Photolysis regulation polymerization method for preparing ultra-high molecular weight fluorine-containing polymer

Abstract

The invention discloses a photolysis regulation and control polymerization method for preparing an ultra-high molecular weight fluorine-containing polymer, and belongs to the technical field of controllable free radical polymerization. A reaction can be carried out in a solution or a body, firstly, a reaction solution composed of the photo-initiation transfer terminating reagent, the fluorine-containing monomer and the solvent is deoxygenated through inert gas or does not need to be deoxygenated, then visible light is used for irradiating the reaction solution for 1 minute to 2 hours at the room temperature, and therefore the fluorine-containing polymer with the ultra-high molecular weight and the low dispersity is obtained. According to the invention, the photo-induced transfer terminator is adopted to realize light-operated free radical polymerization, the pyrazolyl photo-induced transfer terminator is used for preparing the ultra-high molecular weight fluorine-containing polymer for the first time, and compared with the traditional photo-induced transfer terminator, the pyrazolyl photo-induced transfer terminator can quickly and stably generate free radicals under the illumination condition; the polymerization reaction initiation efficiency is greatly improved, so that the polymerization reaction can be completed in a shorter time, the production efficiency is remarkably improved, and a new way is provided for synthesis of high-performance fluororubber / fluororesin.

Description

Technical Field

[0001] This invention belongs to the field of controlled free radical polymerization technology, specifically relating to a photolysis-controlled polymerization method for preparing ultra-high molecular weight fluoropolymers. Background Technology

[0002] Traditional free radical polymerization has advantages such as a wide range of applicable monomers, simple operation, and low cost. However, the molecular weight of the polymer is difficult to control, which limits its widespread application. To solve this problem, controlled radical polymerization technology has emerged. With its unique reversible deactivation mechanism, this method can precisely control the molecular weight, molecular weight distribution, and topology of polymers. It is widely used in the synthesis of functional polymers such as block, graft, and star polymers, greatly expanding the performance and application range of polymer materials.

[0003] However, this method still has certain limitations in the synthesis of high molecular weight polymers. As the target degree of polymerization increases, the polymerization time lengthens, the probability of side reactions rises, and chain termination reactions accumulate, making it difficult to precisely control the molecular weight of the synthesized polymer, resulting in a wider molecular weight distribution. Furthermore, conventional controlled radical polymerization methods typically require the addition of additional radical initiators or catalysts, which not only increases cost but also makes purification more difficult. In contrast, photolysis-controlled polymerization using initiator-transfer terminators does not require additional radical initiators and catalysts and can effectively control the molecular weight and molecular weight distribution of polymers under light conditions, making it a powerful tool for the precise synthesis of high molecular weight polymers.

[0004] Fluoropolymers, with their excellent chemical inertness, low surface energy, and thermal stability, are widely used in numerous fields such as chemical engineering, electronics, and aerospace. However, the preparation of ultra-high molecular weight fluoropolymers often requires harsh reaction conditions and is difficult to synthesize precisely, which greatly hinders further improvement in the performance of fluorinated materials. Therefore, developing a method for the precise synthesis of ultra-high molecular weight fluoropolymers under mild conditions is crucial. This invention develops a method for synthesizing ultra-high molecular weight hemifluoroacrylate polymers using photolysis-controlled polymerization, which is expected to break through the performance limits of such materials and has wide applications in the precise synthesis of polymers, surface modification of materials, semiconductor materials, high-performance coatings, and specialty materials. Summary of the Invention

[0005] The purpose of this invention is to provide an ultra-high molecular weight (M) n >10 6 A photolysis-controlled polymerization method for fluoropolymers with high molecular weight (g / mol) and low dispersion (Đ < 1.3). This invention aims to achieve rapid polymerization and precise synthesis of ultra-high molecular weight and low dispersion fluoropolymers through mild reaction conditions and simple operation.

[0006] The photolysis-controlled polymerization method provided by this invention can be carried out in solution or in bulk. First, the reaction solution, which consists of a photoinitiator, a transfer terminator, a fluorinated monomer, and a solvent, is deoxygenated with argon gas or not. Then, the reaction solution is irradiated with visible light at room temperature for 1 minute to 2 hours to obtain an ultra-high molecular weight and low-dispersion fluorinated polymer.

