Polymerization and Copolymerization of 4-DMVSBCB
The developing history of polymer materials tells that the invention of novel polymers with excellent performance is generally originated from the synthesis of new polymerizable monomers. Among kinds of monomers, benzocyclobutene (BCB) has attracted much interest because of its unique molecular structure and polymer properties [1-2]. The structure of BCB is shown as following with the numbering system, the CA name of which is bicyclo[4,2,0]octa-1,3,5-triene. In terms of the four-membered ring structure, BCBs are able to transform into the reactive
Nowadays, hundreds of BCB based structures have been synthesized, which constituted abundant resource for producing high performance and functional materials. Varying the structures and properties of BCB building blocks and constructing methods would afford materials with tunable or attractive properties and performance. Early prepared BCB monomers were mainly BCB-bicapped structures. As described previously, these compounds can be utilized to generate reactive oligomers which are processable intermediates to prepare polymeric materials. In its most basic statement, the concept of reactive oligomers involves in-situ conversion of low molecular weight compounds into high molecular weight materials. Reactive oligomers are in fact a very old technology with the earliest examples being found in the preparation of organic coatings. This technology however is limited to inferior film-forming properties. To address this problem, more recently, new structures were introduced to make a modification of present structures. Of them, poly (vinyl-benzocyclobutene) has attracted increased attention . In different with the polymers prepared from BCB-dicapped monomers, the BCB unit were placed on the side chains in poly (vinyl-benzocyclobutene). Thanks to their well-controllable structures, properties and performance, these polymers show potential application in nanoparticles preparation, surface modification, apart from microelectronic. On the other hand, early polybenzocylobutenes also suffer from relatively low thermal stability. As a respond to the requirement of high thermal resistance, a new class of polybenzocylobutenes, benzocyclobutene-siloxane resins, has been extensively studied. Through introducing of thermostablepolysiloxane chains, remarkable enhancement in thermal resistance has attained.
Overall, since Finkelstein reported the synthesis of a BCB derivative in 1909 for the first time, the research on BCB has experienced rapid development. The research field has extended from chemistry to materials even physics. Correspondingly, the research targets have also shown a transition from low molecular weight molecules to high molecular weight materials. Furthermore, the applications of BCB-based materials have extended from dielectrics to materials modification, nanostructure construction, and so on. All of these trends suggest a prospective future of BCB related materials. Previously, several excellent reviews have dealt with the development of BCB related science [1-2]. More recently, although new reviews involving the BCB chemistry have been reported [4-5], the progress in BCB related polymers has not been reviewed until now. In this chapter, we will present a review of BCB from 1996, which will concern about two types of newly developed BCB-based materials, poly (vinyl-BCB) and BCB-siloxane polymers.
From a closer look of the structures of the classical BCB resins prepared from BCB di-capped monomers, it can be noted that these cross-linked polymers were prepared by directly taking use of intramolecular Diels-Alder reaction and the functionality of these monomers was more than 2. As a result, cross-linking and polymerization take place simultaneously. Because they lack well-defined structures, conventional BCB-based polymers are difficult to provide controllable properties. Consequently, their applications in many areas are limited. In addition, the network structures are mainly constructed by the polycyclic structure. This highly rigid structure is in general unfavorable for generating higher cross-linking degree.
Polyolefins represent an important class of polymer whose structure could be conveniently controlled by sophisticated methods. More importantly, reactivefunctional groups are able to be introduced into polyolefins conveniently [6-7]. Such built-in cross-linkablefunctionality leads to two-step crosslinking, offering substantial advantages over alternativetwo-component mixtures or one-step formation ofcross-linked, network solids. First of all, chain growth and cross-linkingreactions become independent of one another,either expanding the materials application range orimproving theprocessability of thermally stable polymers. Second, thiscross-linking protocol provides complete control over thechemical structure of the primary polymer, networkstructures and physical/chemical properties of the matrix. Meanwhile, the cross-linking agent is incorporatedas a comonomer into the backbone of the relativehigh molecular weight polymers, allowing thecross-linking density to be directly related to themonomer composition. Hence, constructing new BCB polymers with polyolefin backbone are expected to open a new road towards controllable structure of BCB-based materials.
