tert-Butyl 2-bromoisobutyrate
[CAS 23877-12-5]

Identification

• Classification: Chemical reagent >> Organic reagent >> Ester >> Butyl ester compound
• Name: tert-Butyl 2-bromoisobutyrate
• Synonyms: tert-Butyl 2-bromo-2-methylpropionate; 2-Bromoisobutyric acid tert-butyl ester
• Molecular Formula: C₈H₁₅BrO₂
• Molecular Weight: 223.10
• CAS Registry Number: 23877-12-5
• EC Number: 245-924-7
• SMILES: CC(C)(C)OC(=O)C(C)(C)Br

Properties

• Density: 1.3±0.1 g/cm³ Calc.^*, 1.18 g/mL (Expl.)
• Boiling point: 175.5±8.0 °C 760 mmHg (Calc.)^*, 227.5 - 230.7 °C (Expl.)
• Flash point: 66.8±11.9 °C (Calc.)^*
• Index of refraction: 1.46 (Calc.)^*, 1.436 (Expl.)

^* Calculated using Advanced Chemistry Development (ACD/Labs) Software.

Safety Data

• Hazard Symbols: GHS07 Warning
• Risk Statements: H302-H315-H319-H335
• Safety Statements: P261-P264-P264+P265-P270-P271-P280-P301+P317-P302+P352-P304+P340-P305+P351+P338-P319-P321-P330-P332+P317-P337+P317-P362+P364-P403+P233-P405-P501

Hazard Classification

Hazard	Class	Category Code	Hazard Statement
Skin irritation	Skin Irrit.	2	H315
Acute toxicity	Acute Tox.	4	H302
Eye irritation	Eye Irrit.	2	H319
Specific target organ toxicity - single exposure	STOT SE	3	H335
Chronic hazardous to the aquatic environment	Aquatic Chronic	2	H411

chemBlink Chemical Story

How can a chemist make thousands of polymer chains grow at nearly the same pace? tert-Butyl 2-bromoisobutyrate is a useful answer to that question. This alpha-bromoester is widely used as an initiator in atom transfer radical polymerization, or ATRP, one of the landmark methods for controlled radical polymer synthesis.

Ordinary free-radical polymerization is easy to start but difficult to discipline. Radicals add monomers rapidly, and growing chains can terminate when radicals meet. The result is a population of chains with different histories and often broad molecular-weight distributions. ATRP introduced a reversible activation-deactivation cycle. In a typical copper-mediated system, a dormant alkyl halide end group is reversibly converted into a carbon-centered radical. That radical adds a small number of monomer units before being returned to the dormant halide state. Because most chains spend most of their time dormant, radical concentrations remain low and termination is suppressed.

tert-Butyl 2-bromoisobutyrate is structurally well suited to serve as the first dormant alkyl bromide. Activation cleaves the carbon-bromine bond homolytically in the catalytic cycle, generating a tertiary carbon-centered radical stabilized by the adjacent ester group. If initiation is fast relative to chain growth, many chains begin at roughly the same time. Their number-average molecular weight can then track monomer conversion more predictably than in conventional radical polymerization.

The molecule carries another useful feature: the tert-butyl ester. After polymerization, tert-butyl ester groups can often be removed under acidic conditions to reveal carboxylic-acid functionality. This makes tert-butyl-containing monomers and initiators attractive in routes where a protected acid is useful during polymerization but a polar acid group is desired later.

ATRP also changed how chemists think about polymer architecture. By preserving a reactive halogen chain end, a first polymer block can be reactivated to grow a second monomer, enabling block copolymers. Initiators can be attached to surfaces so polymer brushes grow outward from glass, silica, metals, or other substrates. Small initiator molecules therefore help determine not only chain length but where a polymer begins and what can be built from its surviving chain end.

The memorable feature of tert-butyl 2-bromoisobutyrate is thus not simply that it "starts polymerization." It participates in a reversible molecular traffic system that repeatedly switches chains between active and dormant states. That controlled radical concentration is what turns a very fast radical reaction into a tool for designing polymer molecular weight, end groups, blocks, and surfaces.

References:
1. PubChem, tert-Butyl 2-bromo-2-methylpropionate, CID 90290.
2. Matyjaszewski K., Xia J. Chem Rev. 2001, 101, 2921-2990. DOI: 10.1021/cr940534g.
3. Subramanian S.H., Dhamodharan R. Polymer International. 2008, 57, 479-487. DOI: 10.1002/pi.2362.
4. Kickelbick G., Matyjaszewski K. J Am Chem Soc. 1999, 121, 11912-11921. DOI: 10.1021/ja992732v.

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