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Synthesis of Homopolymers and Copolymers

Christian Slugovc

1.1 Introduction

Ring-opening metathesis polymerization (ROMP) is a versatile chain-growth polymerization technique in which mono or polycyclic olefins undergo ring opening, thereby forming a linear polymer chain. ROMP is typically initiated by group VI or VIII carbene complexes and is capable of forming functionally diverse polymers. Depending on the use of a proper initiator and monomer used, the polymerization is controlled and living, allowing for the precise preparation of diverse polymer architectures with narrow molecular weight distributions. Especially with ruthenium-based initiators, the scope of ROMP is further extended, as most functional groups are tolerated and exclusion of moisture or air is not necessary. These characteristics make ROMP initiated by ruthenium complexes a competitive alternative to living radical polymerization methods. As a consequence, research on ROMP in the last 10 years has been focused on obtaining precision and diversity of macromolecular architectures bearing diverse functionalities. Mostly, ruthenium-based initiators have been used because of their paramount functional group tolerance. Only recently have molybdenum-based initiators staged a comeback because of their ability to provide stereoselective ROMP.

The basic mechanism of ROMP is shown in Figure 1.1. In the initiation step, a metal carbene species undergoes olefin metathesis with the monomer being, in most cases, a strained cyclic olefin. The newly formed carbene complex then performs repeated insertions of the monomer in the propagation step. The initiation rate constant ki should be significantly larger than kp in order to obtain controlled polymerization, that is, every initiator makes a polymer chain. Undesired termination should not occur. The intended termination, upon addition of a proper reactant, leads to the cleaving off and deactivation of the active site and to the introduction of an end group to the polymer chain. Furthermore, undesired side reactions, known as back biting, may occur depending on the nature of the initiator used. In this process, the carbene moiety at a growing polymer chain might react with a double bond from another polymer chain or from its own polymer chain, leading to chain transfer reactions that are detrimental for obtaining polymers with narrow molecular weight distributions and the synthesis of precision polymers.

Figure 1.1 Basic ROMP mechanism.

This chapter focuses on recent development in the synthesis of homo and copolymers via ROMP. Since 2003, several book chapters and review articles covering this field or aspects of this field have been published [1-13]. Therefore, this chapter is not aimed to be a comprehensive review, but rather a concise overview of influential work conducted in the last decade.

1.2 Initiators

Most of the work published involving ROMP is performed using a handful of commercially available ruthenium-based initiators (Figure 1.2). Grubbs first- (G1) and second- (G2) generation ruthenium initiators were state of the art when the first edition of the Handbook of Metathesis was published. Shortly afterward, the high potential of pyridine-containing species (sometimes referred to as Grubbs third-generation ruthenium initiators (G3)) in ROMP was recognized [14, 15]. Nowadays, G3 and its indenylidene analog M31 [16] are the two most often used ruthenium initiators capable of providing controlled living polymerization.

Figure 1.2 Most commonly used initiators for ROMP.

G1 provides, in the case of many monomers, controlled living polymerization [17, 18] but its functional group tolerance is somewhat lower [19] than that of G2, G3, or M31. On the other hand, G2 is more active and provides fast polymerization because of the presence of the N-heterocyclic carbene ligand but does not provide controlled polymerization [20]. In this case, the initiation rate constant (ki) is lower than the propagation rate constant (kp), resulting in a low initiation efficacy and polymers with high molecular weight and broad molecular weight distribution [21]. Nevertheless, both initiators G1 and G2 are still in use for the preparation of polymers even today. The pyridine-bearing initiators G3 and M31 initiate very rapidly [15] and propagate fast (). They provide controlled polymerization and are therefore suitable for the preparation of block copolymers [16, 22, 23]. The initiator G3 bearing 3-bromopyridine as the ligand initiates the fastest; however, it is not particularly stable, so G3 with unsubstituted pyridine ligands is nowadays commonly used.