[0007] The reaction solution contains 1%–100% solids by mass; the molar ratio of the photoinitiator / transfer terminator to the fluorinated monomer is 1:50–20000; the fluorinated monomer is a fluoroalkyl acrylate; the photoinitiator / transfer terminator is a pyrazolyl photoinitiator / transfer terminator; and the solvent is dimethyl sulfoxide, dimethylformamide, tetrahydrofuran, etc.; inert gas protection or no inert gas protection may be used; the wavelength of visible light is 365–600 nm, and the light intensity is 0.1–100 mW / cm². 2 .

[0008] The pyrazol-based photoinitiated transfer termination reagent of this invention has one of the following structural formulas:

[0009]

[0010] Wherein, the electron donor W as a substituent is an N or P heteroatom, and the electron donor Y as a substituent is an O, S, Se or Te heteroatom;

[0011] R1, R′1, R2, and R3 are H and C1~C. 18 Alkyl, C6~C 30 Aryl, C5~C 24 Heteroaryl groups (where the heteroatom in a heteroaryl group is at least one of N, O, or S), etc.

[0012] R4 represents H, C1~C 18 Alkyl groups or substituents capable of generating primary, secondary, or tertiary carbon radicals (such as -CH2CN, -CH(CH3)CN, -C(CH3)2CN, -CH(C2H5)CN, -C(CH3)(C2H5)CN, -C6H 10 CN, -C(CH3)((CH2)2COOH)CN, -C(CH3)((CH2)2COOC n H 2n+1 )CN, -C(CH3)(CH2)2OH, -CH2Ph, -CH(CH3)Ph, -CH(COOH)Ph, -CH(COOC n H 2n+1 )Ph, -C(CH3)2Ph, -CH2COOH, -CH2COOC n H 2n+1 , -CH(CH3)COOH, -CH(CH3)COOCn H 2n+1、 -C(CH3)2COOH, -C(CH3)2COOC n H 2n+1 -C(CH3)2C(NH)(NH2), -C(CH3)2C(NH)N(CH2)2, etc.

[0013] The fluorinated monomer described in this invention is a fluoroalkyl acrylate, and the monomer and its polymer structural formula are shown below:

[0014]

[0015] Wherein, R is a perfluoroalkyl group with 1 to 18 carbon atoms or a fluoroalkyl group containing a few hydrogen atoms (such as -CF3, -CF2CHF2, -CF2CHFCF3, -CF2CF2CF2CHF2, etc.), n is an integer between 0 and 3, and m is a positive integer representing the number of polymerization units.

[0016] The present invention has the following advantages:

[0017] This invention utilizes photoinitiator-transfer terminators to achieve photocontrolled free radical polymerization. For the first time, pyrazole-based photoinitiator-transfer terminators are used to prepare ultra-high molecular weight fluoropolymers. Compared to traditional photoinitiator-transfer terminators, pyrazole-based terminators can rapidly and stably generate free radicals under light irradiation, significantly improving the initiation efficiency of the polymerization reaction. This allows the polymerization reaction to be completed in a shorter time, significantly improving production efficiency. More importantly, these photoinitiator-transfer terminators can perform rapid chain transfer and efficient reversible binding, greatly inhibiting chain termination reactions, thereby preparing ultra-high molecular weight fluoropolymers and providing a new route for the synthesis of high-performance fluororubbers / fluororesins.

[0018] This invention utilizes the characteristics of photocontrolled free radical polymerization to achieve precise control over the structure of ultra-high molecular weight fluoropolymers. By adjusting parameters such as light intensity, wavelength, and duration, the length of polymer chains, degree of branching, and distribution of functional groups can be controlled, thereby preparing fluoropolymers with different structures and properties to meet the diverse needs of different fields for fluoropolymer materials.

