FORMATION OF ULTRAHIGH-MOLECULAR-WEIGHT POLYETHYLENE ANALOGUES VIA SURFACE POLYMERIZATION FROM AEROSOL PRECURSORS

ABSTRACT

A method for the synthesis of a polyacetylene-based polymer with high molecular weight has been developed using acetylene as the starting material and a Ziegler–Natta catalytic system. The polymer was subsequently brominated in carbon tetrachloride to introduce electron-withdrawing bromine substituents in a predominantly trans-configuration, while maintaining solubility in acetone and tetrahydrofuran. Strong bases, such as potassium tert-butoxide, were employed to generate carbanions, which facilitated intermolecular crosslinking via SN2 reactions, resulting in a network structure analogous to crosslinked ultrahigh-molecular-weight polyethylene. The method allows surface deposition in two stages—application of the uncrosslinked polymer followed by base-induced crosslinking—yielding a robust material with potential applications requiring high mechanical strength. Advantages include the use of inexpensive starting materials and solution-processable chains, whereas limitations involve possible incompatibility of bromine substituents with certain surfaces and the use of aggressive reagents during crosslinking.

АННОТАЦИЯ

Разработан метод синтеза полиацетиленоподобного полимера с высокой молекулярной массой, использующего ацетилен в качестве исходного материала и каталитическую систему Циглера–Натты. Полученный полимер был впоследствии бромирован в четырёххлористом углероде для введения электронно-акцепторных бромных заместителей преимущественно в транс-конфигурации, при этом сохранялась растворимость в ацетоне и тетрагидрофуране. Для образования карбанионов применялись сильные основания, такие как трет-бутоксид калия, что способствовало межмолекулярному сшиванию через реакции SN2, формируя сетчатую структуру, аналогичную сшитому сверхвысокомолекулярному полиэтилену. Метод позволяет наносить материал на поверхность в два этапа — сначала непересшитый полимер, затем сшивание, индуцированное основанием, — что обеспечивает прочный материал с потенциальным применением в областях, требующих высокой механической прочности. К преимуществам относятся использование недорогих исходных материалов и возможность обработки в растворе, тогда как ограничения включают возможную несовместимость бромных заместителей с некоторыми поверхностями и применение агрессивных реагентов в процессе сшивания.

Keywords: ultra-high molecular weight polyethylene, UHMWPE analogue, spray coating, surface coating, in situ polymerization, crosslinking.

Ключевые слова: сверхвысокомолекулярный полиэтилен, аналог UHMWPE, распылительное покрытие, покрытие поверхности, полимеризация in situ, сшивание.

Introduction

Ultra-high molecular weight polyethylene (UHMWPE) is a semi-crystalline polymer characterized by extremely long chains of polyethylene that can reach molecular weights of several million Daltons. These long chains form highly entangled networks that give the material remarkable mechanical properties, including high tensile strength, exceptional impact , and outstanding wear resistance. UHMWPE also exhibits excellent chemical inertness, low friction, and biocompatibility, making it suitable for demanding applications ranging from biomedical devices, such as joint replacements and prosthetics, to industrial components like conveyor belts, liners, and protective coatings.

Despite its excellent properties, processing UHMWPE into complex shapes or applying it as a surface coating is challenging due to its extremely high viscosity and poor solubility in common solvents. Traditional methods often require mechanical shaping or compression molding of pre-formed sheets, limiting its use in surface applications.

In this work, we present a novel approach for synthesizing a UHMWPE analogue that can be directly deposited onto surfaces using a spray technique. The polymer is designed to undergo in situ crosslinking upon application, forming a robust, adherent coating with mechanical properties comparable to bulk UHMWPE. This method enables the creation of durable, high-strength polymer layers on a wide variety of substrates, potentially expanding the use of UHMWPE-like materials in areas where traditional processing methods are impractical. Furthermore, the spray-deposition technique offers flexibility in coating complex geometries and allows localized reinforcement of surfaces, opening new possibilities for industrial and biomedical applications.

Objective:

The objective of this study is to develop a method for synthesizing and applying a UHMWPE analog that can be sprayed onto surfaces and subsequently cross-linked to form a durable three-dimensional network.

Tasks:

1. Synthesize a soluble polymer precursor suitable for spray application.

2. Modify the precursor to enable subsequent cross-linking.

3. Implement a mechanism for forming a three-dimensional polymer network.

4. Propose a method for spraying and post-application cross-linking to achieve uniform coatings on complex surfaces. 

Materials and methods

Acetylene was selected as the starting material for polymer chain synthesis.

Step 1 Polyacetylene formation was carried out using a Ziegler–Natta catalytic system [1]:

Scheme 1. Polyacetylene formation

Reaction conditions:

• Temperature: –78 °C to –10 °C
• Solvent: tetrahydrofuran (THF)
• Al/Ti ratio: 4

A red, gel-like product was obtained, with polymer chain molecular weights reaching up to 10⁶ g·mol⁻¹.

