Free-radical polymerizations (FRPs) are processes wherein polymers are synthesized through the successive addition of free-radical building blocks. [29] Various mechanisms can generate free radicals, often involving initiator molecules. These initiating free radicals then add non-radical monomer units, leading to the growth of polymer chains. FRPs serve as primary synthesis routes for a wide range of material composites and polymers due to the relatively nonspecific nature of chemical interactions involved, allowing for versatile polymerization reactions. [29]

FRPs belong to the category of chain-growth polymerizations, alongside coordination, anionic, and cationic polymerizations. [30] The progress of the reaction in FRPs is quantified by measuring the conversion of monomers into polymers. The random nature of events governing the length of each polymer chain results in molecular weight distributions (MWDs) rather than specific molecular weights typical of smaller molecules. Obtaining full MWDs can be achieved through kinetic methods or probabilistic arguments. [31]

Various techniques such as reversible addition-fragmentation transfer (RAFT) polymerizations, atom transfer radical polymerizations (ATRP), nitroxide-mediated radical polymerizations (NMRPs), and stable free radicals (SFRs) are employed in radical polymerizations. These methods are reliable for generating polymers with high molecular weights. [32] - [43] The process of radical polymerization (FRP) involves several key steps, as observed by the formation of polystyrene under the influence of initiators like azoisobutyronitrile (AIBN) or benzoyl peroxide (BPO). [10] Initiation begins with the decomposition of these compounds, yielding stable radicals such as isobutyronitrile and phenyl radicals. While both are capable of initiating styrene polymerization, oxygen-based radicals from peroxides are less effective due to their propensity to abstract hydrogen atoms rather than directly add to double bonds. Propagation proceeds as initiated radicals induce pi bond cleavage in styrene, generating benzylic radicals which continue to add more styrene molecules, forming new benzylic radicals at each step. [44] Notably, region-selectivity occurs during propagation, favoring the formation of stable radicals due to delocalization, particularly through the benzene ring. [44] Termination marks the end of radical reactions, where two radicals combine to eliminate their radical nature, commonly through head-to-head bonding of benzylic radicals. Additionally, termination events may involve one radical abstracting a hydrogen atom from another chain head. [44] Together, these steps explain the elaborate mechanisms governing radical polymerization and the synthesis of polymers like polystyrene.

Based on the most precise mechanism, a radical polymerization can be divided into several individual reaction steps. The kinetics of these processes can be described by kinetic equations for each step, and from these equations, the overall kinetics of the polymerization reaction can be derived.

1) Initiation: This step involves the initiator molecule breaking down into two radicals, typically through a process known as dissociation, with a rate constant represented. These radicals are reactive and can initiate the polymerization process by adding monomers. [1]

2) Growth: The radical formed in the initiation step adds to monomers, initiating the growth of the polymer chain. This step is characterized by a rate constant.

3) Termination: Termination occurs when polymer chains stop growing. It can happen through radical combination or disproportionation. Radical combination involves two radicals coming together to form a stable product (with k_t), while disproportionation involves a radical reacting with a growing chain to produce a saturated chain and an unsaturated chain ( ${k^{\prime }}_t$) as depicted in Scheme 1 . [1]

Cationic polymerization is a highly effective method for producing precise polymers and advanced materials. The advent of living cationic polymerization has opened up new pathways to creating complex polymer architectures that were previously difficult to achieve. Another notable advancement in this field is the use of external stimuli such as light and electricity to control cationic polymerization, which finds applications in surface fabrication, patterning, additive manufacturing, and other areas of advanced material engineering. [48] The field has seen rapid progress, with several reviews focusing on key aspects of living cationic polymerization. These include controlled cationic polymerization catalyzed by Lewis acids, [49] living cationic polymerization mediated by the reversible addition-fragmentation chain transfer (RAFT) mechanism, [50] and photoinitiated and photocontrolled cationic polymerization. [51]

Poly(2-hydroxyethyl methacrylate)