[0019] The photolysis-controlled reaction conditions of this invention are very mild, eliminating the need for harsh reaction conditions such as high temperature and high pressure. The method of this invention can be carried out at room temperature and pressure (room temperature and 1 standard atmosphere), significantly reducing the requirements for reaction equipment and reducing equipment investment. In addition, this method does not require the use of additional catalysts and initiators, avoiding the impact of catalyst and initiator residues on polymer properties, and also reducing raw material costs. Attached Figure Description

[0020] Figure 1 The polymerization product corresponding to Example 1 is polyoctafluoropentyl acrylate (POFPA). 7000 The conversion rate curve of );

[0021] Figure 2 The polymerization product corresponding to Example 1 is polyoctafluoropentyl acrylate (POFPA). 7000 The reaction kinetics curve;

[0022] Figure 3 The polymerization product corresponding to Example 1 is polyoctafluoropentyl acrylate (POFPA). 7000 GPC curve;

[0023] Figure 4 The polymerization product corresponding to Example 2 is polyhexafluorobutyl acrylate (PHFBA). 200 The conversion rate curve of );

[0024] Figure 5 The polymerization product corresponding to Example 2 is polyhexafluorobutyl acrylate (PHFBA). 200 The reaction kinetics curve;

[0025] Figure 6 The polymerization product corresponding to Example 2 is polyhexafluorobutyl acrylate (PHFBA). 200 GPC curve;

[0026] Figure 7 The GPC curve for the molecular weight of the POFPA polymer in Example 3 is shown.

[0027] Figure 8 POFPA, the product regulated in Example 3 1000 Rheological curves of polymer molecular weight;

[0028] Figure 9 POFPA, the product regulated in Example 3 10000 Rheological curves of polymer molecular weight.

[0029] Figures 1-2 The results of monomer conversion curves and reaction kinetic tests for photolysis-controlled polymerization of octafluoropentyl acrylate are presented, demonstrating the high efficiency of this polymerization.

[0030] Figure 3 POFPA is given 7000 The GPC plot of the polymer shows that the polymer still has a narrow dispersion when synthesizing ultra-high molecular weight polymers using this polymerization method.

[0031] Figures 4-5The monomer conversion curves and reaction kinetics test results of photolysis-controlled polymerization of hexafluorobutyl acrylate are presented, indicating that the polymerization efficiency can be significantly improved by using different pyrazol-based photoinitiators and transfer terminators.

[0032] Figure 6 PHFBA is given 200 The GPC diagram of the polymer illustrates that this polymerization method can improve efficiency while ensuring controllable polymer molecular weight dispersion.

[0033] Figure 7 The results of linear molecular weight control for octafluoroamyl acrylate monomer at different target degrees of polymerization (DP) are presented. Polymers with single symmetrical GPC peaks and narrow dispersion were obtained at different DPs, indicating that the molecular weight and dispersion of polymers can be precisely controlled by using this photolysis-controlled polymerization method.

[0034] Figures 8-9 Rheological curves for POFPA with two molecular weights are presented (for POFPA in Example 3, respectively). 1000 and POFPA 10000 ); where POFPA 10000 The storage modulus of the rheological curve is always much greater than the loss modulus, indicating that in the ultra-high molecular weight state, the polymer exhibits rheological properties dominated by elasticity, and ultra-high molecular weight polymers have greater mechanical strength compared with low molecular weight polymers. Detailed Implementation

[0035] To make the objectives and technical solutions of this invention clearer, the substantive content of this invention will be described below in conjunction with specific embodiments; the embodiments listed in this invention are only used to illustrate this invention and are not intended to limit the scope of this invention.

[0036] Example 1

[0037]

[0038] Equation (1): Photolysis-regulated reaction equation of octafluoropentyl acrylate

[0039] Add 0.22 mg of pyrazolyl photoinitiator transfer terminator (3,5-dimethyl-1H-pyrazol-1-dithiocarboxylic acid-2-cyano-2-methylpropyl ester), 1.76981 g of octafluoropentyl acrylate, and 298 µL of dimethyl sulfoxide to a 4 mL reaction flask, mix well to prepare a reaction solution, and the reaction equation is shown in equation (1). Deoxygenate was removed by argon displacement for 30 minutes; then, place the flask at a wavelength of 465 nm and a light intensity of 15 mW / cm. 2 Under a light source, the reaction was carried out at room temperature for 70 minutes to obtain polyoctafluoropentyl acrylate (POFPA). 7000The conversion rate was calculated to be 90% based on infrared spectroscopy detection. (See appendix.) Figure 1 The apparent reaction rate constant is 0.0366 min. -1 See appendix Figure 2 GPC measurements yielded the polymer POFPA 7000 The number-average molecular weight and dispersity are 1620 kg / mol and 1.14, respectively. (See Appendix) Figure 3 .

