Reaction mass of 2,4,6,8,10-pentamethyl-2,4,6,8,10-pentavinylcyclopentasiloxane and 2,4,6,8-tetramethyl-2,4,6,8-tetravinylcyclotetrasiloxane

Hydrolysis

Description of key information

Hydrolysis half-life:

Constituent 1: approximately 5.8 h at pH 4, 1600 h at pH 7 and 11 h at pH 9 at 20 - 25°C (QSAR)

Constituent 2: approximately 1.1 h at pH 4, 63 h at pH 7, and 0.8 pH 9 at 20 - 25°C (QSAR)

Additional information

The requirement to conduct a hydrolysis study for the substance is waived in accordance with Column 2 of REACH Annex VIII because the substance is insoluble in water (predicted solubility 9.1E-06 mg/l at 20°C (Constituent 1); measured solubility <7.3-8.8 ppb at 23°C (Constituent 2)).

The registration substance is a reaction mass of two constituents, Constituent 1: 2,4,6,8,10-pentamethyl-2,4,6,8,10-pentavinylcyclopentasiloxane (CAS 17704-22-2; Vi5-D5) and Constituent 2: 2,4,6,8-tetramethyl-2,4,6,8-tetravinylcyclotetrasiloxane (CAS 2554-06-5; Vi4-D4). For each constituent, the hydrolysis half-lives have been determined using a validated QSAR method. The results are supported by read-across from the fully methylated cyclic siloxane of the appropriate ring-size.

Predicted half-lives of approximately 5.8 h at pH 4, approximately 1600 h at pH 7 and approximately 11 h at pH 9 and 20 - 25°C were determined for Constituent 1, Vi5-D5, using a validated QSAR estimation method. Similarly, hydrolysis half-lives of 1.1 h at pH 4, approximately 63 h at pH 7 and 0.8 h at pH 9 and at 20-25°C were obtained for Constituent 2,Vi4-D4. The result is considered to be reliable and has been used for assessment purposes. The QSAR applies to siloxanes and allows predictions of hydrolysis half-life at pH 7 based on the number of silicon-oxygen atoms in the ring, the ring-strain and the number of Si-H bonds.

The result is supported by read-across of measured data from the methylated cyclic siloxane: hydrolysis half-lives of pentamethylcyclopentasiloxae (CAS no: 541-02-6, D5) and octamethylcyclotetrasiloxane (CAS no: 556-67-2, D4) were read-across to Vi5-D5 (Constituent 1) and Vi4 -D4 (Constituent 2) respectively.

Measured hydrolysis half-lives of 9.3 h, 1590 h and 24.8 - 31.6 h at pH 4, 7 and 9 and 25°C were determined for D5 (Durham and Kozerski 2005). Half-lives of 1.8 h at pH 4, 69 – 144 h at pH 7 and 0.9 – 1 h at pH 9 and 24.6°C (Durham 2006) were determined for D4), in accordance with OECD 111. Similarly, in Durham and Kozerski 2004, hydrolysis half-lives of 33 h at pH 5, 69 h at pH 7 and 0.56 h at pH 9 and 25°C were obtained for D4 in accordance with OECD 111.

The half-lives refer to degradation of parent substance by ring-opening of the siloxane. Full hydrolysis will take longer. The intermediate hydrolysis products are linear siloxanediols. The ultimate hydrolysis product of both constituents of the registration substance is methylvinylsilanediol.

D5 and D4 are cyclic siloxane chains with five and four silicon atoms respectively; the silicon atoms are connected by oxygen atoms and the Si-O bonds are susceptible to hydrolysis. All silicon atoms present are fully substituted with methyl groups. Similarly, Constituents 1 (Vi5 -D5) and 2 (Vi4 -D4) of the registration substance are cyclic siloxane chains with five and four silicon atoms, connected by oxygen atoms, in which the Si-O bonds are susceptible to hydrolysis. All silicon atoms present are substituted with one methyl group and one vinyl group. As well as being structural analogues, both siloxanes have consistent chemical properties including high molecular weight, high log Kow and very low solubility in water.

As the hydrolysis reaction may be acid or base catalysed, the rate of reaction is expected to be slowest at around pH 7 and increase as the pH is raised or lowered. For an acid-base catalysed reaction in buffered solution, the measured rate constant is a linear combination of terms describing contributions from the uncatalyzed reaction as well as catalysis by hydronium, hydroxide, and general acids or bases.

k_obs= k₀+ k_H3O+[H₃O⁺] + k_OH-[OH^-] + k_a[acid] + k_b[base]

 

At extremes of pH and under standard hydrolysis test conditions, it is reasonable to suggest that the rate of hydrolysis is dominated by either the hydronium or hydroxide catalysed mechanism.

 

Therefore, at low pH:

k_obs≈k_H3O+[H₃O⁺]

 

At pH 4 [H₃O⁺] = 10^-4 mol dm^-3 and at pH2 [H₃O⁺] = 10 ^-2 mol dm^-3; therefore, k_obs at pH 2 should be approximately 100 times greater than k_obs at pH 4.

 

The half-life of a substance at pH 2 is calculated based on:

t_1/2(pH 2) = t_1/2(pH 4) / 100

The calculated half-life at pH 2 and 20-25°C is therefore approximately 3 minutes for Constituent 1, and approximately 1 minute for Constituent 2.

Reaction rate increases with temperature therefore hydrolysis will be faster at physiologically relevant temperatures compared to standard laboratory conditions. Under ideal conditions, hydrolysis rate can be recalculated according to the equation:

DT₅₀(XºC) = DT₅₀(T) x e^(0.08.(T-X))

Where T = temperature for which data are available and X = target temperature.

Thus, for Constituent 1, Vi5-D5, the hydrolysis half-life at 37.5ºC and pH 7 (relevant for lungs and blood) is approximately 600 h whilst for Constituent 2, Vi4-D4, the hydrolysis half-life is approximately 23 hours. At 37.5ºC and pH 2 (relevant for conditions in the stomach following oral exposure), hydrolysis half-life is approximately 1.5 min for Constituent 1 and 15 seconds for Constituent 2.

The hydrolysis half-lives of substances used for read-across purposes in other endpoints are discussed below.

Hydrolysis of the read-across substance octamethylcyclotetrasiloxane, D4 (CAS No. 556-67-2)

Data for the substance octamethylcyclotetrasiloxane (CAS 556-67-2) are read-across to the registration substance for appropriate endpoints. The structural similarity and similar physicochemical and environmental fate properties of the two substances are relevant to this read-across. Hydrolysis half-lives for D4 at 25°C are described above. The ultimate hydrolysis product is dimethylsilanediol.  

Hydrolysis of the read-across substance decamethylcyclopentasiloxane, D5 (CAS 541-02-6)

Data for the substance decamethylcyclopentasiloxane (CAS 541-02-6) are read-across to the registration substance for appropriate endpoints. The structural similarity and similar physicochemical and environmental fate properties of the two substances are relevant to this read-across.

Hydrolysis half-lives for D5 at 25°C are described above.  The ultimate hydrolysis product is dimethylsilanediol.