Controlled/Living Polymerization Towards Functional Poly(P-Phenylene Vinylene) Materials† Cite This: Polym
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Polymer Chemistry View Article Online REVIEW View Journal | View Issue Controlled/living polymerization towards functional poly(p-phenylene vinylene) materials† Cite this: Polym. Chem., 2016, 7, 1355 N. Zaquen,a L. Lutsen,b,c D. Vanderzandeb,c and T. Junkers*a,b Poly(p-phenylene vinylene)s (PPVs) are an important class of highly fluorescent polymeric semiconductor materials. Despite their somewhat declining use in optoelectronic applications, PPV synthesis routes were in recent years significantly improved towards controlled/living polymerization. In this way, nowadays well-defined PPV structures that can be implemented in advanced polymer structures have become accessible, finding a potential application in new fields of research. This review summarizes the advances Received 13th December 2015, made and the types of polymers that have recently become available. Most notably, two polymerization Accepted 11th January 2016 approaches are compared, living polymerization towards well-defined PPVs via ring-opening metathesis DOI: 10.1039/c5py01987g polymerization (ROMP) and chain-transfer radical and anionic polymerization in the so-called sulfinyl pre- www.rsc.org/polymers cursor polymerization route. Creative Commons Attribution-NonCommercial 3.0 Unported Licence. 1. Introduction Since the discovery of polyacetylene as the first type of pi- conjugated polymer material in the 1970s,1 a broad variety of – (semi)conducting polymer materials were developed.2 5 With plastic electronic applications such as organic light emitting Fig. 1 General structure of poly(p-phenylene vinylene)s (PPVs). – diodes (OLEDs)6 and organic photovoltaics (OPVs),7 12 these This article is licensed under a materials found a secure position in innovative electronic applications and devices. Among the most common types of Next to step-growth procedures, which mostly provide oligo- – conjugated polymer materials, the class of poly(p-phenylene (phenylene vinylenes),19 22 most predominantly chain-growth 13,14 Open Access Article. Published on 12 January 2016. Downloaded 9/24/2021 12:58:33 AM. vinylene) (PPV) polymers play a special role. PPVs are polymerizations are used to obtain PPVs with moderate to − robust polymers that are comparatively simple in their syn- high molecular weight, ranging between 4000 g mol 1 to − thesis and well-reproducible in their physical characteristics 100 000 g mol 1.23 Since the direct synthesis of PPV is often (depending on the substituents on the 2 and 5 positions at the tedious, in many cases so-called precursor polymerization phenyl core). While initially interesting for their electrolumi- routes are employed,24,25 in which a precursor polymer is nescent properties (the first OLED was made from PPV obtained from an initial polymerization, then converted into material),15,16 their use in optoelectronics has strongly the conjugated polymer in a post-polymerization step. As men- declined over the past years due their somewhat lower per- tioned above, all these synthesis methods are comparatively formance in photovoltaic devices, compared to the use of e.g. popular and straightforward in use; however, with only one pyrrole-based structures (Fig. 1).17 notable exception (metathesis polymerization),26 reactions are However, the synthesis of PPVs has been well studied and a fully uncontrolled. Uncontrolled means in this respect that no broad variety of synthesis techniques have been developed to or only insufficient control over molecular weight is achieved, date, making PPVs very valuable as work-horse materials.18 that end-groups of the polymers are not well defined and that dispersity is often high. As long as the main application of PPV was in optoelectronics, this was not an issue. For these aPolymer Reaction Design (PRD) Group, Institute for Materials Research (IMO), Hasselt University, Martelarenlaan 42, 3500 Hasselt, Belgium. applications, mechanical and optical properties (specifically E-mail: [email protected] the band gap) are of importance, thus ideally requiring good bIMEC associated laboratory IMOMEC, Wetenschapspark 1, 3590 Diepenbeek, control over chain defects and microstructure but less over Belgium molecular weight and end-group functionalities.27 Function- c Design and Synthesis of Organic Semiconductors Group, Institute for Materials ality – when required – was usually introduced via attachment Research (IMO), Hasselt University, Martelarenlaan 42, 3500 Hasselt, Belgium †Electronic supplementary information (ESI) available. See DOI: 10.1039/ of side-chains at the 2 and 5 positions of the phenyl ring, c5py01987g which is also required to keep PPVs soluble and hence This journal is © The Royal Society of Chemistry 2016 