[0040] Example 2

[0041]

[0042] Equation (2): Photolysis-regulated reaction equation of hexafluorobutyl acrylate

[0043] 7.80 mg of pyrazolyl photoinitiator transfer terminator (methyl 3-methoxypyrazole-1-dithiocarboxylic acid isopropionic acid), 1.4897 g of hexafluorobutyl acrylate, and 424 µL of dimethyl sulfoxide were added to a 4 mL reaction flask and mixed thoroughly to prepare a reaction solution. The reaction equation is shown in Equation 2. The solution was deoxygenated for 30 minutes using argon displacement gas. Then, it was placed in a light source with a wavelength of 450 nm and a light intensity of 10 mW / cm². 2 Under a light source, the reaction was carried out at room temperature for 4 minutes to obtain polyhexafluorobutyl acrylate (PHFBA). 200 The conversion rate was calculated to be 71% using infrared spectroscopy. (See appendix.) Figure 4 The apparent reaction rate constant is 0.4070 min. -1 See appendix Figure 5 The obtained polymer PHFBA was measured by GPC. 200 The number-average molecular weight and dispersity were 37.4 kg / mol and 1.05, respectively. (See Appendix) Figure 6 .

[0044] Example 3

[0045] Polymerization reaction with a target degree of polymerization of 500: 3.08 mg of pyrazolyl photoinitiator transfer terminator (3,5-dimethyl-1H-pyrazol-1-dithiocarboxylic acid-2-cyano-2-methylpropyl ester), 1.76981 g of octafluoroamyl acrylate, and 298 µL of dimethyl sulfoxide were added to a 4 mL reaction flask and mixed thoroughly to prepare a reaction solution. The solution was then deoxygenated using argon purging for 30 minutes. Finally, it was placed in a light source at a wavelength of 465 nm and an intensity of 15 mW / cm². 2 Under a light source, the reaction was carried out at room temperature for 30 minutes to obtain octafluoropentyl acrylate (POFPA). 500 The conversion rate was calculated to be 92% by infrared spectroscopy; the obtained polymer POFPA was measured by GPC. 500The number-average molecular weight and dispersity were 129 kg / mol and 1.03, respectively. (See Appendix) Figure 7 .

[0046] Polymerization reaction with a target degree of polymerization of 1000: 1.54 mg of pyrazolyl photoinitiator transfer terminator (3,5-dimethyl-1H-pyrazol-1-dithiocarboxylic acid-2-cyano-2-methylpropyl ester), 1.76981 g of octafluoroamyl acrylate, and 298 µL of dimethyl sulfoxide were added to a 4 mL reaction flask and mixed thoroughly to prepare a reaction solution. The solution was then deoxygenated using argon purging for 30 minutes. Finally, it was placed in a light source at a wavelength of 465 nm and an intensity of 15 mW / cm². 2 Under a light source, the reaction was carried out at room temperature for 40 minutes to obtain polyoctafluoropentyl acrylate (POFPA). 1000 The conversion rate was calculated to be 91% by infrared spectroscopy; the obtained polymer POFPA was measured by GPC. 1000 The number-average molecular weight and dispersity are 255 kg / mol and 1.04, respectively. See Appendix. Figure 7 The obtained polymer was purified and then subjected to rheological oscillation frequency scanning (see appendix). Figure 8 .

[0047] Polymerization reaction with a target degree of polymerization of 3000: 0.51 mg of pyrazolyl photoinitiator-transfer terminator (3,5-dimethyl-1H-pyrazol-1-dithiocarboxylic acid-2-cyano-2-methylpropyl ester), 1.76981 g of octafluoropentyl acrylate, and 298 µL of dimethyl sulfoxide were added to a 4 mL reaction flask and mixed thoroughly to prepare a reaction solution. The solution was then deoxygenated using argon purging for 30 minutes. Finally, it was placed in a light source at a wavelength of 465 nm and an intensity of 15 mW / cm². 2 Under a light source, the reaction was carried out at room temperature for 50 minutes to obtain polyoctafluoropentyl acrylate (POFPA). 3000 The conversion rate was calculated to be 91% by infrared spectroscopy; the obtained polymer POFPA was measured by GPC. 3000 The number-average molecular weight and dispersity were 744 kg / mol and 1.11, respectively. (See Appendix) Figure 7 .

