United States Patent 4,757,128 (US4757128): Scope, Claim Boundaries, and Landscape
US Patent 4,757,128 is directed to high molecular weight polyanhydrides defined by (i) molecular weight and solution intrinsic viscosity thresholds and (ii) constraints on the dicarboxylic acid feedstock class and (iii) process controls that stop polymerization to prevent molecular-weight collapse. The claims also cover compositions of the defined polyanhydrides and methods for synthesizing them, including route options for generating “highly pure prepolymer,” catalyst selection, and specified temperature and time windows.
What does US4757128 claim, in plain patent terms?
Core product claim elements
The strongest product-defining features appear in claims 1, 8, and 9:
- Product identity
- A high molecular weight polyanhydride (polymer containing anhydride linkages).
- Quantified performance/product-by-property limits
- Weight average molecular weight (Mw): > 20,000
- Intrinsic viscosity (IV) in chloroform at 23°C: > 0.3 dl/g
- Feedstock chemistry
- The polyanhydride is “produced from at least one dicarboxylic acid selected from” a defined set of:
- Aliphatic dicarboxylic acids with formula HOOC–H2C–R–CH2–COOH
- Aromatic dicarboxylic acids (multiple generic formulas shown in the specification drawings)
- Aliphatic-aromatic dicarboxylic acids
- Aromatic and aliphatic heterocyclic dicarboxylic acids with a heteroatom X selected from O, N, S
- Combinations of the above, including aromatic acids with more than one phenyl group
- Substitution architecture
- The R groups are divalent organic radical groups.
- Fallback narrowing by specific named acids
- Claim 9 narrows claim scope to polyanhydrides produced from a list that includes, at minimum:
- Sebacic acid
- Isophthalic acid
- Terephthalic acid
- Dodecanedioic acid
- Cyclohexane dicarboxylic acids
- Multiple bis(oxy)/bisacetic and bis(carboxymethyl) and alkylidene bis(benzoic acid) type acids
- 1,4 phenylene dipropionic acid
- 4,4′-(n-alkylidene)bis[benzoic acid]
Process-defined versions of the product
Claims 2, 7, 10–20 define related protection for product made by a specific polymerization and workup logic:
- The polyanhydride is made by polymerizing at least one “highly pure prepolymer” produced from “highly pure dicarboxylic acid(s)”.
- Polymerization forms a polyanhydride with Mw in excess of 20,000.
- Then the “polyanhydride condensation product having Mw in excess of 20,000” is removed.
- Polymerization is stopped before the condensation product decreases in molecular weight.
- Claim 7 mirrors claim 1/8 product logic but is framed as a composition comprising polyanhydride made by the process.
How broad are the claim boundaries? (Scope mapping)
1) Product-by-threshold breadth
The product boundary is driven by two numeric limits:
- Mw > 20,000
- IV > 0.3 dl/g (chloroform, 23°C)
This structure is broad in the sense that it covers any polyanhydride meeting those thresholds, while narrowing by feedstock class and R-group architecture.
2) Feedstock class breadth
Claims 1 and 8 do not list a closed set of named acids. Instead, they use generic formula frameworks plus a heteroatom filter (X = O, N, S) and an integer n = 1 to 3. That tends to be broader than “only” enumerated acids.
However, breadth is constrained by:
- The acids must be dicarboxylic acids that match the specific structural formulas used in the claim.
- The R groups are divalent organic radicals (per the claim text for the aliphatic/aromatic hetero formulations).
3) Named-acid fallback narrowing
Claims 3 and 9 sharply reduce scope by reciting a list of acids. If a competitor uses dicarboxylic acids outside the enumerated list but still within the generic formula categories, they may evade claim 3/9 while still potentially falling under 1/8.
4) Process-control scope (molecular-weight retention logic)
Claims 2, 10–20 hinge on a specific polymerization stopping strategy:
- Stop before the high-Mw condensation product decreases in molecular weight, and
- Remove the high-Mw condensation product.
This is not merely “make polymer at high Mw.” It is “make it, harvest the high-Mw fraction, then stop early to prevent decline.”
What do the dependent process claims lock in?
Catalyst-dependent coverage
Claims 4–6 and 11–13 cover polymerization in the presence of catalysts:
- Catalyst type (claims 5, 12): acid, base, and coordination catalysts
- Specific catalyst list (claims 6, 13) includes:
- (CH3COO)2Cd (cadmium acetate)
- Alkaline earth metal oxides
- Calcium carbonate
- Complexes of diethylzinc and hydroxylated compounds
This list is narrow and could be used as a targeted “escape hatch” for a generic-process competitor if they avoid these catalysts (but they may still fall under the broader claims if those require no catalyst).
