Design of new styrene enriched polyethylenes via coordination copolymerization of ethylene with mono- or α,ω-difunctional polystyrene macromonomers

Abstract

ω-Allyl, ω-undecenyl and α,ω-undecenyl polystyrene macromonomers, well defined in molar mass and functionality, were synthesized via anionic polymerization. Their coordination copolymerization with ethylene with a cationic α-diimine palladium catalyst ${[(\text{ArN}=\text{C}(\text{Me})–\text{C}(\text{Me})=\text{NAr})\text{Pd}{({\text{CH}}_2)}_3(\text{COOMe})]}^{+}{\text{BAr}}_4^{\prime -}$, (Ar=2,6-iPr₂–C₆H₃ and Ar′=3,5-(CF₃)₂–C₆H₃) affords access to a new type of graft copolymers constituted of a polyethylene backbone and polystyrene grafts. It was shown that the environment of the terminal double bond of the PS macromonomers has a huge influence on the polymerization behavior. Indeed, an undecenyl end-group is more reactive than an allyl end-group. The copolymerization of ethylene with α,ω-undecenyl polystyrene macromonomers lead to cross-linking for long polymerization time (18 h at 25 °C). The influence of several parameters (polymerization temperature, ethylene pressure, concentration) on molar masses and macromonomer incorporation yield was also investigated. Macromonomers having the lowest molar masses were the most reactive. The molar mass of the copolymer increased with ethylene pressure. As expected with such a chain walking catalyst, the copolymers presented moderately branched to highly branched structures depending on the ethylene pressure, like for the homopolymerization of ethylene. Finally, rheological investigations of the copolymers showed that a few percentage of polystyrene incorporation can change drastically the mechanical properties of the materials.

Introduction

Since their first utilization in 1958 for the synthesis of graft copolymers [1], macromonomers (polymers with polymerizable entities at one or both chain ends and generally of low molar masses) have raised increasing interest because of their ability to provide an easy access to a large number of (co)polymers of different chemical natures and various controlled topologies (comb-like, bottlebrush, star-like, graft copolymers,…) [2], [3], [4], [5], [6], [7]. These products exhibit tunable solution or solid state properties. Over the years, macromonomers have been (co)polymerized using different polymerization processes. Free-radical and anionic processes were first employed. A lot of work was done on free radical polymerization procedures [8], [9], [10], [11], [12], [13], [14]. Several teams were also involved in the study of the anionic (co)polymerization of macromonomers [4], [15], [16], [17], [18], [19]. Ring opening metathesis polymerization (ROMP) [20], [21], [22], [23], [24], [25], [26], [27], [28] or more recently, new free-radical processes such as atom transfer radical polymerization (ATRP) [6], [7], [29], [30], [31], [32] were also employed to control the (co)polymerization of macromonomers. Finally, it is only in the past 10 years that the (co)polymerization of macromonomers (formed sometimes in situ) has been investigated via transition-metal based processes (Ziegler–Natta type). Metallocenes or late transition metal catalysts have been shown recently to be efficient for the homo- or copolymerization of ω-functionalized macromonomers with α-olefins [33], [34], [35], [36], [37], [38], [39], [40], [41], [42], [43], [44], [45], [46], [47], [48], [49]. Macromonomers terminal double bonds were either styrene-like or olefinic end groups, these latter being not polymerizable by classical free radical or anionic polymerizations. Besides, some palladium complexes containing symmetrical 1,4-diazadiene units with bulky substituents were very efficient for the polymerization of ethylene [50], [51], [52]. The unique feature of these catalysts is to provide access in the absence of any comonomer, to dendritic to hyperbranched to almost linear PEs just by changing the ethylene pressure [53], [54], [55], [56]. The purpose of the present paper was to evaluate first the influence of macromonomer chain length and chain-end environment (allyl or undecenyl) on the reactivity of the terminal insaturation with such a complex. Special attention was given to the influence of ethylene pressure. Some solution properties will be discussed in the second part.