[0048] Polymerization reaction with a target degree of polymerization of 5000: 0.31 mg of pyrazolyl photoinitiator transfer terminator (3,5-dimethyl-1H-pyrazol-1-dithiocarboxylic acid-2-cyano-2-methylpropyl ester), 1.76981 g of octafluoropentyl acrylate, and 298 µL of dimethyl sulfoxide were added to a 4 mL reaction flask and mixed thoroughly to prepare a reaction solution. The solution was then deoxygenated using argon purging for 30 minutes. Finally, it was placed in a light source at a wavelength of 465 nm and an intensity of 15 mW / cm². 2 Under a light source, the reaction was carried out at room temperature for 60 minutes to obtain polyoctafluoropentyl acrylate (POFPA).5000 The conversion rate was calculated to be 91% by infrared spectroscopy; the obtained polymer POFPA was measured by GPC. 5000 The number-average molecular weight and dispersity are 1150 kg / mol and 1.10, respectively. See Appendix. Figure 7 .

[0049] Polymerization reaction with a target degree of polymerization of 7000: 0.22 mg of pyrazolyl photoinitiator-transfer terminator (3,5-dimethyl-1H-pyrazol-1-dithiocarboxylic acid-2-cyano-2-methylpropyl ester), 1.76981 g of octafluoropentyl acrylate, and 298 µL of dimethyl sulfoxide were added to a 4 mL reaction flask and mixed thoroughly to prepare a reaction solution. The solution was then deoxygenated using argon purging for 30 minutes. Finally, it was placed in a light source at a wavelength of 465 nm and an intensity of 15 mW / cm². 2 Under a light source, the reaction was carried out at room temperature for 70 minutes to obtain polyoctafluoropentyl acrylate (POFPA). 7000 The conversion rate was calculated to be 90% by infrared spectroscopy; the obtained polymer POFPA was measured by GPC. 7000 The number-average molecular weight and dispersity were 1620 kg / mol and 1.14, respectively. (See Appendix) Figure 7 .

[0050] Polymerization reaction with a target degree of polymerization of 10000: 0.16 mg of pyrazolyl photoinitiator-transfer terminator (3,5-dimethyl-1H-pyrazol-1-dithiocarboxylic acid-2-cyano-2-methylpropyl ester), 1.76981 g of octafluoropentyl acrylate, and 298 µL of dimethyl sulfoxide were added to a 4 mL reaction flask and mixed thoroughly to prepare a reaction solution. The solution was then deoxygenated using argon purging for 30 minutes. Finally, it was placed in a light source at a wavelength of 465 nm and an intensity of 15 mW / cm². 2 Under a light source, the reaction was carried out at room temperature for 120 minutes to obtain octafluoropentyl acrylate (POFPA). 10000 The conversion rate was calculated to be 91% by infrared spectroscopy; the obtained polymer POFPA was measured by GPC. 10000 The number-average molecular weight and dispersity were 2670 kg / mol and 1.14, respectively. (See Appendix) Figure 7 The obtained polymer was purified and then subjected to rheological oscillation frequency scanning (see appendix). Figure 8 .

[0051] Polymerization reaction with a target degree of polymerization of 20,000: 0.08 mg of pyrazolyl photoinitiator-transfer terminator (3,5-dimethyl-1H-pyrazol-1-dithiocarboxylic acid-2-cyano-2-methylpropyl ester), 1.76981 g of octafluoropentyl acrylate, and 298 µL of dimethyl sulfoxide were added to a 4 mL reaction flask and mixed thoroughly to prepare a reaction solution. The solution was then deoxygenated using argon purging for 30 minutes. Finally, it was placed in a light source at a wavelength of 465 nm and an intensity of 15 mW / cm². 2 Under a light source, the reaction was carried out at room temperature for 120 minutes to obtain octafluoropentyl acrylate (POFPA). 20000 The conversion rate was calculated to be 86% by infrared spectroscopy; the obtained polymer POFPA was measured by GPC. 20000 The number-average molecular weight and dispersity were 5004 kg / mol and 1.05, respectively. (See Appendix) Figure 7 .