2.1. Preparation and properties of vinyl-BCB polymers
2.1.1. Poly (4-vinyl benzocyclobutene)
As a typical vinyl-BCB, 4-vinyl benzocyclobutene (4-VBCB) as shown in Scheme 1, was firstly synthesized by Endo et al.  and more recently by Hawker et al. . Of several well-established polymerization protocols, free radical and anionic methods have been so far demonstrated available for the polymerization of 4-VBCB. Copolymerization of vinyl-BCB with other vinyl monomers such as styrene, methyl methacrylate, and
In the above examples, 4-VBCB was generally used as a reactive functional group, which acts as active sites forcross-linking reactions.Takingpoly(styrene-r-4-VBCB) as an example, the crosslinking structure is composed of cyclic and linear structure (Scheme 2)as suggested in many reports [10, 12, 13]. Moreover, the crosslinking of these polymers were fulfilled by the Diels-Alder reaction of BCB with high atom economy.
The cross-linking of polyolefins by means of BCB presents a considerable significance in stabilizing the polymeric matrix, including enhancing the glass transition temperature (
On the basis of 4-VBCB and derivative poly (4-VBCB), Yang et al. have designed and synthesized a series of new vinyl-BCBs (Scheme 3), including 4-vinylsilylbenzocyclobutene (4-DMVSBCB) , 1-vinylsilylbenzocyclobutene (1-DMVSBCB), benzocyclobutene-4-yl acrylate (4-ABCB) [19-20], and benzocyclobutene-1-yl acrylate (1-ABCB). 4-DMVSBCB was synthesized conveniently by Grignard reaction of 4-bromobenzocyclobutene with vinyldimethylchlorosilane. With the same procedure, 1-DMVSBCB could be synthesized in good yield. 4-ABCB was synthesized through a facile esterification reaction. The detailed routes are outlined in Scheme 3. These new monomers, especially 4-DMVSBCB showed different polymerization behavior compared with 4-VBCB, probably caused by the incorporation of silicon containing group.
2.1.2. Poly (4-vinylsilylbenzocyclobutene)
As a vinylsilane, the polymerization of 4-DMVSBCB with different groups has been studied extensively. In fact, it exhibited almost the same behavior with another vinylsilane compound, 4-dimethylvinylphenylsilane (4-DMVSPh), which was reported to polymerize by the radical, anionic and coordination methods. The homo-polymerization of 4-DMVSBCB is sluggish when initiated by BPO or AIBN, only giving trace amount of poly(4-DMVSBCB) with
As compared with radical polymerization, the anionic polymerization of 4-DMVSBCB affords well-controlled and high molecular weight, and relatively low polydispersity. Anionic homo-polymerization of 4-DMVSBCB wasperformed using
|content of 4-DMVSBCB|
|AIBN 6.35||2.65||St, 2.69||60||48||5.3||45%|
|AIBN 8.74||4.26||St, 1.06||60||48||9.5||29%|
|AIBN 7.92||2.68||4-BrSt, 2.58||60||48||13||48%|
|BPO 7.38||3.19||St, 1.73||75||36||38%|
|BPO 8.68||2.16||MMA, 2.21||75||36|
|BPO 11.46||3.22||VA, 3.16||75||36|
Poly (4-DMVSBCB) shows several outstanding properties. First, it shows superior film forming property. A problem encountered in the film forming process for the low molecular weight polymer is its low viscosity. One solution to this issue was to pre-crosslink the polymer in prior to film forming. In the film forming process of the polymer derived from BCB bis-capped monomer, the pre-crosslinking process is hard to control as gel formation occurred immediately when gel point reached. In comparison, pre-crosslinking process of poly (4-DMVSBCB) is controllable because its polymerization and cross-linking processes were separated and the incorporating ratio of BCB cross-linkablefunctionality into polyolefins was controllable. Second, TGA in air shows that the initial decomposition temperature (
The copolymerization of 4-DMVSBCB with styrene and DMVSPh was also performed (Scheme 4). It is worth noting that the ratios of 4-DMVSBCB to 4-DMVSPh in copolymers showed well accordance with the feed ratios due to their similar structure and reactivity (Table 2). In contrast, when copolymerizing 4-DMVSBCB with styrene, the latter show higher reactivity and incorporation ratio than the former because of the conjugation effect of phenyl group. More importantly, the incorporation of styrene or DMVSPh into poly (4-DMVSBCB) did not lead to obvious reduction in thermal resistance, even when 1:1 of comonomers to 4-DMVSBCB was used.