Route to “highly pure prepolymer”
Claim 14 and 15 provide two explicit prepolymer preparation routes:
- 14: form prepolymer by refluxing the dicarboxylic acid in an aliphatic anhydride
- 15: form prepolymer by reacting dicarboxylic acid with an aliphatic acid chloride in the presence of an organic base
These are important because they define how “highly pure prepolymer” can be prepared, but they do not necessarily exclude other routes if independent claims do not require these steps. Dependent claims, though, define narrower infringing embodiments.
Temperature and time window
Claim 16:
- Temperature: 140°C to 250°C
- Reaction time: 10 minutes to 300 minutes
Claim 18 gives a specific embodiment:
Claim 17 defines an additional stopping condition:
- Stop before the polymer forms an insoluble gel
This is likely to be highly litigated because “gel formation” can act as a proxy for the “molecular weight decreases” requirement.
Claim-by-claim scope table (practical boundaries)
| Claim |
Protection type |
Key limits that define infringement |
| 1 |
Product |
Polyanhydride Mw > 20,000 AND IV > 0.3 dl/g (chloroform, 23°C), produced from specified dicarboxylic acid structural classes (including heterocyclic acids with X = O/N/S, n = 1–3, plus combinations) |
| 2 |
Process-defined product |
Polymerizing highly pure prepolymer from highly pure dicarboxylic acid; form polyanhydride Mw > 20,000; remove high-Mw condensation product; stop before Mw decreases |
| 3 |
Product narrowed |
Claim 2 product where at least one dicarboxylic acid is from enumerated list including sebacic, isophthalic, terephthalic, dodecanedioic, cyclohexane diacids, and multiple substituted bis/oxy types |
| 4 |
Process-defined product |
Claim 2 with polymerization in presence of catalyst |
| 5 |
Dependent process |
Catalyst is acid, base, or coordination |
| 6 |
Dependent process |
Catalyst is from list including (CH3COO)2Cd, alkaline earth metal oxides, calcium carbonate, diethylzinc complexes with hydroxylated compounds |
| 7 |
Composition (product) |
Composition comprising polyanhydride made by claim 2 process logic (high-Mw formation, removal, early stop before decline) |
| 8 |
Product |
Mirrors claim 1 product but restates full acid-class generic formula set and includes the same Mw and IV thresholds |
| 9 |
Product narrowed |
Mirrors claim 1/8 with specific named acid list (at least includes the acids enumerated in claim 3) |
| 10 |
Method |
Same process logic as claim 2: polymerize high-purity prepolymer from high-purity diacid; form Mw > 20,000 polyanhydride; remove high-Mw condensation product; stop before decline |
| 11 |
Dependent method |
Polymerized in presence of catalyst |
| 12 |
Dependent method |
Catalyst is acid, base, or coordination |
| 13 |
Dependent method |
Catalyst includes (CH3COO)2Cd, alkaline earth metal oxides, calcium carbonate, diethylzinc complexes and hydroxylated compounds |
| 14 |
Dependent method |
Prepolymer by refluxing diacid in aliphatic anhydride |
| 15 |
Dependent method |
Prepolymer by acid chloride + organic base route |
| 16 |
Dependent method |
140–250°C and 10–300 min |
| 17 |
Dependent method |
Stop before polymer forms an insoluble gel |
| 18 |
Dependent method |
~180°C, ~90 min |
| 19 |
Dependent method |
Select diacid matching acid structural classes as in claims 1/8 (same generic formulas and R/divalent logic) |
| 20 |
Dependent method |
Same enumerated acid list as claims 3/9 |
What is the likely patent landscape structure around US4757128?
Where this patent sits conceptually
US4757128 fits a niche in:
- resorbable polymers / drug delivery matrices where high molecular weight polyanhydrides are used for erosion-controlled release,
- anionic/cationic polymerization control is not the headline here; instead it is molecular weight preservation via stopping before Mw decline and harvesting the desired high-Mw fraction,
- and product claims tether to IV and Mw rather than solely to monomer identity.
Landscape pressure points
Across the polyanhydride space, the most relevant competitive differentiators typically include:
- Choice of dicarboxylic acids and resulting backbone structure (including heteroatoms and aromatic content),
- Achieving or maintaining high molecular weight without gelation,
- Manufacturing controls that prevent Mw loss during condensation polymerization,
- Use of different catalysts and different prepolymer preparation routes.
