**1. Introduction**

Functional siloxane polymers constitute a large class of reactive materials. Siloxanes with vinyl, silanol, and hydride substitution are the most widely utilized, serving as the basis for the majority of elastomeric silicone products. Functional siloxanes combined with organic monomers form "hybrid" polymers which, despite their demonstrated utility, have comparatively limited commercial applications. Most siloxane polymers are prepared by ring-opening polymerization with high degrees of polydispersion, thereby curtailing their ability to act as precise structural elements. On the other hand, the economics of both the basic building blocks and the polymerization process itself favor equilibrium-derived siloxane polymers. Figure 1 depicts the range of synthetic methods utilized to prepare siloxane polymers.

Due to the intrinsic process as well as the structural control that it provides, Living AROP-derived polymers provide the potential for a broad range of hybrid organic– inorganic materials. Briefly, AROP-derived materials provide a mechanism for translating macromolecular synthetic methods normally associated with organic polymers (and excluded from inorganic polymers) into hybrid polymer structures. Among these AROPderived siloxane polymers, perhaps the most technologically significant are monofunctional

**Citation:** Goff, J.; Sulaiman, S.; Arkles, B. Applications of Hybrid Polymers Generated from Living Anionic Ring Opening Polymerization. *Molecules* **2021**, *26*, 2755. https://doi.org/10.3390/ molecules26092755

Academic Editors: Sławomir Rubinsztajn, Marek Cypryk and Marek Cypryk

Received: 23 March 2021 Accepted: 22 April 2021 Published: 7 May 2021

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**Copyright:** © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

and heterobifunctional silicone macromers. Siloxane macromers are defined as siliconcontaining species with a single functional polymerizable group which, although used as monomers, possess sufficiently high molecular weight and enough internal monomer units to be considered polymeric. In another sense, they are siloxane building blocks derived from Living AROP. Siloxane macromers enable the use of technologies other than those associated with siloxane polymerization—e.g., techniques associated with the wider range of synthetic organic polymerization technologies—to incorporate siloxane-associated properties: e.g., techniques associated with the wider range of synthetic organic polymerization technologies. Notably, only controlled "living" AROP provides a path to siloxane polymers with sufficiently controlled structures and functionality to behave as macromers in polymerization with organic monomers. Siloxane macromers thus enable the introduction of selected siloxane properties into higher order structures via macromolecular engineering.

**Figure 1.** Synthetic methods for preparation of siloxane and siloxane-hybrid polymers.

Although the advent of living anionic ring-opening polymerization can be traced back approximately 50 years, commercial applications of this technology have only reached the marketplace within the last 20 and, by our estimate, production has exceeded 100 tons per annum only in the last 5. The availability and range of polymers with tailored molecular weight, polymer backbone structure and basic architecture, and both functional and nonfunctional alternatives have grown enormously, stimulated by symbiotic application and development efforts. This review relies heavily on patent literature; the global collection of patents, of which there are over 10,000,000 in the US alone, is one of the most comprehensive collections of technical information in the world but is often neglected in scholarly reviews. Nevertheless, the impact of patent technology on how materials are prepared and utilized may equal or exceed that of academic literature.

#### *1.1. Fundamentals/Building Blocks/Architectures*

The "living" anionic ring-opening polymerization of cyclosiloxanes is in fact more properly described as kinetically controlled ring-opening polymerization. The features that define the "living" aspects of the polymerization are: a quantitative initiation (as shown in Scheme 1), and the fact that the rate of polymerization propagation is significantly greater than that of the polymer chain randomization processes, particularly the reversion of the degree of polymerization driven by equilibration or "back-biting" processes (as shown in Scheme 2) [1].

**Scheme 1.** Kinetically Controlled Polymerization.

**Scheme 2.** Thermodynamic Equilibrium Driven Processes (i.e., Reversion, Backbiting, Chain-scrambling).

> Both initiation and back-biting are driven by the catalytic opening of a siloxane bond. The evolution of Living AROP has depended on the recognition of classes of cyclic siloxane monomers that possess ring strain, as well as on "weak" catalysts which are able to rapidly cleave the Si-O-Si bonds of the strained monomers but are relatively slow and ineffective at cleaving Si-O-Si bonds in unstrained systems. The difference in reaction kinetics provides an opportunity to deactivate the catalyst before significant equilibration effects are observed, resulting in the scalable preparation of polymers with polydispersities approaching 1.

> The differential polymerization of ring-strained cyclics, as opposed to unstrained cyclic siloxanes, was apparently observed in early industrial development, as is made particularly clear in the example of fluorinated silicones generated from 3,3,3-trifluoropropylmethylcyclic siloxanes [2]. Attempts to polymerize cyclic tetrasiloxanes were ineffective due to the fact that the reversion kinetics apparently matched those of polymerization for the unstrained cyclic tetramer. The polymerization of cyclic trisiloxanes, on the other hand, was effective due to the ability to deactivate the catalyst before significant reversion could occur. The potential for weak catalysts such as lithium phenoxide to produce polymers of ring-strained monomers was first recognized by McVannel [3], while the quantitative, selective formation of a lithium initiator generated from the reaction of organolithium reagents with cyclic trimers was studied by Frye [4], who, surprisingly, observed that, in a 1:1 molar stoichiometry, *n*-butyl lithium reacted with hexamethylcyclotrisiloxane (D3) to form lithium n-butyldimethylsilanolate, leaving 2/3 of the D3 unreacted. Lee and Frye also noted that polar "promoters" would then cause polymerization to proceed [5]. Finally,

Fessler reported the relative effectiveness of promoters in the living polymerization process and provided mechanistic insight into siloxane-silanolate reactions that could result in shifts between triad, Gaussian, and redistribution products, as shown in Scheme 3 [6].