2.1.3. Poly (1-vinylsilylbenzocyclobutene)
In comparison with 4-DMVSBCB, where the silicon atom bonds with benzene ring (Bz), 1-DMVSBCB exhibit a structure change that the silicon atom bonds with cyclobutene (Ct) (Scheme 5). This appeared slight structural change leads to a significantly alteration in polymerization behavior, especially for anionic polymerization. The anionic polymerization did occur for 1-DMVSBCBbut required several days. In addition, it was noted that the polymerization did not proceed except using
Benzocyclobutene-4-yl acrylate exhibits effective homo-polymerization and co-polymerization by free radical methods as its similar property with methyl acrylate (Scheme 6). BCB units are able to be incorporated into a series of polyarylates or other polyolefins by copolymerization. Related studies  showed that the incorporation ratio of 4-ABCB in copolymer could reach 6.7 mol% when using 4.7 mol% feed ratio of 4-ABCB. This result indicates that 4-ABCB exhibit higher polymerization activity than MA.
In recent years, apart from its conventional application in coating, adhesive, etc, polyarylates have been studied for use in a wide range of other areas such as gas separation membranes, hydrogels, tissue engineering, drug delivery, dentistry, and shape-memory materials for cardiovascular applications. Particularly, the potential applications of polyarylates as shape memory materials, which are capable of memorizing temporary shapes and recovering their permanent shape upon thermal activation, have attained much attention . For polyarylates to possess shape memory properties, it is necessary to have a permanent network which can be achieved via chemical or physical cross-linking. Interestingly, the cross-linking of polyarylates by the reaction of BCB generates nanoparticles which serve as physical cross-linkers . As a result, physical cross-links were generated, apart from chemical cross-links. This unique cross-linking architecture would be beneficial to broaden the list of cross-linking methods used in the production of SMPs.
Benzocyclobutene-1-yl acrylate (1-ABCB) exhibits similar radical polymerization behavior with 4-ABCB (Scheme 7). 1-ABCB also shows higher activity than MA as 1-ABCB/MA copolymerization results indicate that 1-ABCB/MA ratio in copolymer is higher than 1-ABCB/MA feed ratio. Interestingly, the initial ring-opening temperature of poly (1-ABCB) is approximately 170 oC, which is lower than that of 4-subsituted BCB.
The purpose of designing benzocyclobutene-siloxane structure is to introduce the excellent thermostability of siloxane materials into BCB structure. The benzocyclobutene-siloxane materials are expected to possess integrated properties of polysiloxane and BCB related materials. As described above, one representative resin developed earlier is poly (DVS-BCB). However, poly (DVS-BCB) shows poor controllability of the property of pre-crosslinked polymer used for film-forming. To solve this problem, Yang et al. have prepared a mixture of poly (4-VBCB-
On the other hand, the application of poly (DVS-BCB) have been limited largely in several standard interconnects due to their lower thermal stability and higher fragility relative to polysiloxanes. It implies that a simple combination of siloxane and BCB structure scarcely affords materials with high performance as expected. Structural analysis shows that the relatively low thermostability of poly (DVS-BCB) may be due to the incomplete crosslinking of DVS-BCB, the presence of residue ethylene moieties which are prone to take oxidation reaction, or the BCB-siloxane alternating copolymerization structure which destroys the thermally stable Si-O-Si backbone. Yang et al. have prepared a new siloxane-BCB monomer, 1,1,3,3-tetramethyl-1,3-bis[2’-(4’-benzocyclobutenyl)]vinyldisiloxane (DES-BCB) (Scheme 9). Ethyl was introduced instead of vinyl in DVS-BCB, which might enhance the thermal oxidation stability and decrease the fragility of cross-linked polymers simultaneously. However, TGA of poly (DES-BCB) shows a relatively inferior thermal stability with 5 % weight loss temperature of 380 oC, indicating that alkene oxidation would not be responsible for the inferior thermal stability of poly (DVS-BCB).