Poly(2-hydroxyethyl methacrylate), known as pHEMA, is a polymer highly valued for its biocompatibility, clarity, hydrophilicity, and remarkable durability, resisting crack propagation and bearing heavy loads effectively. [62] Originating from the toxic monomer HEMA, pHEMA undergoes a non-toxic transformation during polymerization. Its pivotal role in the development of modern soft hydrogel contact lenses traces back to the groundbreaking work of Otto Wichterle and Drahoslav Lim in the 1960s. [63] Initially utilized as an orbital implant, the favorable biological and mechanical properties of pHEMA prompted its exploration for contact lens applications. [64]

Since then, pHEMA has found versatile applications across biomedical fields, including soft contact lenses, bone tissue regeneration, drug delivery systems, artificial corneas, wound dressings, and prostheses. Soft contact lenses made from pHEMA offer flexibility and oxygen permeability crucial for eye health, in contrast to rigid gas-permeable materials like PMMA used in hard contact lenses. [62]

To bolster the mechanical strength of PHEMA-based hydrogels, a uniform network with robust ionic coordination interactions has been devised by Wang et al. [65] Leveraging the biocompatible monomer maleic acid, MA, which contains two carboxyl groups capable of forming numerous coordination bonds with Fe³⁺, facilitates the creation of binding sites evenly distributed along the backbone chains. [65] The pronounced polarity and cis replacement structure of MA ensure this distribution, effectively impeding the separation of PHEMA chains and yielding a homogeneous network. Thus, the pairing of MA and Fe³⁺ was chosen by the investigators to synthesize an enduring PHEMA-based hydrogel network. [65] Central to this approach was the establishment of strong coordination interactions between the carboxylate groups of copolymerized maleic acid (MA) units and Fe³⁺. This interaction prevented the phase separation of PHEMA chains in water, resulting in a uniform network structure. [65] This uniformity mitigated stress concentrations, while the dynamic coordination crosslinking mechanism efficiently dissipated energy, maintaining network elasticity. The culmination of these factors yielded hydrogels with exceptional tensile strength (3.44 MPa), elastic modulus (14.22 MPa), and toughness (4.17 MJ/m³). These values surpass those of pure PHEMA hydrogels by 22.7, 43.1, and 24.2 times, respectively. [65]

To synthesize low molecular weight PHEMA, controlled radical polymerization techniques are indispensable. Among the most successful methods are reversible addition fragmentation chain transfer (RAFT) and atom transfer radical polymerization (ATRP). Both have been effectively employed in HEMA polymerization, including the synthesis of HEMA-based block copolymers. [66] However, limitations exist that hinder their widespread use in biomedical applications.

In RAFT, a RAFT agent, often a thiocarbonyl-based compound, governs polymerization. [67] To achieve low molecular weight polymers, high concentrations of RAFT agents are necessary. If not removed post-polymerization, these agents can impart coloration or toxicity to the polymers. [66] ATRP, on the other hand, utilizes a transition metal catalyst, typically a Cu(I)/(II) system, in relatively high concentrations. [35] The imbalance between Cu(I) and Cu(II) necessitates high catalytic concentrations. [68] [69] Removing the copper post-poly-merization poses challenges despite advancements in catalyst removal techniques. [70]

Advancement in ATRP, known as “activators regenerated by electron transfer (ARGET) ATRP,” addresses these challenges. This method employs an excess reducing agent to regenerate Cu(I) from Cu(II), maintaining an appropriate Cu(I)/(II) balance. [71] ARGET ATRP reduces copper concentrations to parts per million (ppm) levels, simplifying catalyst removal by polymer precipitation. [72] Moreover, ARGET ATRP demonstrates tolerance to limited oxygen levels, eliminating the need for extensive deoxygenation steps. Various monomers and reducing agents have been successfully utilized in ARGET ATRP. [72]

