Polybutadiene photovulcanization: A novel approach using phenacylbromide under uv-a irradiation for the production of robust thermosets

Abstract

The invention of synthetic polymers has greatly influenced today's lifestyle by supplying products that enhance convenience, functionality, and overall quality of life. Due to their exceptional versatility, polymers are essential in everyday life, as they may be synthetically engineered to exhibit a broad spectrum of qualities, from stiffness to flexibility, according on the specific application. The increasing need for these materials accelerates advancements in polymer science and technology globally, encompassing both university research facilities and industry production. Typical instances demonstrating the extensive use of polymers included textiles and fibers, packaging films and furniture as flexible plastics, optical lenses and instrument covers as stiff plastics, and rubber bands and tires as elastomeric materials. Rubber has been utilized for centuries due to its exceptional mechanical properties. The global demand for natural and synthetic rubber has steadily increased, with total production exceeding 28.8 million metric tons in 2023. This remarkable polymer not only influenced the trajectory of wars and civilizations but also remains a fundamental component of modern industrial mechanization. However, as rubber shaped history, it was also reshaped by it. The increasing and often unmet demand for natural rubber (NR), coupled with its supply limitations and geographic dependency, catalyzed a transformative solution: the invention and widespread adoption of synthetic rubber (SR). Natural rubber consists of repeating cis-1,4-isoprene units, whereas synthetic rubbers are composed of various repeating units, such as butadiene, styrene, acrylonitrile, or chloroprene, depending on the desired properties and applications. Among these, polybutadiene-based synthetic rubbers include polybutadiene (pBD), styrene-butadiene rubber (SBR), and acrylonitrile-butadiene-styrene (ABS), all of which utilize butadiene as a key monomer to enhance elasticity, durability, and chemical resistance, and are widely utilized in tires, belts, gaskets, adhesives, seals, and various industrial applications due to their versatility and durability. The development of advanced polymerization methods, such as anionic and emulsion polymerization, and the introduction of Ziegler-Natta catalysts for selective double-bond production revolutionized synthetic rubber manufacturing, enabling cost-effective production, enhanced material properties, broader industrial applications, and a significant rise in its integration into everyday products and technologies. One of the defining characteristics of rubber materials is their three-dimensional network of covalent bonds, formed through chemical crosslinking, which imparts elasticity, durability, and resistance to deformation. Prior to the development of crosslinking methods, the use of natural rubber was severely limited; it was brittle in winter and soft and sticky in summer, restricting its application to simple products like erasers. This limitation was overcome in 1839 when Charles Goodyear discovered "accidentally" the vulcanization process, wherein rubber chains are chemically bonded via sulfur crosslinks under heat, resulting in a thermally stable, elastic material that revolutionized the rubber industry. Although thermal vulcanization with sulfur remains the dominant industrial process, it often requires high temperatures and prolonged curing times, which can be limiting for temperature-sensitive applications. To address these challenges, alternative crosslinking approaches, such as photochemical curing (photocrosslinking), have gained attention due to their milder conditions, rapid reaction rates, and spatial control. Polybutadiene (pBD), with its high content of unsaturated C=C bonds, is particularly suited for photocrosslinking processes, offering an efficient route to rubber networks without the need for elevated temperatures. Under UV irradiation, hydrogen abstraction from allylic positions adjacent to double bonds generates radicals, which initiate chain propagation and crosslinking reactions. Photo-vulcanization of pBD and related rubbers has been achieved using various initiator systems, including thiol-ene click chemistry, and radical photoinitiators, offering versatile solutions for advanced elastomer applications. Phenylacylbromide (PhABr), also referred to as 2-bromoacetophenone, is a distinctive radical photoinitiator that undergoes homolytic cleavage upon UV-A irradiation, generating bromine radicals (Br•) and acetyl radicals (PhCOCH2•). These reactive species facilitate hydrogen atom abstraction, generating radical active centers on polymer chains. An additional feature that differentiates PhABr from conventional photoinitiators is the formation of acetophenone as a byproduct following hydrogen abstraction by the acetyl radical. Acetophenone is not only a well-known Type I photoinitiator capable of further hydrogen transfer but also acts as an efficient photosensitizer for generating singlet oxygen (¹O₂) upon energy transfer to molecular oxygen in its triplet ground state. Singlet oxygen is a highly reactive species capable of selectively oxidizing unsaturated polymers by forming hydroperoxides, thereby introducing a secondary oxidation-crosslinking pathway during prolonged irradiation. This dual reactivity, involving both radical initiation and singlet oxygen production, distinguishes PhABr as an exceptionally efficient initiator system, capable of driving both crosslinking and oxidative modifications simultaneously in unsaturated elastomers like polybutadiene. This study demonstrates an efficient approach for the photovulcanization of liquid polybutadiene (pBD) under UV-A irradiation by using phenacyl bromide (PhABr) as a dual-action photoinitiator. After UV-A irradiation, phenacyl bromide (PhABr) produce bromine (Br•) and phenylacyl radicals (PhCOCH2•) that are able to abstract a hydrogen from polybudatiene chain to form a macroradical which goes to crosslinking by finding another macroradical. Moreover, after hydrogen abstraction of phenylacyl radicals (PhCOCH2•), acetophenone is produced which is one of the common examples of Type II photoinitiator and is capable of further hydrogen transfer and very well known as photosensitizer for generating singlet oxygen (1O2) upon energy transfer to molecular oxygen in triplet state (3O2). We first solve polybutadiene and phenacyl bromide (PhABr) in chloroform in the dark conditions. Then solution solvent casted into proper petri cap and the sample underwent UV-A light for 2 hours long to form a highy transparent, modaretly elastic, smooth and robust film. The chemical and physical characteristics of film are examined in great detail. The homolytic cleavage of PhABr and the subsequent chemical modifications in the polymer chain were analyzed using NMR, FTIR, and liquid UV spectroscopy. The changes along the polymer chain were examined using all three spectroscopic methods, and the resultant spectra exhibited consistent and complimentary findings, facilitating a clearer and more reliable interpretation of the structural modifications. The solid-state UV-Vis spectroscopy demonstrated high optical transmittance of throughout the visible region. To investigate thermal behaviours of crosslinked pBD films, Differential Scanning Calorimetry (DSC) is used and thermograms showed a significant increase of the glass transition temperature (Tg) from a theoretical baseline (-98 ℃) to -20℃, with the 10 wt% loading. Mechanical testing confirmed the changes by showing a linear increase in Young's modulus and tensile strength as the material transforms from a viscous liquid into a rigid thermoset. Tensile study indicated that higher concentrations yield more stiff films, as expected. To clarify crosslinking, gel fraction experiments is conducted with most of the common chemicals in ordinary lab and confirmed the formation of a robust 3D network (Gf> 87% in THF and CHCl3). Furthermore, surface analysis (SEM and WCA) provided evidence that the photovulcanization is spatially uniform, producing smooth, defect-free films. A significant drop in the water contact angle from 99º to 86º at higher loadings revealed the increment in surface polarity due to photooxidation. The film characteristics and reaction process were extensively examined utilizing an extensive selection of characterization methods.