In their further work, a new BCB-siloxane polymer with polysiloxane backbone and BCB pendant groups was reported. This polymer was prepared by Pt/C or H2PtCl6catalyzedhydrosilylation between poly (methylhydrosiloxane) and 4-DMVSBCB or poly (methylvinylsiloxane) and 4-dimethylhydrosilylbenzocyclobutene (4-DMHSBCB) (Scheme 10). The incorporation ratio of 4-DMVSBCB pendant groups is tunable, andthe maximum ratio is up to 72 %. The corresponding cross-linked BCB-siloxane resins exhibit an initial decomposition temperature of 429 oC, showing a superior thermal resistance over poly (DVS-BCB) resins and other common polysiloxanes.
Fan et al  have prepared two novel siphenylene-siloxane copolymers bearing latent reactive BCB groups by polycondensation procedure of 1,4-bis (hydroxydimethylsilyl) benzene with bis (dimethylamino) methyl (4’-benzocyclobutene) silane and bis (dimethylamino) methyl [2’-(4’-benzocyclobutenyl)vinyl] silane (Scheme 11). The BCB-based monomers were synthesized from Grignard and Heck reaction, respectively. Copolymers with high molecular weight (
In their further work , two new methylphenylsiloxane monomers terminated with cross-linkable benzocyclobutene functionalities, 1,1,5,5-dimethyldiphenyl-1,1,5,5-di[2’-(4’-benzocyclobutenyl)vinyl]-3,3-diphenyltrisiloxane (BCB-1) and 1,1,3,3-dimethyl-diphenyl-1,1,3,3-di[20-(40-benzocyclobutenyl)vinyl]disiloxane (BCB-2), were prepared (Scheme 12). These two monomers possess similar structure with DVS-BCB. Correspondingly, they show a simultaneous polymerizing and cross-linking process. Cured BCB-1 and BCB-2 exhibited good thermal stability (
3. The application of BCB-based materials
3.1. Low dielectric materials
The performance of integrated circuit devices, historically limited by characteristics of the transistors, has become limited by the back-of-end-of-line (BEOL) signal delay of interconnect caused by wire capacitance, crosstalk and power dissipation. As one of the major technological strategy to decrease signal delay, developing new generation of low dielectric materials as alternatives of SiO2 have received much attention in recent years. Besides low dielectric constant, a qualified dielectric materials must satisfy other requirements, including high thermal stability, high mechanical strength, high breakdown field above 2 MV cm-1, low moisture absorption, low thermal swelling coefficient or film stress, good adhesion to various matrix, ease of damascence processing or etching, reactive inertness, good film forming property, etc. Among kinds of the accessible alternatives, some polymers with outstanding and promising mechanical and physicochemical properties have been widely studied . In the past few years, the studies were mainly concentrated on the following polymers: polyimides, BCB resins, poly (binaphthylene ether), polydiphenyl, polyquinolines, polynorbornene, silicon-containing polymers and fluorinated polymers.
Although several potential applications were discovered for BCB-based materials, only the use as low dielectric materials was accessible for application until now. One of the reasons for the intense research on BCB-based low dielectric materials is that the thermal activation would generate a highly reactive intermediate to form clean products, which are beneficial for designation of high precision curing processes. The application of BCB resins in microelectronic should start from Cyclotene . So far, several formulations of these BCB-based resins have been commercialized for thin-film dielectric applications and are currently used in the fabrication of liquid crystal displays, GaAs integrated circuit interconnect, interlayer connection and chip bumps. Although BCB-based resins displayed several advantages over many other polymeric materials, their use in standard interconnect is limited by their relatively low thermal stability than the required process temperatures.
Table 2 lists the thermal and dielectric performances of BCB-based materials reported in the past few years. Most of them show low dielectric property with dielectric constant below 2.65, it is worth noting that poly (4-DMVSBCB) possesses the lowest dielectric constant probably due to the absence of polar atoms in this carbosilane structure. Moreover, these new BCB-based materials show higher thermal stability except poly (4-vinylBCB). Most of them could be considered as potential alternatives of poly (DVS-BCB).