[0052] The comparison of Examples 2 and 3 demonstrates that this method effectively controls the polymerization process and precisely regulates the molecular weight and dispersion of the polymer by adjusting the type and amount of the photoinitiator-transfer terminator and the reaction time, thereby meeting the synthesis requirements of different target degrees of polymerization. Furthermore, the number-average molecular weight and dispersion measured by GPC show that the molecular weight distribution of the ultra-high molecular weight polymer is relatively concentrated and narrow, further illustrating the high precision and reliability of the controllable synthesis of ultra-high molecular weight fluoropolymers in this invention. Comparison of rheological curves for different molecular weights shows that the ultra-high molecular weight fluoropolymers synthesized using this method have higher mechanical strength compared to low molecular weight fluoropolymers. Combined with the excellent chemical stability of fluoropolymers, they hold promise for applications in encapsulation materials, high-performance coating materials, semiconductor materials, and aerospace specialty materials.

[0053] The raw materials and equipment used in this invention are all commonly used in the field; the methods used in this invention, unless otherwise specified, are all conventional methods in the field.

[0054] The above description is merely a preferred embodiment of the present invention and does not constitute any limitation on the present invention. Any simple modifications, alterations, and equivalent transformations made to the above embodiments based on the technical essence of the present invention shall still fall within the protection scope of the present invention.

Claims

1. A photolytic regulation polymerization method for preparing an ultrahigh molecular weight fluorine-containing polymer, comprising the steps of: first, removing oxygen from a reaction solution composed of a photoinitiated transfer termination reagent, a monomer, and a solvent by argon or not, and then irradiating the reaction solution with visible light at room temperature for 1 minute to 2 hours, thereby obtaining an ultrahigh molecular weight and low dispersity fluorine-containing polymer.

2. A photolytic, regulated polymerization process for the preparation of ultrahigh molecular weight fluoropolymers according to claim 1, characterized in that: The solid mass content of the reaction solution is 1-100%, the molar ratio of the photoinitiated transfer termination reagent to the monomer is 1:50-20000; the monomer is fluoroalkyl acrylate, the photoinitiated transfer termination reagent is a pyrazolyl photoinitiated transfer termination reagent, and the solvent is dimethyl sulfoxide, dimethyl formamide or tetrahydrofuran; the polymerization can be carried out under inert gas protection or without inert gas protection; the wavelength of visible light is 365-600 nm, and the light intensity is 0.1-100 mW / cm 2 .

3. A photolytic, regulated polymerization process for the preparation of ultrahigh molecular weight fluoropolymers according to claim 1, characterized in that: The structure of the pyrazolyl photoinitiated transfer termination reagent is shown in one of the following formulas, ； wherein the electron donor W as a substituent is a N or P heteroatom, and the electron donor Y as a substituent is an O, S, Se, or Te heteroatom; R1, R′1, R2, and R3 are H and C1~C. 18 Alkyl, C6~C 30 Aryl or C5~C 24 Heteroaryl, wherein the heteroatom in the heteroaryl group is at least one of N, O, and S; R4 represents H, C1~C 18 Alkyl groups or substituents capable of generating primary, secondary, or tertiary carbon radicals; substituents capable of generating primary, secondary, or tertiary carbon radicals are -CH2CN, -CH(CH3)CN, -C(CH3)2CN, -CH(C2H5)CN, -C(CH3)(C2H5)CN, and -C6H. 10 CN, -C(CH3)((CH2)2COOH)CN, -C(CH3)((CH2)2COOC n H 2n+1 )CN, -C(CH3)(CH2)2OH, -CH2Ph, -CH(CH3)Ph, -CH(COOH)Ph, -CH(COOC n H 2n+1 )Ph, -C(CH3)2Ph, -CH2COOH, -CH2COOC n H 2n+1、 -CH(CH3)COOH, -CH(CH3)COOC n H 2n+1、 -C(CH3)2COOH, -C(CH3)2COOC n H 2n+1 -C(CH3)2C(NH)(NH2) or -C(CH3)2C(NH)N(CH2)2.

4. A photolytic, regulated polymerization process for the preparation of ultrahigh molecular weight fluoropolymers according to claim 1, characterized in that: The fluorine-containing monomer is a fluorine-substituted alkyl acrylate, and the structure of the monomer and the polymer is shown in the following formulas, ； wherein R is a perfluoroalkyl group having 1 to 18 carbon atoms or a fluorine-containing alkyl group containing a small number of hydrogen atoms, n is an integer between 1 and 3, and m is a positive integer representing the number of polymerization units.

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