dimethyltrimethylene carbonate) [90]. 

Mono-Cp titanium derivatives show reactivity as catalyst precursors for olefin polymerizations, particularly for the polymerization of styrene and functionalized monomers. A review highlighting the developments in the design and applications of non-metallocene complexes, including mono-Cp derivatives, as catalyst systems for a-olefin polymerization has appeared.440 Titanium complexes bearing Cp in addition to chloro ligands and activated by aluminum... 

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