3.2. Vinyl-BCB used for the functionalization of Carbon Nanotube
The methods that have been explored for the functionalization of the sp2 network of the carbon nanotubes (CNTs) include addition reactions ofazomethineylides, bromomalonates, nitrenes, nucleophiliccarbenes, and free radicals. In 2007, Baskaran et al. prepared functionalized CNTs by (4+2) cycloaddition of BCB for the first time . The stained ring of the BCB is susceptible to thermal ring opening to form
|Poly (4-DMVSBCB)||380||428||< 2.45|
3.3. Applications in Organic Light Emitting Diodes (OLED)
OLEDs have attained much research interest in the past few decades due to their promising applications in full color displays and solid-state lighting. Device fabrication, involving the construction of hole-transporting layer (HTL), phosphorescent emitting layer (EL), and electron-transporting layer (ETL) multilayers structure, shows great impact on the ultimate performance. There are usually two methods to fabricate the layers. One is high vacuum vapor deposition of small molecules, and another is solution processing of polymers or dendrimers. The solution processing technique presents advantages in thermostability. However, it requires that each deposited layer must resistant to the solvent that used to deposit subsequent layers. Perhaps the most elegant strategy to address these issues involves the development of cross-linkable materials. To date, most of the reported thermally cross-linkable hole-transport materials are based on perfluorocyclobutane (PFCB) cross-linking unit. Recently, a novel thermally cross-linkable hole-transporting polymer for solution processible multilayer OLEDs was designed . This amorphous polymer was prepared from the radical copolymerization of vinyl-BCB with 4-(N-(4-Vinylphenyl)-N-(4-methylphenyl)amino)-4’-[N-phenyl-N-(4-methylphenyl)amino]biphenyl initiated by AIBN (Scheme 14). It contains ~9 mol % of BCB with the
3.4. Applications in organic field-effect transistors
Organic field-effect transistors (FETs) are promising in enabling disposable electronics and flexible electronics technologies. Fabricating ultrathin defect-free gate dielectric layer with a high quality interface to the semiconductor layer is crucial for polymeric FETs. In recent years, several polymer dielectric materials including spin-on silesquioxanes, photoresist, polyimide and poly (vinylphenol) (PVP) were investigated. However, these polymers are not wholly satisfactory because they still cannot give conformal and pinhole-free films in the sub-100-nm regime. Consequently, present spin-on polymer dielectrics are typically limited to more than 300 nm in thickness, thus demanding a large gate voltage to operate. Recent works show that DVS-BCB possesses most of the requisite gate dielectric properties . Furthermore, defect-free films down to a few tens of nanomaters in thickness are attainable by a simple solution casting technique. The fabricated devices can operate at a low voltage with a field-effect mobility of few 10-4 cm2/Vs. They can be continuously operated at 120 oC, showing a good operating stability.
4. Perspective of BCB related chemistry and materials
4.1. Potential polymerization methods
Although most of the vinyl-BCBs can be polymerized by anionic and radical polymerization methods, their copolymerization with styrene, MMA, ethylene and so on encountered a big challenge as the great difference polymerization activity between them. Coordination polymerization method as a powerful tool has promoted a great development of polymer chemistry in the last decades. However, the coordination polymerization of vinyl-BCB monomers has not been reported yet. In fact, most of these monomers mentioned above are capable of polymerizing through coordination polymerization in terms of its similar structure with styrene and acrylate. Of course, the coordination polymerization of vinyl-silylBCB, such as 4-DMVSBCB and 1-DMVSBCB, is more difficult due to the Lewis acid property of silicon element and the large steric hindrance. The coordination polymerization of several simple vinylsilane compounds, indeed, has been studied. The firstly used catalysts were Ziegler-Natta catalysts, which however failed to induce accepted polymerization performance. An acceptable result was obtained till 2008 when Nomura et al has demonstrated a successful copolymerization of vinyltrialkylsilanes with ethylene catalyzed by nonbridged half-titanocenes . Thus, in this regard, a large number of works including new high-efficiency catalysts developing and in-depth polymerization behavior and mechanism studying are highly desired in the field of vinyl-BCB related polymer chemistry.