Polym. Chem.,2016,7, 1355–1367 | 1355 View Article Online Review Polymer Chemistry processable.28,29 Although controlled synthesis of PPVs for optoelectronic applications could be of interest from a funda- mental point of view,30 the major focus in this review is placed on biomedical application-driven controlled synthesis of PPVs. As PPVs and their derivatives have been mostly replaced in electronic applications, the question arises whether they can be used in other fields of application as well. An outstanding – feature of PPVs is their inherent fluorescent properties,31 33 hence PPVs are excellent candidates for biomedical appli- cations, where fluorescent properties are used broadly for imaging and in fact, a steep rise in the usage of conjugated polymers in bioimaging and theranostic applications has been – seen in recent years.34 40 PPVs play a prominent role here due to their good performance with respect to fluorescence but also due to their high reliability in properties (good reproduci- bility of polymerizations, especially when using micro-reactor technology)41 and the relative purity in which they are obtained, in the absence of toxic catalysts and heavy metals. However, in order to employ these materials in biomedical research, their complexity and functionality must be inherently increased. To interact in – or to mimic – biological processes Scheme 1 (a) Simplified olefin metathesis mechanism and (b) conver- and environments, the ability to self-assemble is required, as sion of cyclic alkenes to polymers in ROMP. ffi Creative Commons Attribution-NonCommercial 3.0 Unported Licence. is the potential to e ciently couple bioreceptors or other materials to the conjugated materials. In other words, con- trolled/living polymerization strategies have become a neces- architectures, giving access to a broad variety of interesting sity for the further evolution of conjugated polymers. For materials. A variety of catalysts as well as monomers are various types of conjugated polymers, living polymerization employed, with the Grubbs and Schrock catalyst as well as the strategies such as, for example, the Grignard metathesis norbornene monomer (NBE) and its derivatives being the polymerization (GRIM), were introduced,42 which gives access most prominent ones.51,52 to complex block copolymer structures and which allows ROMP mechanism applied to the synthesis of PPVs. With chain-end functionalization of polymer chains. respect to conjugated polymers, ROMP features a significant This article is licensed under a Here, we now focus on the strategies that can be used to advantage: via the olefin metathesis reaction, a vinylic bond is control polymerizations leading to PPVs, namely on the ring- directly obtained in the main chain of the polymer. Hence, the opening metathesis polymerization (ROMP) and the so-called pi-conjugated main-chain structure of PPV can in principle be Open Access Article. Published on 12 January 2016. Downloaded 9/24/2021 12:58:33 AM. anionic and radical precursor polymerization routes. This directly made available via ROMP. In fact, polyacetylene can be review is hence an update on a summary of (uncontrolled) PPV accessed via ROMP of cyclooctatetraene.53 Over the last few polymerization techniques that our group published a few decades, PPVs have mainly been synthesized by the polymeriz- years ago.18 ation of a bis(halomethyl) benzene via the addition of a strong base (e.g. NatBuO or LiHMDS; see section 2.2).18 Still, several attempts have been made to use ROMP for the synthesis of 2. Controlled synthesis of PPVs PPVs. In principle, a relatively high synthetic effort is required to reach this goal as monomer synthesis is tedious. Yet, ROMP 2.1 Ring-opening metathesis polymerization is inherently a living polymerization process (if transfer reac- Ring-opening metathesis polymerization (ROMP) is a chain- tions are absent) and not only is the conjugated polymer growth polymerization process where unsaturated cyclic system directly accessed by ROMP, but also block copolymers olefins are converted into polymeric materials. The reaction is and high-end-group functionality can be directly achieved. The based on olefin metathesis, a process that found its origin in first research in this field was performed by Kumar et al.,54 in – the 1950s.43 48 Here, a metal–carbene (alkylidene) complex which 1,4-divinylbenzene was treated under vacuum using a undergoes reversible [2 + 2] cycloadditions with olefins, tungsten alkylidene Schrock catalyst,55 yielding an insoluble thereby completing metathesis via metallacyclobutane inter- PPV oligomer in good yields and with low molecular weight 49,50 mediates, as shown in Scheme 1. When employing ring- (degree of polymerization, DPn, of 6). A follow-up of this work strained cyclic alkenes, olefin metathesis leads to a polymeriz- by Fox et al.56 confirmed