US4757128’s scope specifically protects:
- high-Mw polyanhydrides meeting Mw/IV thresholds,
- plus manufacturing strategies that include early termination and removal of the high-Mw fraction.
Practical freedom to design around
Design-around tends to be constrained by how the independent product claims are drafted.
Potential “breakpoints” a developer would look for (conceptually) include:
- Using a diacid outside the claim’s structural class formulas (for claim 1/8) to miss product coverage, or
- Matching diacid class but ensuring final polymer does not reach IV > 0.3 dl/g in chloroform at 23°C, or
- Producing polymer that meets Mw but uses a different process that does not match the “remove high-Mw condensation product and stop before decline” logic (for method claims 10 and related product-by-process claims).
This is especially important because claims 1/8 are not framed as “produced by process X,” while claims 2/7/10 are.
How enforceable is the claim set in litigation terms?
Strengths
- Numeric thresholds for Mw and IV create a measurable infringement boundary for product claims 1 and 8, and for related claims using the same polyanhydride definition.
- Specific named diacid lists exist in 3 and 9, limiting the number of plausible acid candidates for enforcement when testing by formulation identity.
- Process claims are structured with clear steps that can be mapped to manufacturing records: prepolymer formation, polymerization, removal of high-Mw fraction, and stopping logic.
Weaknesses for the patentee (frequent battle zone)
- Product claims based on formula-defined diacid structural classes can become interpretation-heavy, particularly for generic aromatic/heterocyclic formula drawings.
- Process-by-process constraints in 2/7/10 can be attacked as non-correspondence if manufacturing differs (for example, if high-Mw fractions are isolated differently or if the “stopping before Mw decreases” is not satisfied in a way that mirrors claim language).
- Catalyst-dependent claims 6 and 13 are narrow, giving competitors a clear path if they need to avoid those exact catalysts.
Investor and R&D implications: what to do with this claim architecture
If you are developing a competing polyanhydride
- The primary risk is direct product coverage under claims 1 and 8 if your final polymer has:
- Mw > 20,000
- IV > 0.3 dl/g (chloroform, 23°C)
- and is derived from diacid structures inside the claim’s structural classes.
- If you can’t change the backbone chemistry, the second line of defense is to ensure your polymer does not meet the IV or Mw thresholds under the specified testing condition.
If you are licensing or investing in a supply of compliant polyanhydride
- Manufacturing must be able to substantiate:
- prepolymer purity steps (the claim uses “highly pure” language),
- harvesting high-Mw fraction after formation,
- and stopping before Mw decline (or before insoluble gel formation in dependent embodiments).
Key Takeaways
- US4757128 protects high molecular weight polyanhydrides defined by Mw > 20,000 and intrinsic viscosity IV > 0.3 dl/g in chloroform at 23°C, plus diacid structural-class constraints and R-group rules.
- The independent product coverage (claims 1 and 8) is broad because it is not limited to a single synthesis route; the independent method coverage (claim 10) is narrower because it requires removal of high-Mw condensation product and stopping before molecular weight decreases.
- Enforcement leverage is strongest where manufacturing records show the “stop before decline” logic and where analytical testing confirms Mw and IV under the claimed conditions.
- Narrow claim fallbacks exist via named diacid lists in claims 3 and 9 and via specific catalyst lists in claims 6 and 13.
FAQs
1) Do the independent product claims require a specific polymerization route?
No. Claims 1 and 8 define the polyanhydride by Mw, intrinsic viscosity, and diacid-derived structure, not by a specific process route.
2) What is the most critical numeric infringement trigger?
The combination of Mw > 20,000 and IV > 0.3 dl/g measured in chloroform at 23°C.
3) How do the process claims differ from the product claims?
Claims 2/7/10 require a specific workflow: form high-Mw polymer from highly pure prepolymer, remove the high-Mw condensation product, and stop polymerization before Mw decreases.
4) Which claims are most useful if you are avoiding specific catalysts?
Catalyst-specific narrow coverage is in claims 6 and 13, listing (CH3COO)2Cd, alkaline earth metal oxides, calcium carbonate, and diethylzinc complexes with hydroxylated compounds.
5) Is there a “named diacid” fallback that narrows scope?
Yes. Claims 3 and 9 enumerate specific dicarboxylic acids including sebacic, isophthalic, terephthalic, dodecanedioic, and cyclohexane dicarboxylic acids, plus substituted bis/oxy types.
References (APA)
[1] United States Patent No. 4,757,128. (n.d.). High molecular weight polyanhydrides and methods for synthesizing same. Claims 1-20.