Earlier work demonstrated that free radical method could not initiate the polymerization of vinyl-silylBCB. Nevertheless, it does not mean that the radical polymerization definitely fails to give products with high molecular weight and poor yields.Theoretically, the living free radical method could stabilize the initiated monomer free radicals, thus extending the lifetime of them and consequently enhancing polymerization activity. In addition, living radical polymerization of silyl-functionalized vinyl monomers with large steric hindrance, for example vinylcyclicsilazane, has been indeed reported . The use of RAFT agents with anionic characteristic displayed excellent activity, giving the product with molecular weight up to 11300 g/mol. Thus, further studies regarding the radical polymerization of vinyl-silylBCB should include the investigation of living polymerization, such as RAFT and ATRP. Moreover, these methods can also be used for the polymerization of vinyl-BCB to afford improved and controlled molecular weight, enabling the generation of random even block copolymers.
4.2. Low temperature cross-linking
Although the cross-linking by BCB ring-opening reaction presents several superiorities over common cross-linking ways, its high ring-opening temperature would induce the deformation of films due to the generally low
Currently, the nanomaterials with unique chemical structure and corresponding high performance and attractive functions have been attracted increased attention. Hence, exploiting vinyl-BCB based materials with nanostructure would be an interesting research topic because of the cross-linkable characteristic upon heating and the unique cross-linking behavior of BCB. Based on the well-established in-situ methods to construct nanostructures from polymerization of olefins, similar nanostructures based on poly (vinyl-BCBs) could be easily constructed. These BCB-based nanomaterials can be potentially used as adsorption materials, optic-electronic materials, coating materials, and so on.
More recently, Harth et al. present a new BCB based cross-linking structure that allows a 100 oC lower ring-opening temperature in comparison with conventional ones . They found that the incorporation of BCB based cross-linking unit by copolymerization shows a limited molecular weight control. Thus, as an alternative, they explored a grafting-on method by grafting a low-temperature cross-linking unit, 2-(1,2-dihydrocyclobutabenzen-1-yloxy)ethanol, onto a linear acrylate polymer backbone (Scheme 16). To afford the required end group functionality, a convenient method was adopted to convert the alcohol to an amine for grafting onto the polymer. The polyacrylate was prepared via reversible addition-fragmentation transfer (RAFT) polymerization. A polymeric precursor with a 75,000 Mwpolyacrylate with a PDI of 1.21 was obtained using AIBN catalyst towards the RAFT initiator. The amine was grafted to poly (acrylic acid) through chloroformate activation chemistry and the 5 % conversion ratio of the carboxylic acid functional groups to functionalized benzocyclobutene units was obtained. Monitoring by 1H NMR show that the ring-opening was initiated at the temperature higher than 100 oC and the optimal temperature is above 130 oC at which the polymeric precursor could be fully converted. Dynamic Light Scattering (DLS) and TEM confirmed the formation of nanoparticles from the cross-linking of linear copolymers. TEM and DLS suggest that the mean diameter of nanoparticles is approximately 4.3 ± 0.8 and 7.3 ± 0.5 nm, respectively.
In the past few years, the approach to the utilization of benzocyclobutenes to construct polymers has experienced significant change. BCB-related polymers based on two-step cross-linking technology, represented by poly (vinyl-BCB)s, has brought remarkable improvement in controllability of materials structures and properties. Apart from this, their excellent polymerization compatibility with traditional olefins allows the convenient incorporation of BCB functionalities, significantly improving properties of original polymers and extending the potential application of BCB-related polymers. Besides, a series of BCB-siloxane polymers with novel structures exhibiting excellent thermal stability and low dielectric properties have been prepared. Indeed, some of these polymers are promising alternatives of poly (DVSBCB) and would be widely used in microelectronic devices. These exciting works are just a beginning. A new window has been opened for BCB related science and technology. The future development will mainly focus on the following areas: 1) searching for more suitable BCB-related dielectric materials; 2) utilizing BCB to construct novel structures and improve performance of original materials; 3) exploiting more applications suitable for BCB-related polymers.
The authors would like to thank the financial support through the National Natural Science Foundation of China-NSAF and China-NSFC, and Open Project of State Key Laboratory Cultivation Base for Nonmetal Composites and Functional Materials for Southwest University of Science and Technology. Prof. Yang would like to thank students from his research for their contributions.
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