Despite the advantages, using ATRP in polar solvents can compromise polymerization control, resulting in higher molar mass dispersity. [73] Nevertheless, PHEMA has been synthesized using ATRP in methanol or methanol/water solvent systems, albeit with slightly higher molar mass dispersities compared to nonpolar media. [73] Overcoming the challenge of synthesizing low molecular weight PHEMA with minimal catalyst levels, recent studies have employed ARGET ATRP at ambient temperatures. Optimization of ligand to copper ratios, reducing agent concentrations, and catalyst levels have enabled controlled polymerization, yielding water-soluble PHEMA with minimal catalyst content. [73]

Polystyrene

Polystyrene (PS) is a polymer derived from the monomer styrene, a liquid hydrocarbon sourced from petroleum. [74] Typically solid at room temperature, PS can be melted at higher temperatures for molding or extrusion purposes, then cooled to regain its solid form. Styrene, being an aromatic monomer, imparts aromatic characteristics to PS. [74]

BASF pioneered the commercial production of PS in the 1930s, and since then, it has found extensive use across various plastic products. In geosynthetics, PS is predominantly employed in its expanded form, comprising approximately 5% PS and 95% air. [74] This expanded PS boasts low thermal conductivity due to the voids filled with trapped air, making it ideal for insulation in building structures. PS for architectural and engineering applications can also be extruded into standardized shapes or formed into sheets with diverse patterns. Previously, expanded PS contained chlorofluorocarbons, but with environmental concerns in mind, safer blowing agents are now utilized. [74]

Polystyrene is a versatile material widely used in medical applications due to its affordability, low density, clarity, and dimensional stability, along with its compatibility with radiation sterilization. It comes in two primary forms: crystal polystyrene and high impact polystyrene (HIPS). [75] Crystal polystyrene finds use in labware like petri dishes and tissue culture trays, while HIPS is utilized in thermoformed products such as catheter trays, heart pump trays, and epidural trays. Both types are also employed in respiratory care equipment, syringe hubs, and suction canisters, competing favorably with PVC, polypropylene, and acrylics in labware and packaging for kits and trays. [75]

Crystal polystyrene resins, supplied typically as one-eighth-inch granules, are clear and glassy. When biaxially oriented, known as oriented polystyrene (OPS), they transition from brittle to flexible and durable. [75] OPS is formed by stretching the polystyrene sheet transversely, toughening what would otherwise be brittle. Injection-molded crystal grades serve various purposes, from cutlery and drink cups to medical labware and housewares. High-heat crystal grades are employed in applications like medical products and packaging. Extruded high-heat crystal grades are used in foam sheets for meat trays and foam board stock for insulation. [75]

High impact polystyrene grades are enhanced with polybutadiene elastomers, with high-impact grades containing around 6% - 12% elastomers and medium-impact grades about 2% - 5%. HIPS resins offer easy processing, good dimensional stability, impact strength, and rigidity. Some high-performance HIPS grades now rival more expensive engineering resins, finding applications in appliances and consumer electronics. Injection-molded HIPS resins are utilized in appliances, premium office accessories, and toys, while extruded HIPS resins are common in food packaging, dairy containers, and disposable tableware. [75] Polystyrene exists in three forms: atactic, isotactic, and syndiotactic. However, commercially available polystyrene mainly comprises atactic polystyrene. [75]

Another study explored the fabrication of a highly stable hyper cross-linked polystyrene (PS) organogel using γ-ray irradiation in tetrachloroethylene solvent, a novel eco-friendly synthesis method. [76] The degree of crosslinking, hydrophobicity, and swelling behavior of the PS organogel in various solvents was investigated. Additionally, the capacity of the PS organogel to selectively absorb chlorinated pollutants (CPs) from synthetic petroleum wastewater effluents was optimized using response surface methodologies (RSM), considering factors like pH, CP concentrations, temperature, PS amount, and absorption time. [76] Tests were conducted to assess the applicability and recyclability of the PS organogel for decontaminating real petroleum wastewater effluent, evaluating its economic viability in practical scenarios. Through in-situ cationic polymerization, a cross-linked PS organogel with 91% crosslinking was synthesized. The PS organogel exhibited high selectivity for chlorinated molecules over other organic molecules, with 80 - 184 times greater absorption capacity. [76] Response surface modeling confirmed the PS organogel’s ability to absorb CPs effectively across a wide range of pH and temperature conditions. ANOVA analysis validated the developed CPs absorption model onto the PS organogel, highlighting its accuracy. [76]