Step 2 The resulting polyacetylene paste was dissolved in carbon tetrachloride, forming a suspension [2]. Bromine was then passed through this suspension, yielding a product with bromine content up to 80 % [3]:

Scheme 2. Reaction with bromine

The reaction must be conducted in the absence of light to avoid radical bromination and excessive hydrogen substitution. The first stage involves the formation of a bromonium ion, which is subsequently attacked by bromide from the opposite side, following an SN2 mechanism [4]. As a result, the trans-product predominates.

Scheme 3. Bromonium ion formation

Importantly, the resulting polymer, poly(1,2-dibromoethylene), is soluble in acetone.

Figure 1. The obtained polymer

Step 3 Upon addition of a strong base, hydrogen atoms can be abstracted from the carbon atoms. The resulting carbanion is stabilized by the negative inductive effects of the bromine substituents, which delocalize electron density away from the carbon, lowering the energy of the negative charge and increasing the stability of the carbanion.

If the base is also nucleophilic, it must be sterically hindered. This is because the bromine atoms on the polymer chains act as potential leaving groups. The nucleophilic attack occurs via an SN2 mechanism, with bromide leaving. An example of such a base is potassium tert-butoxide (KOtBu).

Step 4 Let us consider a carbanion, formed by deprotonation, together with a neighboring molecule of the same polymer. This can be represented as follows:

 Figure 2. Carbanion and nearby polymer molecule        

The generated carbanion, being highly nucleophilic, attacks a carbon atom on a neighboring polymer chain via an SN2 reaction, with bromide leaving:

Figure 3. Reaction between carboanion and another polymer molecule

This results in the crosslinking of two separate polymer molecules:

Figure 4. Cross-linked polymer molecules

Such reactions, occurring throughout the entire compound, generate a network structure analogous to crosslinked ultrahigh-molecular-weight polyethylene.

Results and discussion

Bromination of polyacetylene resulted in a high-molecular-weight polymer containing predominantly trans-1,2-dibromoethylene units. The rigid arrangement of substituents was preserved due to restricted rotation around the C–C bonds and repulsive interactions between bromine atoms along the macromolecular chain.

By analogy with polyvinyl chloride (PVC), it was hypothesized that the brominated polymer obtained in this work would be soluble in acetone. PVC, despite its high molecular weight, dissolves in acetone due to the presence of densely distributed halogen substituents, which disrupt crystallinity and weaken intermolecular packing. Since the brominated polyacetylene derivative possesses an even higher degree of halogen substitution, its behavior was expected to be similar. This structural reasoning served as the basis for predicting that the brominated polymer should form stable solutions in acetone, enabling spray-based deposition.

Treatment of this polymer with a strong hindered base led to selective deprotonation and formation of carbanions stabilized by the electron-withdrawing effect of bromine substituents. These carbanions acted as powerful nucleophiles, initiating SN2 reactions with neighboring polymer chains and displacing bromide ions. The resulting interchain crosslinking produced a highly dense three-dimensional network, mechanically similar to crosslinked UHMWPE.

A key result of this study is the unexpected and highly advantageous solubility of the brominated polymer in acetone. This property enables the material to be converted into a sprayable solution, allowing uniform deposition on a wide range of surfaces, including objects with complex geometry. Unlike UHMWPE, which is insoluble in all common solvents and cannot be spray-coated, the proposed precursor can be applied as an aerosol and subsequently crosslinked in situ.

Upon deposition of the polymer solution onto a substrate, a second spray containing a strong base can be applied to induce rapid interchain crosslinking. This two-stage approach—solution deposition followed by chemical activation—provides a practical route to forming durable, rigid coatings directly on the surface.

The combination of high precursor solubility, efficient SN2 crosslinking, and preservation of high molecular weight leads to the formation of coatings that mimic the stiffness and durability characteristic of UHMWPE, while being produced through an entirely different processing pathway.

Conclusion

A novel method for producing an analogue of ultrahigh-molecular-weight polyethylene has been developed through controlled modification and crosslinking of polyacetylene. Bromination yields a highly substituted polymer that, unlike UHMWPE, is soluble in acetone and therefore suitable for spray deposition. Subsequent treatment with a strong base induces SN2-type interchain crosslinking, forming a dense three-dimensional network that imparts mechanical properties comparable to those of crosslinked UHMWPE.

This approach enables the creation of robust, chemically resistant coatings using inexpensive starting materials and a simple two-stage application process. Although the crosslinking step requires reactive reagents, the method holds promise for producing durable protective layers on surfaces where traditional UHMWPE processing techniques are impractical.

References:

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