Polyisobutylene HRD-750 is a low-molecular-weight highly reactive polyisobutylene (HR-PIB) supplied as a clear, viscous liquid. The product designation HRD-750 identifies a nominal number-average molecular weight of 750 g/mol and a terminal vinylidene content that typically exceeds 70 mol% when measured by 1H NMR olefinic integration in deuterated chloroform. The material carries CAS 9003-27-4 and functions primarily as a chemical intermediate for ashless dispersant chemistry, not as a finished lubricant additive. Conventional low-molecular-weight polyisobutylene of equivalent Mn contains mostly internal olefinic groups and typically shows terminal vinylidene content below 10 mol%; HRD-750 is therefore classified as a highly reactive grade because the α-terminal vinylidene group is accessible to thermal ene functionalization.

The specification envelope for incoming material is summarized in Table 1. These values are representative of the product class and should be verified against the certificate of analysis for each batch, because prolonged heating in non-inerted transfer lines can increase moisture and shift the olefin distribution.

Incoming material control should include gel-permeation chromatography in tetrahydrofuran with refractive-index detection and calibration against narrow-dispersity polyisobutylene standards, because calibration against polystyrene can introduce systematic bias in the reported Mn. The terminal vinylidene content is determined by proton nuclear magnetic resonance using the integrated olefinic signals at δ 4.6–4.8 ppm normalized against the methyl proton signal at δ 1.0–1.2 ppm. Water content above 50 mg/kg is not normally handled by simple decantation because HRD-750 forms a single-phase organic liquid at ambient temperature; in-line vacuum drying at 80–100 °C under 10–20 kPa is used when moisture exceeds specification.

The olefin distribution is not fully described by total vinylidene content alone. The remaining terminal unsaturation includes β-terminal structures and internal trisubstituted olefins, which are less reactive in the thermal ene reaction. If the 1H NMR signal at δ 5.15 ppm from trisubstituted internal olefin exceeds 15 % of the total olefinic area, maleation conversion may fall below the expected range. This is a routinely observed batch-control issue in PIB plants where catalyst residues are not fully deactivated. Batch-to-batch variation in Mn of ± 30 g/mol is commonly observed; this variation changes kinematic viscosity by approximately 0.8–1.2 mm²/s at 100 °C and should be correlated with the PIBSA acid number before process adjustments are made. Dispersity index above 2.2 may indicate contamination with higher-molecular-weight polymer that can form insoluble gels during storage.

What Distinguishes HRD-750 from Conventional Low-Molecular-Weight Polyisobutylene?

The most operationally significant difference is the concentration of reactive terminal vinylidene groups. Conventional PIB of similar molecular weight typically contains less than 10 mol% terminal vinylidene and cannot achieve a useful maleic anhydride adduct in a direct thermal ene process; it generally requires chlorination or a metal-catalysed route that introduces residual chlorine and generates effluent. HRD-750 is specified at ≥70 mol% terminal vinylidene, allowing conversion to polyisobutenyl succinic anhydride in a chlorine-free thermal process at 200–230 °C with a maleic anhydride-to-terminal vinylidene molar ratio of 1.2–1.5:1.

Compared with higher-molecular-weight HR-PIB grades, HRD-750 provides lower kinematic viscosity and higher molar concentration of terminal olefin per unit mass. This reduces pressure drop through static mixers and gear pumps in continuous PIBSA processes and permits higher throughput at equivalent reactor volume. The trade-off is that the resulting PIBSA has a shorter polyisobutylene segment, which changes the solubility profile of the final succinimide dispersant in high-viscosity synthetic basestocks; formulators may need to adjust the amine stoichiometry and the extent of post-treatment.

In incoming material specification, total unsaturation by iodine value is not a substitute for terminal vinylidene content. A batch with an iodine value of 20–25 g I₂/100 g can fail terminal vinylidene specification if the internal olefin fraction is elevated. The olefin distribution is controlled by the polymerization catalyst system and quench procedure; this is the principal reason that two PIB products with the same Mn and similar iodine value can behave differently in maleation. The distinction is also observed in plant operations: the chlorine-free route reduces corrosion risk in overhead condensers and avoids formation of zinc chloride or iron chloride deposits in esterification reactors.

Where published data for HRD-750 in a specific downstream formulation is limited, direct side-by-side maleation trials under identical molar ratios and reactor residence times are recommended because the reactivity difference can be attenuated by dissolved oxygen or by inhibitor residues in the base polymer.

Continuous production of polyisobutenyl succinic anhydride from HRD-750 is conducted in a nitrogen-blanketed stirred reactor or wiped-film evaporator. The terminal vinylidene group reacts with maleic anhydride through a thermal ene mechanism; the processing window is narrow because insufficient temperature leaves unreacted polymer while sustained operation above 240 °C increases oxidative degradation products and darkens the PIBSA. In a 2.0 m³ stirred reactor with a 4:1 height-to-diameter ratio and a hot-oil jacket capable of maintaining ±2 °C, the charge is typically heated to 210–225 °C and held for 4–6 h after the maleic anhydride feed is completed. Residual maleic anhydride is removed at 180–210 °C under 2–10 kPa; nitrogen sparging at 0.5–1.0 L/min per 100 kg reaction mass is used in the stripping operation. Under these conditions, the terminal vinylidene conversion for Mn 750 HR-PIB is commonly reported in the range of 75–90 %, with the exact value dependent on reactor geometry, sparge rate, and the level of residual antioxidant.