At ambient temperature, anionic polymerization has been utilized to produce branched polystyrenes with diverse molecular weights and structures. This method involved incorporation of a distyryl branching comonomer into a conventional sec-butylithium-initiated polymerization of styrene. [77] By controlling the length of the primary chains within the branched polymers and strategically placing the branching points along these chains, variations in glass transition temperature were achieved. [77] Interestingly, there was no direct correlation between the molecular weight of the branched polymer and its glass transition temperature. Instead, the length of the individual chains composing the branched macromolecules strongly influenced this property. [77]

The synthesis of polymers poses various challenges that affect their production and properties. One significant challenge is achieving high molecular weight polyacrylate polymers using controlled/living radical polymerization techniques like RAFT, ATRP, and NMP, which have been successfully reported by only a limited number of specific methods. [78] [79] Redox polymerizations, initiated by a redox reaction between oxidizing and reducing agents, also present challenges in polymerization processes. [80] Additionally, the synthesis of heterocyclic polymers through triple-bond-based polymerizations faces technological significance along with synthesis challenges. [81]

Difficulties also arise in copolymerization processes involving nonconjugated α,ω-diolefin/propylene due to the low reactivity of high molar mass macromonomers, leading to low efficiency and process complexity. [82] The synthesis of unsubstituted polythiophene is challenging due to its insolubility despite promising properties, hindering its production. [83] Incorporating cellulose nanocrystals into polymer latex particles can lead to nanoparticle aggregation, processing difficulties, and weak mechanical properties. [84]

Furthermore, challenges in polymer synthesis extend to the production of polymeric thin films with high electrical conductivity, where processing difficulties and inhomogeneous distributions of components limit their utility. [85] The development of porous polymer-silica composites for thermal insulating materials faces challenges related to high production costs and extreme fragility, impacting their commercial applications. [86] Additionally, the synthesis of self-assembled graphene oxide microcapsules in Pickering emulsions for self-healing epoxy coatings may encounter difficulties in solidification and reaction maintenance within the microcapsules. [87] The challenges in polymer synthesis encompass various aspects such as achieving high molecular weight, coping with low reactivity of macromonomers, addressing insolubility issues, managing nanoparticle aggregation, ensuring homogeneous distributions, and overcoming production costs and fragility concerns. These challenges highlight the complexity and intricacies involved in synthesizing polymers with desired properties and functionalities.

Polymer characterization allows scientists to unlock information about the properties of polymer materials. As polymer materials have innumerable applications, polymer characterization is hugely relevant to a wide range of sectors and industries. In attempt to identify a polymer we usually sought for the 1) chemical identity-this answers the question of what polymer is this? What additives are present? Is there any contamination? 2) morphology—that is amount of crystallinity and optical properties 3) molecular weight—molecular weight distribution 4) solution behavior-viscosity, flow, and gel 5) thermal behavior-glass transition, melt and decomposition temperature, melt viscosity and 6) mechanical properties—tensile strength, modulus, impact, creep and dimensional stability.

Scientists rely on a variety of techniques for polymer characterization as depicted in Table 1 .

Chromatography, an analytical technique used to separate mixtures into individual components, is one of the most useful tools for polymer characterization.