**Scheme 3.** Ion-pair Complexation by Dimethoxyethane (DME) Promoter Favors Chain-End Over Internal Scrambling [5].

A comparison of a GC (gas chromatograph) of siloxane macromers with equivalent MW and PDIs, showing triad and Gaussian distributions, is provided in Figure 2 (author's work). In contrast to anionic polymerization with K+ and Na+, in which there is little differentiation between chain scission points, the Li+ redistribution mechanism favors chain termini: i.e., despite chain-end scrambling, narrow polydispersity is maintained.

**Figure 2.** Comparison of silicone macromers demonstrating polymer termini scrambling (GC of methacrylate functional macromer (MCR-M11) with a nominal MW of 1000 showing shift from Triad to Gaussian distribution with a change in promoter. (Author's work)).

During the same period when Lee and Frye's report was published but separate from the discoveries relating to ring-strained siloxane monomers and lithium-based initiators, there was significant interest within the silicone industry in generating block copolymers, with lithium silanolate-based initiators being shown to lead to the sequential polymerization of cyclotrisiloxanes. The combination of the growing interest in forming block copolymers, the development of siloxanes with strained cyclic structures, and quantitative lithium catalyzed polymerization underlies the publication by Saam [7,8] that reviewers point to as establishing the potential of "living" AROP siloxanes—in which he clearly demonstrated initiation, promotion, narrow MW distribution, and the ability to form block copolymers. It was recognized at that time that these polymers had to be terminated before equilibrium processes dominated in order to maintain the target molecular weight, as visualized in Figure 3. Functionalized termination reagents were later used to create

siloxane macromers [9,10], although a process for preparing these macromers was not reported until 10 years later [11].

**Figure 3.** Time-dependent ROP polymer growth of strained versus unstrained cyclosiloxanes.

The term "macromer" is a contraction of the word macromonomer and refers to a relatively high molecular weight species with a single functional group which, although used as a monomer, has sufficient internal monomer units to be considered a polymer. The earliest commercial siloxane macromers contained methacrylate functionality and found commercial utility in the formation of organic–inorganic hybrid polymers (Scheme 4). Their termination, or "capping", functionality was derived from the use of methacryloxypropyldimethylchlorosilane. The general structure for a siloxane is depicted in Scheme 5. Variations of the basic structure are depicted in Schemes 6–8.

**Scheme 4.** Polydimethylsiloxane macromers with symmetric architectures, produced via the anionic ring-opening polymerization of a cyclic trisiloxane and subsequent coupling with a functional dihalogen-substituted silane reagent, have recently been described.

**Scheme 6.** Symmetric macromers can contain a pendant methacrylate group bisecting the polydimethylsiloxane backbone.

**Scheme 7.** Symmetric silicone macromer with methacrylate functionality bisection.

A second class of functionality can be introduced into macromers by using novel initiators, thereby yielding telechelic polymers in which the second functional class—e.g., hydroxyl—is located at the telechelic polymer termini, which are equidistant from the first functional class.

**Scheme 8.** Carbinol terminated telelechelic with macromeric methacrylate functionality.

More recently, living AROP has been combined with the concept of functional initiators to generate both monodisperse telechelic and heterobifunctional siloxanes.

This historic overview has only given a condensed description of the chemistry, structure, and function of siloxanes derived via living AROP. For those interested in more details regarding the chemical aspects of siloxanes derived from living AROP, the following references should be consulted [1,12,13]. While the bulk of the literature and commercial applications utilize a lithium anion as a weak base component of the initiator, an intriguing recent series of reports utilize substituted cyclic guanidines in combination with water or silanol that act as initiators for living AROP [14,15].

The following tables summarize monomeric building blocks (Table 1), the initiators (Table 2), functional terminations, and architectures that have shown practical utility in commercial applications.

#### *1.2. Monomeric Building Blocks*

The fact that there are only five monomeric building blocks for the living AROPderived polymers that represent virtually all commercial applications as well as the vast majority of published reports is a consequence both of practical synthetic routes [16–18] and the sluggish rates of polymerization reported for cyclotrisiloxanes with greater organic substitution [19]. A preparation has been reported [20] for the simple and highly desirable monomer trimethylcyclotrisiloxane, but its practical isolation has not ye<sup>t</sup> been described. This has led to interest in pentamethylcyclotrisiloxane [21] and hydridotetramethylsiloxanylethylsubstituted cyclotrisiloxanes [22], which can be used directly as monomers for polymerization or reacted with various olefins to form more elaborately substituted trisiloxanes. Similarly, vinylpentamethylcyclotrisiloxane has been prepared, and offers an advantage over trivinyltrimethylcyclotrisiloxane in cases where isolated vinyl substitution on the polymer chain is desired [23]. The primary cyclic monomers used in the production of macromers on both commercial and research levels are listed for convenience in Table 1. Other cyclic monomers have been reported in the literature, and include higher hexaalkylcyclotrisiloxanes [24], chloropropylmethylcyclotrisiloxanes [25], and substituted hexaarylcyclotrisiloxanes [26]. Specialty monomers with limited reference include D4 [27], acrylate [28], cyclic ether [29], trimethylsiloxy-substituted [30] cyclotrisiloxanes, and similarly substituted strained carbosiloxanes [31].


**Table 1.** Cyclic Siloxane Macromer "Building Blocks".

#### **Table 2.** Initiators.