Field observations on a production-scale wiped-film evaporator with 6 m² heated surface indicate that residual moisture above 50 mg/kg produces unstable feed pressure and requires in-line vacuum dehydration at 80–100 °C before the reaction section. The use of gear pumps with clearances designed for 30 mm²/s at 80 °C reduces cavitation during transfer from storage to the reactor. The progress of the maleation reaction can be followed by Fourier-transform infrared spectroscopy using the anhydride carbonyl band at 1780–1860 cm⁻¹; disappearance of the vinylidene C–H out-of-plane deformation near 890 cm⁻¹ is used as the primary endpoint indicator. The reaction mass should be sampled for kinematic viscosity and acid number at 30 min intervals after the expected endpoint, because the thermal ene reaction can enter a diffusion-limited regime if the PIBSA viscosity exceeds 150 mm²/s at 100 °C. If the viscosity rises above this limit, additional maleic anhydride does not increase conversion but increases the load on the stripping system. Foaming during stripping is suppressed by maintaining condenser temperature at 100–120 °C and limiting nitrogen sparge to 0.8 L/min per 100 kg.

The resulting PIBSA is subsequently reacted with polyamines such as triethylenetetramine or tetraethylenepentamine at 150–170 °C to form succinimide dispersants. The stoichiometry is controlled by the carbonyl-to-amine ratio and the desired total base number of the finished engine oil; typical succinimide treat rates in passenger-car motor oil are 3–8 wt%, with total base number measured by ASTM D2896 and sludge/varnish performance evaluated by ASTM D6593. The shorter polyisobutylene segment of HRD-750-derived PIBSA reduces thickening at low temperature; kinematic viscosity of the finished oil at 100 °C and high-temperature high-shear viscosity at 150 °C are measured by ASTM D445 and ASTM D4683.

When HRD-750 Is Used in Adhesive and Sealant Compounding

In butyl sealant and hot-melt adhesive compounding, HRD-750 functions as a low-viscosity reactive diluent and rheology modifier. The addition level is typically 5–25 phr, depending on the base elastomer Mooney viscosity and the required open time. A formulation containing 100 phr butyl rubber, 20 phr HRD-750, and 120 phr calcium carbonate filler is processed on a 40:1 L/D twin-screw extruder at barrel temperatures of 180–190 °C; the HRD-750 reduces compound Mooney viscosity from approximately 65 MU to 48 MU when tested in accordance with ISO 289-1. This viscosity reduction permits higher filler loading without exceeding the torque limit of the drive motor and without increasing the screw speed above the shear-sensitive range for butyl rubber.

Tensile and tear properties of the cured compound are evaluated in accordance with ASTM D412 and ASTM D624. Because HRD-750 contains reactive terminal olefin groups, residual unsaturation can participate in peroxide cure systems; the cure kinetics are monitored by moving-die rheometry in accordance with ISO 6502. The use of HRD-750 in extruded sealant profiles often requires adjustment of the curing package because the low molecular weight of the reactive diluent increases the concentration of chain ends available for radical attack. Peroxide levels should be re-optimized on a moving-die rheometer per ISO 6502 rather than carried over from a conventional PIB-based formulation. Published data for HRD-750 in high-displacement acetoxy-cure adhesive systems is limited; formulators should validate fracture toughness and shear strength per ISO 527-2 and ASTM D1002 on primed substrates.

Storage, Handling, and Incompatibility Limits

Storage of HRD-750 above 60 °C for extended periods or in vented containers exposed to oxygen can increase carbonyl content and shift the olefin distribution. Bulk storage should be maintained under a dry nitrogen blanket with oxygen below 0.5 vol% and water content below 50 mg/kg in the liquid. Transfer lines should be heat-traced to 50–80 °C and equipped with gear pumps rated for viscosities of at least 20 mm²/s at 50 °C to prevent cavitation. The material is incompatible with strong oxidizers and can undergo exothermic degradation when mixed with free halogens; closed-loop filtration with 10 µm nominal stainless-steel mesh is recommended before charging to reaction vessels.

Combination with amine-based additives before complete maleation should be avoided because residual amines compete for maleic anhydride and reduce the PIBSA yield. The product should not be stored in copper or copper-alloy lines for prolonged periods if trace metal pickup is a concern; 316L stainless steel or high-density polyethylene is preferred. For formulators requiring regulatory compliance, the material may be evaluated under 21 CFR 178.3570 for incidental food-contact lubricants and is subject to registration under REACH (EC) No 1907/2006; end-use confirmations are the responsibility of the converter or blender.