A variety of thermal analysis techniques are used to characterize polymers and gain insight into structure and behavior. Differential Scanning Calorimetry (DSC) is one of the most common and measures how much heat is needed to increase the temperature of a polymer sample. Data helps manufacturers and product developers determine key thermal transition temperatures. These include glass transition temperature (T_g), melting temperature (T_m) and crystallisation temperature (T_c). [88] Other useful thermal analysis techniques for polymer characterization include Thermogravimetric analysis (TGA), thermomechanical analysis (TMA) and Dynamic Mechanical Analysis (DMA). From widely used materials like nylon to complex polymeric nanocomposites, modern spectroscopy is redefining how polymer materials are characterized.

Of particular interest, the use of Near-Infrared (NIR) spectroscopy and NIR imaging for polymer characterization is now common. Near-Infrared (NIR) spectroscopy and imaging have become widely utilized for characterizing polymers. This method involves examining electromagnetic spectra in the NIR range and finds broad application in industries like engineering and manufacturing. [89] NIR light can deeply penetrate polymeric substances, making this technique suitable for analyzing products of different sizes, from micrometers to centimeters. In industrial settings, NIR spectroscopic imaging is valuable for tasks like quality assurance, examining polymer films, and identifying specific components within numerous parts. [89]

Microscopic polymer characterization is a potent method for uncovering surface structures and patterns within polymer materials. By employing sophisticated instruments, researchers can look deeper than what’s observable with the naked eye, examining individual atoms constituting polymers. In recent years, traditional optical microscopes have been supplanted by advanced techniques like Scanning Electron Microscopy (SEM) and Transmission Electron Microscopy (TEM). These electron microscopy methods offer superior resolution, enabling analysts to produce high-detail images of polymer samples.

Polymers have a higher molecular weight than other materials, which means they can exhibit unusual deformation behaviors. It’s this characteristic that makes many polymers elastic and viscous. Rheometric polymer characterization allows researchers to analyze the viscoelastic properties of polymers, optimize manufacturing formulas and improve product performance. [90]

Strength, elasticity, viscoelasticity, and anisotropy stand among the crucial characteristics defining polymers, collectively termed mechanical properties. These properties are shaped by interactions among polymer chains, dictating how they respond to external forces. The capacity of chains to elongate and reorient under stress further influences mechanical behavior. [91] The occurrence of crazing, marked by whitening when a polymer undergoes stress, also plays a role in determining mechanical properties. Typically, polymers are classified as elastomers, plastics, or rigid polymers based on their mechanical attributes. [91]

FTIR, or Fourier Transform Infrared Spectroscopy, serves as a powerful tool for answering the fundamental question, “What is this?” It offers a relatively user-friendly approach, allowing for the rapid analysis of a diverse range of materials. FTIR employs four primary sampling methods: Transmission, Attenuated Total Reflectance (ATR), Diffuse Reflectance, and Specular Reflectance. Regardless of the method used, the sample is exposed to an infrared energy beam. Through absorption and transmission of this energy, the sample reveals its unique chemical structure. Consequently, FTIR generates a distinctive spectrum for each compound, facilitating accurate identification and analysis. [92]

Nuclear magnetic resonance (NMR) spectroscopy is a highly versatile and powerful technique that has expanded into nearly all subdisciplines of chemistry and materials science. Commonly, an NMR spectrum presents peaks at different chemical shifts corresponding to nuclei of different chemical environments. allowing structural analysis and compositional characterization. More sophisticated and multidimensional NMR techniques connect NMR parameters such as chemical shift, line width, couplings, and relaxation rates to the structure and dynamics of the polymer chains at the atomic level. [93]

Common one-dimensional (1D) NMR techniques (e.g., ¹Hand¹³C) are often used to provide information on reaction conversion, functionalization, product purity, and monomer content. 1D NMR is particularly useful in elucidating the composition of copolymers, where recent advances in quantitative NMR measurements coupled with high-resolution mass spectrometry have enabled sequence-level characterization. [94]

Atomic Force Microscopy (AFM) has become a crucial tool to examine the structure, mechanics, and activity of various molecules with high precision. [95] Since its invention, AFM has found widespread use not only in fields like materials science, engineering, and medical research. [96]