Other ruthenium initiators have also been used for ROMP, mostly designed to meet special requirements (Figure 1.3). Complexes 1 and 2 have been used as water-soluble ruthenium initiators [24, 25]. Complex 3 was aimed at fluorescence marking of ROMP polymers [26]. Complex 4, bearing a fluorinated phosphine ligand, has been used to demonstrate the feasibility of phase transfer activation in ROMP [27]. Complexes 5 and 6 are examples for initiators that can be activated upon irradiation with UV light. Complex 5 needs a tandem approach for its activation, that is, it must be used in combination with a photo acid generator [28]. Complex 6 is a commercially available example of an UV-activated initiator for ROMP [29, 30]. Activation of latent initiators upon a proper stimulus is an important research branch in ROMP. Other triggers such as UV irradiation, mechanical force [31], addition of acids [32] or anions [33], and higher temperature [32, 34] have also been investigated.

Figure 1.3 Ruthenium initiators designed for special applications (Mes = mesityl).

Complex 7 (Figure 1.4), initially synthesized by Förstner [35], was recognized by Grubbs [36] to provide cyclic polymers, which is very interesting, as their synthesis is hard to accomplish with conventional polymerization methods. Upon addition of the monomer, this initiator produces a polymer chain, which remains attached to the initiator at both ends (Figure 1.4). This intended situation leads eventually to intramolecular chain transfer via olefin metathesis, back biting, and with the growing polymer chain releasing the cyclic polyolefin and the initiator 7. This process was termed ring expansion metathesis polymerization (REMP) [36, 37]. The use of REMP allowed the preparation of cyclic polyethylene (PE) from cyclooctene (COE) (8) and subsequent hydrogenation of the resulting polymer [36]. Minor variations of the structure of the initiator, that is, varying the lengths of the tether, led to major effects on the kinetics of the polymerization. A tether length of 6 or longer provides rapid molecular weight growth, while tether lengths smaller than 6 gives competitive rates of propagation and initiator release. These tether length levels are valid for initiators bearing unsaturated NHC ligands, and a saturated NHC ligand generally leads to a higher activity at a given tether length [38, 39].

Figure 1.4 Mechanism of REMP and the monomers used.

Most work on REMP was done with COE (8) or cyclooctadiene (COD) (9) but functionalized monomers can also be used. A dendronized macro-monomer 10 (Figure 1.4) has been used under REMP conditions, leading to the formation of cyclic nanostructures with a diameter of 35-40 nm [40]. Recently, a REMP-derived cyclic macro-initiator derived from monomer 11 was used to prepare cyclic brush copolymers by combining REMP with triazabicyclodecene-catalyzed ring-opening polymerization of a cyclic ester [41]. Furthermore, REMP processes were developed for the synthesis of functional cyclic polymers, cyclic polymer brushes, and cyclic gels [42, 43].

1.3 Monomers

The most commonly used monomer for ROMP is norbornene (NBE) and its derivatives because their high degree of ring strain affords rapid polymerization. Most importantly, a whole family of NBE derivatives are easily accessible via the Diels-Alder reaction of cyclopentadiene (CPD) or furan and an olefin, for example, readily commercially available acrylates. Some challenges in controlled living polymerization arise because of the fact that endo-substituted NBEs polymerize much slower than their exo-substituted counterparts. In some cases, preferably diastereo-pure monomers have to be used, for example, ROMP of monomers 12 and 13 using G2 (Figure 1.5) [44-46].

Figure 1.5 Amino acid-functionalized monomers (12, 13), reactivity of disubstituted NBE derivatives (disubstituted NBE model), active ester-bearing NBE for post-polymerization functionalization (14), isoxazolino NBE derivative (15), NBE with functional unit for cell adhesion (16).

An NBE-based monomer is composed of the polymerizable group (the strained bicyclic moiety) and a functional unit (F) that is attached via an anchor group (A), in many cases via a spacer (S) (Figure 1.5). The anchor group, in addition to providing a synthetically feasible connection to functional unit, also influences the reactivity of the monomer during propagation [47, 48]. Furthermore, substitution of the norbornene with anchor groups...