In AFM, a sharp tip attached to a flexible cantilever is used to scan the surface of non-conductive samples. This scanning process can be done in different modes: contact mode, where the tip maintains a set distance from the sample surface while scanning; semi-contact mode, where the tip briefly touches the sample surface at the bottom of each oscillation; and non-contact mode, where the tip hovers just above the sample surface without making direct contact. [97] By precisely controlling the interaction between the tip and the sample surface, AFM enables researchers to obtain detailed information about morphology, biomechanics, and contraction activity. This ability to characterize samples at such a fine scale has made AFM an indispensable tool for understanding the materials and for advancing research in various scientific disciplines. [97]

The Atomic Force Microscope (AFM) operates on the principle of measuring surface topography by scanning a sharp tip across a sample while maintaining a constant interaction force between the tip and the sample. [97] The system consists of a cantilever with a sharp tip attached to it. As the tip interacts with the sample surface, the cantilever deflects as shown in Figure 4 . To measure these deflections, an optical amplification system is employed. A laser beam is directed onto the cantilever, and the reflected beam is detected by four photodiodes arranged in a cross configuration. [97] Any movement of the cantilever tip alters the angle of reflection, which is detected by the photodiodes. During scanning, as the tip moves across the sample surface, the interaction force between the tip and the sample is kept constant using a feedback mechanism. This ensures accurate and consistent measurements of surface features. [97]

Differential Scanning Calorimetry (DSC) is frequently combined with other electro-analytical methods to analyze the thermal characteristics of polymers. DSC measures the heat flow produced when a sample undergoes various temperature changes, providing insights into important properties like glass transition temperature, crystallization behavior, and melting point. [98] The history of how a polymer is treated thermally significantly affects its glass transition temperature and crystallinity. Annealing, for example, involves heating a semi-crystalline polymer to a temperature higher than its glass transition but lower than its melting point. This process allows the polymer chains to move and crystals to form, which can greatly increase the material’s crystallinity. Manufacturers who use injection molding capitalize on this understanding by controlling the temperature of the mold to influence the crystallinity of semi-crystalline thermoplastics. By carefully adjusting the mold temperature, they can ensure that the polymer fills the mold and cools in a way that achieves the desired level of crystallinity. This control over crystallinity is crucial for meeting the specific physical requirements of the molded parts. The thermogram obtained during the analysis provides material properties including temperatures at the glass transition (T_g), onset of melting, peak of melting (T_m), and enthalpy of fusion (ΔH) used to calculate crystallinity. These measurements can be specific to a particular polymer, confirming the polymer identity. [99]

In principle differential scanning calorimetry (DSC) is a method used to measure how much heat flows into or out of a material as its temperature changes continuously. [100] In a DSC unit, there are two calorimeters placed in a furnace block to minimize heat loss. This setup allows for measurements in either temperature-linear (TL) or temperature-modulated (TM) scanning modes. Even when using the same technique, different glass transition temperature (Tg) values can be obtained by employing various methods and protocols, such as heat flux and power compensation calorimeters, as well as different scanning programs. [101]

Each calorimeter typically consists of a small pan, often made of aluminum for good heat conduction, with a temperature sensor underneath to track temperature changes. The sample calorimeter holds the material being analyzed, while the reference calorimeter contains an identical pan without any material. [101] This configuration enables accurate measurement of heat flow in the sample compared to a baseline reference as depicted in Figure 5 .

Thermogravimetric Analysis (TGA) is another widely used thermal analysis technique. It works by measuring weight changes when the polymer is exposed to different temperatures. Increases or decreases in weight can be used to predict how a polymer will behave when exposed to harsh environments. Data can also be used to predict how polymeric materials will react to processes such as oxidation and decomposition. TGA is often carried out in a well-controlled atmosphere such as a vacuum or an inert gas filled environment to control the nature of the thermal decomposition by preventing oxidation. The measurements can be presented in plots that are either temperature-dependent or time-dependent where changes in mass can usually be attributed to material volatility or degradation. [93]